How Does a Gas Furnace Work?

Easy Guide to gas furnace troubleshooting


Understanding How a Gas Furnace Works and the Sequence of Operation

Hey guys, how are you doing?  Today I’m going to describe for you the sequence of events that needs to happen for your furnace to start blowing warm air into your house. We’ll start at the thermostat and go all the way to the blower turning on, forcing air into the rooms of your home. Furnace troubleshooting is the topic coming up today on Fox Family Heating and Air.

Furnace Troubleshooting Safety

First, I want to give fair warning to anyone watching this that isn’t already an experienced technician in the HVAC industry.  This furnace troubleshooting video is for educational purposes only.  Fox Family Heating and Air does not recommend anyone other than a professional start opening up the furnace to try to diagnose the failure going on with your system.

There are high and low voltages that can shock a person.  There are also lots of moving parts that can damage body parts—namely, hands and fingers.  The furnace also produces hot surfaces within the furnace compartments and around the housing, potentially causing severe burns.  An actual flame produced by the ignition and startup of a gas furnace can cause severe burns and damage to a person or property.

When your house reaches a point where the heat needs to come on to keep you comfortable, a series of components work in a specific order to produce that heat.

The Thermostat

The thermostat is the first part of the sequence that engages, making the furnace work. There’s 24-volt power at the R terminal of the stat already.  Within the workings of the thermostat, 24 volts closes a switch at the W terminal.  That signal is sent to the control board back at the furnace.

The Furnace Control Board

The control board is a printed circuit board with various switches, resistors, and terminals that act as the quarterback of the heating system.  It calls the plays as they need to happen.

Low Voltage Wires

The control board has a terminal block with screws on it, with a set of thin low voltage wires coming from the thermostat.  Typically, the colors of these wires are red, yellow, white, green, and blue.  The wires are going to R (red), Y (cooling), white (heat), green (blower motor), and blue (common).

Note: the wire colors don’t matter here. They’re still copper on the inside of the sheathing.  So if we use a brown wire for R at the control board, brown needs to be hooked up to R at the thermostat.

The Inducer Motor

Once the control board receives the thermostat signal to turn the heat on, it tells the inducer motor to come on.  The inducer motor is a major component that removes the carbon monoxide from the flame of the gas furnace.  It draws the spent gasses into the metal or PVC flue pipe, which transfers those fumes from the furnace to the atmosphere through the roofline.  You may have seen the metal pipe sticking out of the roof of your house in the winter, exhausting steam into the air. That’s the exhaust we’re talking about here.

The Pressure Switch

This safety device proves that the inducer motor is on and doing its job properly.  If it’s not, the sequence shuts down and retries again.  This pressure switch is actually measuring the suction the inducer motor is producing and sends a signal back to the board, letting it know that startup is working so far.

Roll Out and High Limit and Pressure Switches

Meanwhile, other low voltage safety switches are sending a signal of all-clear back to the board.  There are a couple of “roll-out” switches and a high-temperature limit switch that must confirm to the control board all is well there, too.  The wires leading to the roll-out, high limit, and pressure switches are usually all wired in the same series circuit with each other as a safety control.  If any of these safety switches sense anything wrong with the heating system’s startup, the sequence stops, and retries.

The Ignition Sequence

Next, three components engage in lighting the flame and proving that it is lit. When the pressure switches and other safeties tell the control board all is well, the board starts the ignition sequence.  First, the board sends a signal to the ignitor.  This could be a hot surface ignitor that glows orange or a spark ignitor, which produces an arc between two metal forks lasting for several seconds.  (My blog post and video discussing why hot surface ignitors fail might be of use for you, too.)

Whether the ignitor glows or sparks or not, the next component, 24 volts, is sent to the gas valve, which opens the diaphragm inside of it. It opens, allowing natural or propane gas to flow on to through the metal burner assembly.

The Heat Exchanger

The gas now flowing through multiple orifices in the burner assembly reaches the ignitor, causing a flame to ignite and burn in a controlled fashion straight into the firebox or heat exchanger.  For the purposes of this post, we’ll call it the heat exchanger.

Crossover channels within the burner assembly allow the gas to flow from the first burner to the last one, where the flame pours over a thin metal safety rod called the flame sensor.  The flame meeting the rod creates a millivolt DC signal to the control board that allows the gas valve to remain open.  No flame being sensed means gas is flowing uncontrolled throughout the furnace cabinet, which is not good.

At this point, we have power, a good thermostat, a functioning inducer motor, ignition, flame, and flame sensor to verify it.

A delay now occurs to allow the heat exchanger to warm up so cold air isn’t sent through the ducts and into the air.  The heat exchanger is a hollow metal box with individual chambers.  The flame pours into each chamber, warming the metal to an extreme temperature.

Once hot enough, the air that flows over and around the metal box warms quickly from room temperature to about 100 to 140 degrees.  The temperature is set by the manufacturer and must be closely adhered to.  This will keep the system operating safely and to proper specs.

The Blower Startup

After this delay completes, the blower starts up, sending forced room temperature air over the correct speed of the metal heat exchanger.  If the air is sent over too fast, the air entering the room won’t be warm enough.  Too slow of air or not enough air and the system gets too hot.  Too hot means the high-temperature limit we discussed earlier will open, telling the control board something’s not right. So, this blower motor has to be dialed in just right.

Furnace Troubleshooting Tips

Here are some things that can happen when the furnace isn’t starting up correctly.  The following troubleshooting tips are not all-inclusive and are not to be taken as scripture that what is going with the furnace you’re working on is the problem.  These are general problems only.

