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.

Recap

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.

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