No power to the board – If the unit is plugged in correctly and the breaker at the main panel is in the on position, there should be power to the furnace control board.  A transformer can fail between the outlet and the control board, and they can and do regularly.  The board with proper power can send the high and low voltage signals it needs to be the quarterback and run the plays.

Power, thermostat, no inducer motor – Low voltage power is sent from the control board to the R terminal at the thermostat.  Assuming you have 24 volts there, the thermostat closes the W switch, which now has 24 volts applied to it.  If the 24-volt signal is getting back to the control board’s W terminal, the control board will send the high voltage signal to the inducer motor.  If voltage is getting to the inducer motor but it doesn’t run, you likely have a bad inducer motor or capacitor for the inducer motor if it has one.  If you’re not getting voltage to the inducer motor from the board, you have a bad board or faulty wiring connection between the two.

Power, thermostat, inducer, no ignition – If the inducer motor is running, the ignitor should start glowing or sparking.  The gas valve should open, allowing the gas to flow, the gas flame should crossover to the other burners in line, and a signal should be received at the flame sensor telling the board everything is good to go.

As with many components in furnace troubleshooting, if the part is getting power but not operating, it’s likely failed.  If it’s not getting power from the control board, it’s likely a bad board.  I have a great video on why control boards fail for more information.

Power, thermostat, inducer, ignition, flame, sensor, but no blower – If everything works as it’s supposed to, except the blower motor hasn’t turned on after the flame ignited after about a minute or so, something is going on there.  If the motor is getting power but not working, the motor or its capacitor may have failed.  If the motor is not receiving power from the board, the board is likely bad.  Not all blower motors have capacitors, either.  This is especially true for systems made in 2020 or later.

Power thermostat, inducer, ignition, flame, sensor, blower, shuts down on high limit or roll-out – Lastly, if the blower motor comes on and the system starts heating, but after a few minutes or even several minutes the system shuts down, the high-temperature limit switch may have opened causing the system to retry again, after the heat exchanger cools off.  If the chamber that houses the heat exchanger gets too hot, this high limit switch will shut down the system.

So What Causes a Shutdown?

First, we have to check that the blower speed settings are correct.  Next, the air filter could be dirty, ductwork could be too small or even collapsed, or the evaporator could be clogged with dirt. (Check out one of my most popular videos that shows what kind of problems a dirty evaporator coil can create.)

All of these items have one thing in common:  not enough air flowing over the heat exchanger.  This causes the inside temperature of the furnace to go over the recommended setting established by the manufacturer.

Although many things can go wrong with the gas furnace, sometimes in combination with each other.  Not much else can go wrong unless something in this sequence goes wrong.  I sure hope this answers some questions you have about troubleshooting a gas furnace.  Be safe and use your head out there. Don’t get in over your head if you don’t know what you’re doing.

Thanks so much for stopping by, and we’ll see you next time!

Don’t miss our videos related to this topic:


Troubleshooting a Furnace Pressure Switch: 5 Easy Things to Check

Troubleshooting a Furnace Pressure Switch

Today we’re going to talk about troubleshooting a furnace pressure switch.  I want to expand on our recent gas furnace troubleshooting series by going into each part of a furnace’s operation sequence.  I’ll fill you in on what the pressure switch does and why it’s important.  And to wrap things up, I’ll give you five easy things to check when you’re troubleshooting a furnace pressure switch.  That’s coming up here on the Fox Family Heating & Air Blog.

First, I want to give fair warning to anyone watching this that is not already an experienced technician in the HVAC industry.  This video is for educational purposes only.  Fox Family Heating & Air does not recommend anyone other than a professional to start opening the furnace up and trying to diagnose the failure going on with your system. 

There are high and low voltages that can shock a person.  There are also lots of moving components that can damage body parts—namely, hands and fingers.  The furnace also produces hot surfaces within the furnace compartments and around the housing, which can cause severe burns.  An actual flame produced by the ignition and start-up of a gas furnace can cause severe burns and damage to a person or property.

The Furnace Sequence of Events

First, as a technician, you must know the sequence of events for a gas furnace to start properly. It’s very straightforward, and you should have this memorized before you can even consider being qualified to troubleshoot.

  1. Power to the furnace control board
  2. Thermostat signals the call for heat
  3. Inducer motor kicks on
  4. Pressure switch proves the inducer operates correctly
  5. Ignitor activates
  6. Gas valve energizes
  7. Flame pours across burners
  8. Flame sensor proves all burners are lit
  9. The blower forces air through the ducts

First, the Inducer Motor Starts

When a furnace begins a new cycle, the inducer motor is the first thing you should see kick on.  120 volts is applied through the wires coming from the control board.  This starts the inducer motor for up to 60 seconds before anything else even happens. It’s a safety feature that creates a negative pressure or draft which purges the heat exchanger of any poisonous gasses, namely the byproducts of combustion.  It makes the air inside the hollow tubes of the heat exchanger cleaner when the flame kicks on. When we have cleaner air inside the heat exchanger at the time of combustion, the furnace’s efficiency increases.

Next Up, the Furnace Pressure Switch Activates

Next, a safety device called a pressure switch activates when the diaphragm inside it recognizes the inducer motor’s suction or purging action.  The pressure switch is a normally open switch that closes upon the manufacturer’s specifications for required negative pressure.

If the inducer turns on and is working normally, the furnace pressure switch should activate. There’s no time lag on this either.  The inducer motor creates this draft quickly.  Within 5 to 7 seconds in most cases – and the rest of the furnace starts up from there.

If the pressure switch doesn’t activate, the furnace will then shut down, wait a bit, and try again.  If the pressure switch doesn’t close after 3 to 5 tries, the control board will stop sending voltage to the inducer motor, essentially locking it out from attempting it anymore.

You can tell the system is on some sort of safety lockout when the furnace’s fan (or blower) pushes room temperature air through the ducts and into your rooms.   No one likes cool air blowing into their house when it’s heating season, so this happens to alert the occupant that the system isn’t working right, and they should call an HVAC company to come out and troubleshoot the system.

How to Troubleshoot a Furnace Pressure Switch

Let’s assume the inducer motor is running correctly. But the pressure switch doesn’t seem to be closing.  Using your meter, you can trace the 24 volts coming from the control board, through the safeties, and on to the pressure switch.  Place one lead on ground or a solid piece of metal attached to the furnace.  Place the other on the incoming terminal of the switch.  If you have 24 volts on the incoming terminal but not at the terminal leaving the switch, you can assume the pressure switch has not closed. 

Another way of doing this is by testing with your leads across the two terminals.  When the pressure switch is open, your meter will read 24 volts. When the switch closes, you’ll read 0 volts.

Remember, the pressure switch doesn’t close until the inducer motor comes on and provides the necessary suction for the pressure switch to close. The required suction is listed on the pressure switch.

Furnace Pressure Switch Not Closing

When we’re troubleshooting a furnace pressure switch, we can do a few things.  We can take our manometer and make sure the inducer motor is creating the vacuum by hooking up the meter’s hose directly to the collection chamber that the pressure switch tubing is connected.  Take that pressure switch hose off and put your manometer’s hose on the same port.

Once you put the hose on and start the system up, the inducer comes on, and the manometer should start reading the induced draft as it begins to rise.  This number on the meter needs to be greater than the number on the pressure switch.  For example, suppose you’re testing a pressure switch that closes at -0.7 inches water column. In that case, the suction reading on the meter should be around -1″ wc.  It could be less, it could be more, but it can’t be less than the number on the pressure switch.  Meaning, if you’re reading -0.4″ wc, something is causing a low pressure reading. 

Five Easy Things to Check

Let’s check out some of the things that could be going on here. Some of the more common reasons I’ve seen furnace pressure switches either fail or not close to allow the rest of the system fire up are:

  1. A clogged port on the collection chamber to the pressure switch
  2. An obstruction in the flue
  3. A diaphragm that’s ruptured or stuck
  4. The pressure switch hose is damaged
  5. The pressure switch hose has water in it

1. A clogged port on the collection chamber to the pressure switch

On the collection chamber, check to see if the port itself is clear of any calcium deposits, dirt, or other debris that would prevent air from flowing through the port.  If there is, take a small wire — like some thermostat wire — and clean that port out.  Whatever the substance is, it should be brittle enough to be scraped off, allowing the port to become clear.

2. An obstruction in the flue pipe

Remember, the inducer motor causes a draft to allow the gasses to be drawn out of the heat exchanger and into the flue pipe where it terminates outside the building—usually the roof.  I have found that bees, wasps, and birds like to build their nests in and around the flue pipe. It’s not likely to happen during the winter, but for sure can happen over the summer.  So, if the season is early and the furnace hasn’t been run yet, it’s good to check at the roof vent. It’s also not uncommon to see that the nest (or the bird) has fallen all the way down to the base of the pipe, where it meets the furnace. 

3. A diaphragm that’s ruptured or stuck

Commonly, the pressure switch fails because the diaphragm inside the casing has become stuck or it has ruptured.  Ruptured diaphragms can sometimes make a flapping noise.  Stuck diaphragms just won’t budge on the required draft.  Sometimes a little tap with your finger on the flat part of the casing will free the stuck part, and that’s great!  But your switch is on borrowed time, and nine times out of 10, the switch will fail again. If I tell you it will fail tomorrow, it will last for years.  If I tell you it will last for years, it’ll fail tomorrow.  I recommend trying to convince the customer to replace the pressure switch now, so there are no surprises.

One way to see if the pressure switch is stuck open or closed is to breathe lightly into the hose leading to the switch. You’ll hear the diaphragm open and close.  It doesn’t mean the switch will work properly, but it gives you more information to troubleshoot a furnace pressure switch.

Because pressure switches are almost impossible to rebuild, a new switch should be ordered.

4. A pressure switch hose is damaged

There have been diagnostic service calls that I’ve been on where I could tell the port was clean, the flue pipe was clear, the inducer motor was pulling a proper draft, and the diaphragm was functional.  But, the pressure switch would still not send 24 volts across to the other terminal.  Is the hose itself in good condition?  Rats like to chew these hoses up and leave holes in them. Other hoses can become brittle and crack.  Keep some extra tubing in your service van for cases like this.

5. A pressure switch hose has water in it

Another possibility is that water could be stuck inside the tubing to the pressure switch.   Condensing furnaces send the flue gasses up the pipe, but latent heat will turn that gas into moisture that flows back down the flue pipe and into the inducer motor assembly.  Remember that naturally flowing water flows downward.  If there is a low spot in the hose leading to the pressure switch, you’ll find it won’t close.  Try draining the hose by unplugging it from the port.  Just be careful; there can be a lot of water in the hose so maybe have a bucket handy. 

Installing a Furnace Pressure Switch

During the pressure switch installation, you want to make sure it’s mounted in the correct position.  The pressure switch you took out was in a vertical position for a reason.  Diaphragms don’t activate as easily when they must fight gravity.  I once replaced a pressure switch with a universal and mounted it horizontally, parallel with the ground, and the switch failed a few days later.  Of course, it didn’t happen when I tested it the day I installed it, but another tech had to come out and fix the problem a few days later.

I could have avoided this had I read the section in the installation instructions that said not to mount it in the horizontal position.  I never did that again!

OEM switches will usually just screw right back into their old spot, but universals sometimes need to be creatively mounted. You may need to use a longer hose to get to the switch—another good reason to have an extra hose on your van.  And make sure there are no dips in the hose, so water doesn’t accumulate, causing the switch to fail again in a couple of days.


So, just to recap.  When a furnace begins a new cycle, the inducer motor is the first thing you should see kick on. A safety device called a pressure switch activates when the diaphragm inside of it recognizes the suction or purging action of the inducer motor.  The pressure switch is a normally open switch that closes upon the manufacturer’s specification’s required negative pressure.

If the pressure switch closes normally, the rest of the furnace sequence of operation will continue.  If the pressure switch won’t close, the system will go into safety mode, try a couple more times, and eventually just start blowing cold air in your room, letting you know something is wrong with your furnace.

Be patient and check the things we went over today before condemning a pressure switch.  It could be one of a few things.

Thanks so much for stopping by, and we’ll see you on the blog next time.

Don’t miss our video series related to this topic:

Troubleshooting a Furnace Gas Valve

Troubleshooting A Furnace Gas Value

10 Easy Things to Check When Troubleshooting a Furnace Gas Valve

Hey guys, today we’re going to talk about troubleshooting a furnace gas valve.   I wanted to expand on our recent gas furnace troubleshooting series by going into each part of a furnace sequence of operation.  I’ll describe what the gas valve does and why it’s important.  And towards the end, I’ll give you ten things to check when you’re troubleshooting a furnace gas valve.  That’s coming up here on Fox Family Heating & Air.

The Furnace Sequence of Events

First, as a technician, you have to know the sequence of events that occurs for a gas furnace to start up properly. It’s straightforward, and you should have this memorized before you can even consider being qualified for troubleshooting a furnace gas valve.

  1. Power to the furnace control board
  2. Thermostat signals the call for heat
  3. Inducer motor kicks on
  4. Pressure switch proves the inducer operates correctly
  5. Ignitor activates
  6. Gas valve energizes
  7. Flame pours across burners
  8. Flame sensor proves all burners are lit
  9. The blower forces air through the ducts

First, the Inducer Motor Starts

When a furnace begins a new cycle, the inducer motor is the first thing you should see kick on.  One hundred twenty volts are applied through the wires coming from the control board.  This starts the inducer motor for up to 60 seconds before anything else even happens.

Next, a safety device called a pressure switch activates when the diaphragm inside it recognizes the inducer motor’s suction or purging action. 

When the “all clear” signal arrives at the control board, high voltage is sent to the ignitor – be it a hot surface ignitor or a spark ignitor.  The hot or sparking ignitor stands in the way of the gas that is getting ready to pour over it. 

This is Where the Gas Valve Comes Into Play

Modern gas valves typically have a printed circuit board in them that receive a 24-volt signal to activate the valve inside of it.  Remember the video I did on printed circuit boards?  If not, I’ll attach it below so you can brush up on what they are and the things that can go wrong with them.

This sequence will happen in three stages – and even if one step of this doesn’t perform, each part is still going to do its thing sequentially once the board gives the signal.

So, after the board senses the pressure switch and inducer motor are working:

  1. 120 volts is given to the ignitor (on some package units, it’s 240 volts.)
  2. 24 volts is given to the gas valve.
  3. The flame sensor starts detecting if there is a flame or not.

The ignitor is supposed to come on for a set amount of time: 30 to 60 seconds. (See our video on ignitors for an in-depth explanation of this topic.)

Next, the gas valve opens.  The gas coming from the utility company or the propane tank in the back yard is free to flow on to the ignitor.  That gas valve is what’s regulating the flow of the gas.

The flame sensor senses whether the flame is correctly burning.  At the opposite end of the burner assembly, the flame sensor also stands in the way of the flame.  The rod, which should be cleaned annually, by the way, will heat up and send a millivolt signal down to its ceramic base and on to the control board.

Only a certain amount of gas can be allowed to pass through the manifold and on to the burners.  The manufacturer of the furnace determines what that will be.  It is pretty standard, though—about 3.5″ water columns (wc).  The natural gas pressure coming from the street is somewhere around 7″-10″ wc, but the gas valve itself specifically allows that 3.5″ wc onto the burners. 

There are some situations and equipment where I’ve been told to bring the outlet pressure down to 3.25″ wc.  But I only did it on the advice from the technical support rep from that equipment.  Specifically, it was Ruud equipment.  The rollouts were getting too hot because the hood covering the flame would trap the heat and make the safety open.  Modifying the hood and adjusting the gas pressures were recommended to us, which seemed to fix it.

Furnaces differ, so please check your furnace installation and service guide for your system’s specifics.  This is something you don’t want to get wrong.

The gas valve is adjustable.  And usually, the installer of the equipment will dial in the outlet pressures on start-up.  Because the gas valve manufacturer – Emerson, White-Rodgers, Honeywell, and other valves makers will usually have it pre-set to that 3.5″ wc, some installers forget to do this.  We can’t assume the valve is correctly adjusted each time. That’s why you can have issues with your furnace related to your gas valve – because it wasn’t set up right by the installer during its first use.

Troubleshooting a Furnace Gas Valve

If 24 volts is coming from the board to the gas valve terminals and you don’t hear that little clicking noise the internal valve makes, you could have a bad gas valve.  To double-check, take the leads off to the gas valve and check there.  Got 24 volts?  Then something downstream of that 24 volts is not working. 

What’s the next thing that’s supposed to be working?  The printed circuit board or electric solenoid attached to the gas valve isn’t telling the valve to open, OR that gas valve board IS telling it to open, but the valve is stuck somehow.

If something is wrong with the internal components of the gas valve, it should be replaced. The gas valve cannot be repaired in the field. Only the gas valve manufacturer or someone certified by the gas valve manufacturer can make these repairs.

Some people will literally take a wrench and bang on the gas valve to get it to open up.  This is extremely dangerous.  Gas is nothing to toy with.  If you decide to try this and it kicks on, please replace the gas valve now rather than later. 

If we try to fix these ourselves and something goes wrong with the gas valve, and it somehow caught the house on fire, the investigation could come back to the furnace.  If they wanted to know who last worked on it and what was done to it, the gas valve manufacturer could claim innocence, and the homeowner’s insurance could deny the customer’s claim.  I know that sounds a little drastic, but it could happen.  Why put yourself in that situation?

I see people try to fix control boards, ignitors, and such, but we shouldn’t try to fix gas valves ourselves with such a sensitive instrument.

Here are ten things we can check when we think we have a bad gas valve before condemning it:

  1. Check the wires to the gas valve.  Are they cracked or frayed?  That could mean a couple of things.  You have a REALY old furnace, or something could have scorched the wires—things like that.  Replace the wire and continue your diagnostic.
  2. Check the coil at the gas valve.  If you check the coil’s resistance by putting your two-meter leads on each terminal and you get a reading of OL, you have a bad coil. There are more complicated things here but let’s keep this straightforward. 
  3. The gas coming into the valve should be at utility line standards.  It’s around 7″-10″ wc for natural gas in my neck of the world. There’s a port on the inlet side to check it.
  4. You may have plugged burner orifices.  A furnace that’s been off all summer can be the victim of a spider spinning a web inside the burner orifices.  Now, that’s a tiny spider, I know, but I promise, it happens!  Take a small piece of thermostat wire and gently poke inside the holes of the orifices attached to the manifold and try to fire up the system again.
  5. The flame might be coming on for a few seconds but then shutting off.  Is there a dropout of voltage or gas pressure to the gas valve?  That’s something to check for sure.  And you can do that by putting a “T” fitting in line with the hose to connect to your manometer.  Check the inlet and the outlet side to see if the pressure is dropping on either side of the valve. 
  6. Another reason the flame could drop out after only a few seconds of burning is the flame sensor.  If the sensor doesn’t detect the flame, the control board will signal the gas valve to shut down.
  7. If the flame does anything but shoot directly into the hollow metal heat exchanger, a safety can trip.  One safety trip is the rollout switch.  Sometimes you’ll get a little part of the flame that drifts off to the left or right, sending the switch off.  That doesn’t mean you should remove the switch.  It means you need to fix the problem.  Clean the end of the burner assembly nearest the heat exchanger.  Rust will sometimes build up on the crossover channels.  Use a wire brush to clean and see if that solves it.  Then place the burner correctly into the channel.
  8. The other safety trip that can cause the system to cut the gas off to the valve is the high limit switch.  If the furnace runs for a few minutes, then shuts off, something could be causing the inside of the furnace to get too hot. The first thing I would check is to see is if the evaporator coil is dirty.   I have a great video that shows what a dirty evaporator coil looks like and what it takes to clean it.
  9. The other reason the high limit could open is the blower motor speed could be set too low.  Check your installation guide as a reference for where the settings should be.
  10. Check the ductwork too.  These last three have all dealt with airflow.  If the return duct is crushed, then we’ll have low airflow again.  Visually check the return duct and feel around it if it looks questionable.  If the duct is not perfectly round, then this could be the problem. The furnace is suffocating.

What else should folks check when troubleshooting a furnace gas valve?  Leave me a comment down below to share your expertise.

When you’re installing the new gas valve, there are few things to keep in mind. It’s a like-for-like change out, but gas leaks are a serious issue, so make sure to use some pipe dope or pipe tape to seal the fitting. 

Also, don’t bend the manifold when you’re trying to remove the gas valve or put the new one back on.  Use two wrenches to get a proper hold on the manifold and the gas valve.

I strongly recommend not over-tightening the gas valve to the manifold.  You could bend the manifold, but also remember, someone might have to get that thing off someday, and you’d be creating a challenging situation for a tech that has to come out and service it in a few months.  Some guys get a little over the top and really crank down on it.  Not necessary. 

Check for gas leaks with an electronic gas sniffer or soap bubbles.  This will assure you the fittings are snug and leak-free.  And don’t forget to check the outlet side when the gas valve is on.  It doesn’t help when the valve is off because no gas is flowing through it.

If it’s a natural gas set-up, the spring that comes inside the valve will already be the right one.  If you’re using LP gas, you’ll need to make sure you put the right spring in it. It’ll come in the box.  Check the manifold orifices to ensure they are the right ones for LP too. And put the sticker on the gas valve that says LP.  This will help future HVAC technicians when they service the furnace.

And lastly, check the gas pressure on the new valve after you’ve replaced it.  I can’t say it enough. It’s simple to do with the right tools, don’t just change the valve and not check the pressures.

When it comes to troubleshooting a furnace gas valve, there’s also a setting for low fire on two-stage units that needs to be checked.

If the gas pressure is too low, your furnace’s efficiency will go down.  More condensation than usual will build up because the air in the air-fuel mixture will be too high.  The condensation can cause corrosion, possibly creating the need for a heat exchanger replacement in the future.

High gas pressure can be just as bad for your furnace because it dramatically increases the furnace’s overheating risk. When this happens, high limit switches will start opening, causing intermittent operation.  It can also crack your heat exchanger since it’s only rated to handle a certain amount of heat.  And cracked heat exchangers can introduce the spent gasses inside the heat exchanger to be carried along with the heat blowing into the house.

So, to recap.  When a furnace begins a new cycle, the inducer motor is the first thing you should see kick on. A safety device called a pressure switch activates when the diaphragm inside it recognizes the suction or purging action of the inducer motor.   Next, the three parts of the ignition sequence begin.  The ignitor kicks on, the gas valve opens, and the flame sensor senses that the flame exists.  If this all goes well, you have heat blowing into the house about a minute later when the blower kicks on.

What else should folks check when troubleshooting a furnace gas valve?  Leave me a comment down below to share your expertise. Thanks so much for stopping by, and we’ll see you at the next blog post.

Don’t Miss Our Video Series on This Topic and Related Videos:

Diagnosing a Bad Blower Motor: 8 Easy Things to Check

Diagnosing a Bad Blower Motor

Hey HVAC techs! I’m Greg Fox, and today we’re going to talk about diagnosing a bad blower motor. I want to expand on our recent gas furnace troubleshooting series by going into each part of a furnace sequence of operation. This week, it’s the blower motor.

This blog post is all about the commonly used permanent split capacitor (PSC) blower motor. The capacitor is a storage bucket of electrons that helps regulate the voltage going to the motor as it starts up and continues running through the cycle. Today’s capacitors typically last 5 to 10 years. But as always, it can last much longer.

Before you start taking wires off, something I’ve learned over time is:

• I like to use my work phone to take a picture of the wires attached at the control board, so I won’t forget what taps the original wires were attached to. It makes things much easier for me when deciding which color of wires to put where.

• The other thing I do to make things easier for me when I’m disassembling a blower from its housing is to use a marker to identify where the wires were routed as they come from the motor to the control board. This way, you can position the blower and its belly band in the right spot the first time!

Sequence of Operation

So, what needs to happen for the blower fan motor start?  In air conditioning or cooling mode, the thermostat calls for the AC to kick on by connecting 24 volts from the R terminal to the Y terminal. But the thermostat also gives 24 volts to the G or Fan terminal. The Y terminal being energized kicks on the outdoor unit, while simultaneously the G terminal being energized turns on the air handler’s blower motor.

In heating mode, the 24 volts at the R terminal connects only to the W terminal to start the sequence of events that happens for the furnace to start up. It energizes the W terminal but NOT the G terminal in heating mode because the control board tells the blower when to turn on. That’s usually 30 to 60 seconds after the flame has ignited to start heating the furnace. The delay in starting it is because we don’t want cold air coming through our ducts while we wait for the heat exchanger to warm up. Smart eh?

Once the low voltage signals have made it to the control board terminals, a series of connections wind themselves through the printed circuit board, where a switch will energize to send 120 volts, or in some cases 240 volts on to the motor. Check your wiring diagram to be sure which motor you have.

Eight things that you can do to diagnose a furnace bad blower motor:

1. Make sure it’s getting power to it – if you don’t have the proper voltage coming in from the board, start by checking voltage at the outlet, then move on to the transformer, making sure high voltage is coming in, and 24 volts is going out to the board.

If you have power to the board and no power is going out to the motor (and everything else is operating normally in the furnace sequence) you may have a bad control board. You can test this by jumping out 120 volts to the blower motor wires; this will let you know if the blower motor works or not.

2. Ensure the capacitor attached to it is good – whether you do it before you jump out the blower motor or not, quickly check the capacitor to see if it meets manufacturer specs. If it doesn’t, replace it and retest.

3. Make sure the motor isn’t getting too warm to the touch – Some motors just get old and dirty. If the blower gets too hot, it can lock up and stop spinning or still spin but have an open winding inside the motor that doesn’t allow the circuit to be complete.

4. Check the amp draws and compare them to the data provided by the furnace sticker. It’s sometimes necessary to use a mirror or pull the motor out to check the motor’s amp rating. If you have access to it,  the furnace’s service facts manual will list the OEM motor specs. And they vary quite a bit. 4 to 12 depending on the size of the furnace and type of motor. If the amp draws are too high, either the motor could be going bad, causing intermittent operation, or the static pressure pushing back on the blower squirrel cage is so great it’s causing the motor to strain to provide the air needed.

5. The motor won’t spin – If the rotor that holds the fan blower wheel won’t spin or is hard to spin, it’s a good sign that the blower motor is shot. Once again, if the motor is getting proper voltage and the capacitor is good, but the blower won’t freely spin, it may have seized, which is not uncommon.

6. The motor spins but is making a scraping or screeching noise – You could have proper voltage to the motor, but it has this god-awful screeching noise. Check to make sure the squirrel cage isn’t rubbing against the side of the blower housing. Center the wheel inside the blower housing and re-tighten the locking nut to the rotor shaft, so it doesn’t slide anymore.

7. The spine of the blower wheel could be separated from its fins, which cup the air and throw it into the ducts. The air pushing back against it will cause metal on metal friction, creating the noise. The spine attached to the rotor shaft is spinning much faster than the separated wheel itself. Replace the blower wheel and retest.

8. Dirty blower motors and the importance of filters – I’ve seen blower wheels get so dirty that you need to clean them just to get air moving again. The cups are filled with so much dust, dirt, skin, and hair that it weighs down the wheel and creates a motor strain. Sometimes when it’s bad like this (and you should tell your customer this before you perform the blower cleaning), the clean wheel becomes light enough again that the motor burns itself out because all this time, it’s been used to spinning a heavier weight.

You could clean everything and get it going again, and two weeks later, get called out because the blower’s not working because it spun out. If you covered your bases by communicating properly with the customer, your company wouldn’t have to pay for a new blower motor. It could look shady if you didn’t—just something to think about.

I hope this blog post has given you some useful pointers to use out in the field. Pinpointing a bad blower motor can be one of the more intimidating parts of diagnosing a furnace. But if you know when it’s supposed to turn on in the sequence of operation, you can go from there by checking the things listed in this post.

Thanks so much for stopping by, and we’ll see you at the next blog post.

Don’t miss our videos related to this topic:

Expect Great Things at Your Fox Family Air Conditioning Tune-Up

Expect Great Things at Your Fox Family Air Conditioning Tune-Up

What Is an Air Conditioner Tune-Up, Anyway?

An air conditioning tune-up is what responsible homeowners do to maintain their home’s HVAC system.   It’s a thorough cleaning and testing of the air conditioning system, to ensure that it’ll work when you need it this summer.  We operate with the understanding that a clean system will run longer than a dirty system.  To read more about keeping your system running during the summer, check out our blog post on this topic.

Not every HVAC company in town will perform an Air conditioning tune-up.  I would say it’s because they feel they’re not very good money generators for them.  I get it, wiping down air conditioners and testing parts aren’t very exciting for some.  But I don’t think they understand the opportunity they have to create a relationship with someone and their HVAC system.

I like to develop relationships with my customers by taking care of their AC system every spring.  If we can perform an air conditioning tune-up every spring, for years and years, I know my customers will, at the very least, allow us to provide a quote for a new system when the time comes.  At the same time, our customers get to work with Sacramento’s most honest air conditioning company.  Our technicians will only bring up parts or repairs that will make the system return to factory standards, help it last longer, and make it safer for their families.

What Happens at My Air Conditoning Tune-Up?

When we are on the way, our technician will call you and let you know.  We are proud to park on the street in our bright white vans with the Fox Family logo on them.  After you open the door and allow us to come in, we usually start at the air filter and thermostat.

I like to ask if there are any areas of the house that need any attention.  Do any rooms not get the right amount of air?  Does it cool the house down to your satisfaction in the middle of summer?  Questions like this can establish how you like your system to run.  Because not every homeowner is the same, right?

Once I know what’s going on in your mind as a homeowner, we will turn on the AC system, together, at the thermostat, listen for the air to come on, and walk outside to make sure everything out there is at least running.  Now we all know the system was running when we arrived!

From here, you’ll be able to sit back and do whatever you need to do while the tech goes out and runs through a list specifically designed for your type of air conditioning system.

Step One of Your Air Conditioning Tune-Up

We usually start out at the air handler in the attic or closet.  As a responsible business owner, if the furnace is in the attic, I need my techs to get in and out quickly.  It’s hard to ask a tech to spend time cleaning a furnace in a hot attic.  The furnace gets physically cleaned during the furnace tune-up, rather than during an AC tune-up, but there are some really important things to check here, so we try to get in and get out effectively and safely.

The most important thing we’re testing is the temperature difference between your supply and return air ducts.  If it’s not where it needs to be, we have a series of checks we will do to get it right.  A quick look at the evaporator coil can make a huge difference in the comfort of your home this summer.  If it’s dirty or clogged it will make your system underperform.

As part of the AC tune-up, we also need to make sure your blower and the flywheel is clean and ready to run a lot in the coming summer months.  The tech will pour water down the drain lines to make sure the condensate drains properly.

We always offer the option of a condensate safety switch to protect your home from potential damage.  The secondary drain pan under the evaporator coil in the attic is a potential source of problems as well, so we make sure it’s ready for any emergencies.

Additionally, we’ll make sure the metering device for your refrigeration system is mounted properly while checking for any obvious refrigerant leaks in the copper tubing.  We’ll also check for proper insulation levels in your attic because it creates such an effective barrier between the hot air in the attic and the cool air you’re trying to keep in your house.  It pays to have a thick layer of insulation up there!

Step Two of the Air Conditioning Tune-Up

Once we’re done in the attic — and I really only want my techs up there maybe fifteen minutes on warmer days — we’ll head to the outdoor unit where the majority of the AC tune-up is done.  Here, we test the components inside the panel, focusing on things like your refrigerant levels to ensure your system isn’t running too long, unnecessarily.  The high and low voltage electrical running the AC needs to maintain a certain sizing, workmanship, and integrity.

Checking Items During Your AC Tune-Up

All in all, we check about 35 items on the outdoor unit and 20 items on the indoor unit.  If you happen to have a packaged unit that sits on the side of the house or even on the rooftop, we still check all 55 items.

After we check the entire AC system, we’ll let you know if there are any parts that need to be replaced.  Our trucks are stocked with almost every part you need for your AC to get back up and running properly the same day.

If your system is running well, we get right to work washing your AC.  Many air conditioning manufacturers are switching to materials like micro-channel which can’t be washed with soapy or chemical solutions, and we pay attention to things like that.  If you have a dog that runs around in your back yard, we try not to use soapy solutions that drain into the area around the AC, so your best friend doesn’t get sick.

We are so thorough cleaning the AC, it’s not uncommon to see a tech vacuuming out your AC to get rid of the sticks and other debris that can be a nuisance to a healthy air conditioner.

What’s the Benefit of an AC Tune-Up?

When the air conditioning tune-up is complete, you’ll have peace of mind knowing that your system is in tip-top shape.  Making sure a professional completes these steps every year will really pay off in the later years of your HVAC system’s life.  As a technician, my 20-year-old system is so clean, it runs like a champ.  It’s old and loud, but it keeps my house cool just fine!  Even on 105-degree days.  Why?  Because I take care of it.

If you’re curious and would like to learn more about how your air conditioner works, check out my blog post on this topic.

So, give Fox Family a call here in the greater Sacramento Valley area.  We would be honored to service your HVAC system in the year ahead!

Thanks for checking in on our blog.  See you next week!


Don’t miss our video on this topic:

Yes, It’s Air Conditioning Tune-Up Season in Sacramento

It's Air Conditioning Tune-Up Season in Sacramento

Fox Family Heating and Air Begins Air Conditioning Tune-up Season in Sacramento

The weather in Sacramento is usually pretty predictable, but this year in February no one needed assistance with their heating systems.  Afternoons are beginning to reach the mid-60’s, so February is actually a great time to start thinking about your AC  tune-up.

Fox Family Heating and Air offers an air conditioning tune-up unlike the others in town. We have a piece of paper that lists everything we will be checking on your AC tune-up visit.

We start out at the thermostat and make sure it displays properly, is mounted correctly, and that you understand how to use your thermostat and are comfortable using it. We’ll check the batteries while we are there too. Some thermostats don’t have batteries and are connected through the “C” terminal. Now we’ll turn your AC on and listen. Listening to the AC system run is like music to the ears of an HVAC technician.

Next, we move on to the filter and make sure it’s clean. If you have a washable type, we’ll wash it for you and let it dry as we continue on with the air conditioning tune-up.

On we go to the air handler or furnace wherever it is in your house. It’s usually either in the garage, a closet or in the attic. Some houses have packaged units that are on the rooftop of the home, or sometimes on the ground. Either way, I’m sure you’ve seen that thing so let’s go find it. While we are here in the attic we’ll check the surroundings and ensure all the ductwork looks connected, is strapped properly, is sealed at the plenums, and delivering air to each room as designed.

We also check insulation levels out in the attic because it’s your air conditioner’s best friend when it comes to saving energy. We are looking for R-38 levels of insulation since that is the code standard set by cities and counties in the area. If you are not even close to those levels, we’ll notify you when we come back down.

Here is a list of components we check on the air handler during the AC tune-up. We clean and check all of these items (except the blower assembly and the evaporator coil.  Additional charges apply on these two items just because a good amount of labor is involved in cleaning them).  This ensures your system can run as efficiently as possible. Clean = Good!

  • Door safety switch
  • Temperature difference between return and supply air
  • Blower capacitor rating
  • Blower wheel is balanced
  • Overall blower assembly cleanliness
  • Lubricating the bearings on some blower motors that still have oil ports
  • Condensate drain lines direct water away from the HVAC unit to protect the house
  • Condensate drain slope is proper
  • Is a condensate safety switch present
  • Secondary drain pan condition (Rusted?)
  • Condensate pump if applicable
  • Evaporator coil condition
  • UV air purification system bulb
  • Refrigerant leaks at the evaporator coil
  • TXV is mounted properly

Then we head to the outdoor unit where we wash the unit from any debris that might have plugged the condenser coils over the winter. At the condenser we check and clean all of the following components:

  • Outdoor temperature
  • Your refrigerant type
  • High voltage service disconnect for proper electrical code safety
  • Tighten the lugs in the electrical disconnect
  • Ensure proper wire size to the AC
  • Max fuse rating of the outdoor condenser
  • Min circuit ampacity
  • Fuses in the service disconnect mounted properly
  • Refrigerant leaks visible around the AC
  • Proper amount of suction line insulation on refrigerant lines
  • Condenser fan FLA
  • High voltage readings at the contactor
  • Low voltage readings at the contactor
  • Compressor start assist present?
  • Electrical connections tightened
  • Compressor run amps
  • Condenser coil condition
  • Compressor capacitor condition
  • Lubricate the fan motor if applicable
  • Is the outdoor unit level?
  • Wash / wiped down the outdoor unit
  • Leaves out of the bottom of the unit

We really enjoy the opportunity to come out and take care of your HVAC system! It has always been a passion for Greg Fox to keep his clients’ systems clean and operational for the upcoming season. We actually have a club membership where we come out and give you:

  • 2 pre-paid precision HVAC tune-ups per year
  • No dispatch fee EVER!
  • 15% off all parts and labor
  • Front of the line priority service

What’s the cost for an AC tune-up membership?

It’s kind of a no-brainer to let us come out and maintain your system for just $14.95 a month or $179.40 per year. We offer it at such a low price just because we want to develop relationships with our clients so when it does come time to change your system out, you’ll choose us to do the work. Talk to your technician and see how he can help you.

Check out our video related to this topic: