Defrost Mode On A Heat Pump

Defrost Mode On A Heat Pump

What Happens in Defrost Mode on a Heat Pump?


There’s something mystical when it comes to the heat pump system.  We know it runs like a normal air conditioner in the cooling season, but when we get into heating season, some extra components come into play and we get confused or lose track of the sequence of operation for a heat pump.

So let’s go over some basics, that even I have to review from time to time because heat pumps are not my strongest suit.  I have a video called Basic Heat Pump Operation that you might want to refer to if you need an even more stripped-down version of heat pump operation.  This one’s going to focus on the defrost function and what we should be checking to diagnose a heat pump that’s not working or might be frozen over in heating mode on a cold day.

So let’s quickly review some things we learned in the last video.  In cooling mode, the heat pump works just like an air conditioner.  The refrigerant cycles through the system and basically makes the indoor evaporator coil, a cold coil, and the outdoor unit’s coil the hot coil.  We remove the heat from inside the house at the outdoor unit and pull it out to the outdoor coil to be released into the atmosphere.

In heating mode, a reversing valve reverses the flow of refrigerant to make the indoor coil the hot coil and the outdoor coil the cold coil.  So, we’re trying to extract heat from the outside and bring it inside, which can be done down to a certain outdoor temperature.  After that, there is very little heat in the air to extract, so heat strips will kick in to supplement that effort.  

A regular occurrence with a heat pump in the heating season is for the outdoor unit to go through a defrost cycle.  You can imagine that cold outdoor coil interacting with the cold outdoor temperatures can cause some freezing.  Anytime that outdoor coil gets below 40° or so, the outdoor coil being the cold coil develops frost on it.  It can’t keep operating this way, or that frost will develop into a straight-up ice block!

So, we want to melt this frost by essentially switching back into cooling mode.  Because remember, in cooling mode, the outdoor coil becomes the hot coil.

You’ll notice when you wire in the low voltage on a heat pump, you’re not just wiring in two wires like on a normal AC condenser.  Single stage heat pumps need five wires running outside to them.  Red for “24 volts”, Blue for “Common” which can be labeled B or C on defrost boards, Orange for the “Reversing Valve” or the O terminal,  Yellow for “1st stage compressor” or the Y terminal, and something like a black or brown for the X2 terminal or “Emergency Heat.”  

Notice I didn’t say “Y for cooling” because the same Y terminal is energized whether we’re in cooling or heating mode.  We’re essentially energizing the compressor and fan on the outdoor unit.  Whether we want to be in cooling or heating is up to the “O” terminal being energized or not.

Heat Pump Wiring

Remember, these wires can be any color coming from the indoor air handler to the outdoor heat pump.  All wires are copper inside.  So, for the Y terminal at the heat pump defrost board, if a wire with purple sheathing leaves the Y terminal at the indoor air handler, then the other end of that purple wire should be tied into the Y terminal at the heat pump.  It doesn’t matter what color that wire is.

The reason we have so many wires coming to the outdoor unit is to relay signals given from the heat pump to the air handler when it goes into defrost.

The defrost board is the quarterback for this whole play too.  For the defrost cycle to begin, two things have to happen.  A sensor attached to the outdoor refrigerant coil (the copper coil)(or aluminum) has to get down to 26° F, and a second requirement is that the defrost board has to agree that the compressor has run the required amount of time.  On the equipment I usually work on, it’s either 45 or 90 minutes.  

When those two requirements have been met, a contact on the defrost board closes, completing a circuit to read 24 volts at X2 so the heat strips at the air handler will come on.   Inside at the air handler, the fan still blows, which means there is cold air coming out of the ducts.  But the air handler’s heat strips come on to neutralize the cold air.  

That same circuit closing causes the O terminal to have 24 volts which reverses the flow of refrigerant to cooling mode.  You’ll hear when that happens too because the reversing valve makes a pretty noticeable whooshing sound when the change in directions happens.  We explain more about the reversing valve in another video.

The third thing that happens when that circuit completes is a set of contacts open to stop the outdoor fan motor.   This is to help warm the coils up faster.  Because if we were drawing cold air across the outdoor coils when we were trying to warm them up, it would be counterproductive.

You would think the reversing valve would energize to go into heating mode, but on 90% of the systems out there, not having 24 volts to the reversing valve causes the system to default to heating mode.  In most parts of the country, having heat is more important than having cool air, so the reversing valve on a heat pump defaults to heating mode.  Here in California during the summer, we would strongly debate that.

So what have we done here?  What voltages should we be reading at their respective terminals as the board triggers the defrost cycle?

  • 24 volts can be read between C and R on the defrost board.
  • 24 volts can be read between C and O.
  • 24 volts can be read between C and Y.
  • 24 volts can be read between C and X2 or whatever the emergency heat terminals happen to be labeled on your equipment.
  • Also, the high voltage wires (usually labeled D1 and D2) on the defrost board leading to the outdoor fan motor, will only be sending 120 to the motor instead of 240.  So, one of those terminals will have 120 to ground and the other will have 0 volts to ground.

What needs to happen for the demand defrost cycle to complete?  When the liquid temperature leaving the outdoor coil reaches about 50 degrees, the defrost termination relay on the defrost board opens.  If the temperature doesn’t rise to that point after 10 minutes, an override switch will open, and de-energize the relay which will terminate the cycle.

One last time the reversing valve makes a big whooshing sound and switches the flow of refrigerant back to heating mode, the outdoor fan turns on, the heat strips inside turn off, and the indoor coil becomes the hot coil again.

When defrost has completed and the system has gone back into heating mode, here are the voltages you’ll read back at those same terminals from earlier.

  • 24 volts can be read between C and R on the defrost board.
  • 0 volts can be read between C and O.
  • 24 volts can be read between C and Y.
  • 0 volts can be read between C and X2 or whatever the emergency heat terminals happen to be labeled on your equipment.
  • Also, the high voltage wires on the defrost board leading to the outdoor fan motor, will be reading 120 to ground on each terminal. 

If you find that the outdoor heat pump is turning into a giant ice ball, there are a few things to check before condemning the defrost board.  After the system has been turned off a while and the ice has melted, let’s make sure the coils are clean.  Restricted airflow across the indoor coil or the outdoor coil can cause the ice build-up.


If the coils are clean, we need to check the refrigerant levels.  If those are good, then something’s going wrong with the defrost operation.  It could be the refrigerant line or ambient sensors, the actual board itself, or the reversing valve that is malfunctioning. 

Most of the time the temperature sensors are permanently attached to the defrost board, so if they’re not reading correctly, the whole board would have to be replaced.  Installation guides have tables that show the resistance the sensors should be reading at certain temperatures.  Using your meter and some super thin leads will help you determine the readings.

Remember, the defrost board sends 24 volts to the reversing valve at the O terminal.  Is that 24 volts reaching the solenoid on the reversing valve?  No? Then check the wire connections.  If they’re good. Then the defrost board itself is likely bad.  

Yes, you do have 24 volts?  Then something is going wrong with that solenoid and or the valve itself.  But the defrost board is doing its job.

Just like with control boards on a furnace, if the board is giving the proper voltage to the motor and the motor isn’t working, it’s not the board.  If the board isn’t giving the proper voltage, then it’s the board or something else upstream of it.

See!  Defrost boards aren’t that hard, huh?

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


Don’t miss our video on this topic:

Which AC System Is The Best?

which AC system is best Thumbnail

One of the most common questions I get as an HVAC contractor in Sacramento is, “Which AC system is the best?” I see a ton of articles online about this topic – many that someone who’s not even in the HVAC industry wrote!  Some compensated blog writer wrote it or gave you a list of top-rated systems.  Systems they’ve never even touched. These bloggers are telling people that nationally recognized economy line systems are better than the systems that are truly going to last you a long time.


Short and sweet, three companies have the best reputation over several decades of manufacturing, in no particular order, they are: 

  • Trane
  • Carrier
  • Lennox 


Now, when I say Trane, I also mean American Standard.  And when I say Carrier, I also mean Bryant.  Lennox is Lennox.  But, American Standard systems are made in the same factories, on the same production lines as Trane systems.  Bryant is made in the same factories, on the same assembly lines as Carrier.  The difference?  The tag is on the side of the unit.  American Standard does have a different shell around the sides of the outdoor AC unit, but that’s it.  I know this because I toured the factory where they make them.  I’ve seen the process.  (And it’s very cool!)

So, the same high voltage contactor that’s in a Carrier is in a Bryant air conditioner.  The same inducer motor on a Carrier is in a Bryant furnace.

But isn’t Carrier more expensive than Bryant? And isn’t Trane more expensive than American Standard? Until now, I noticed that Bryant was a little cheaper, although they were engineered exactly the same.  But now, in 2021, I see a very marginal price difference—the same with American Standard and Trane.  Lennox systems are priced right along with these brands as well.  So if you get different prices from contractors giving you bids, it’s because of that contractor’s overhead or desired profit margins.  Not because one is more expensive than the other to the contractor.

Knowing this the list really looks like this, in no particular order:

  • Trane (or American Standard)
  • Carrier (or Bryant)
  • Lennox


We sell Trane as our premium line and Coleman or Payne as our economy line.  But my goal here is to try to stay as neutral as possible here so you don’t feel like I’m trying to sway you one way or the other.  You’ll hear me talk about some brands being better than others, and I mean no offense to anyone or any manufacturer.

But you’ve got to take this sort of advice from someone who’s installed all of them at one point or another and serviced the equipment out in the field. 


Before I list the rest of the systems,

I want to mention air conditioning systems come fully assembled at the factory and are ready to work. However, it takes experienced technicians to modify the unit per the manufacturer’s instructions to conform to your specific home’s demands.  The last steps of installing it “in the field” and adding whatever additional parts to bring it up to proper building code in your area is up to the contractor you choose.  That’s an important point because buying a Trane, Carrier, or Lennox includes buying it from a professional, detail-oriented, reliable contractor that you trust and are comfortable with bringing it to life.  If someone is going to install it for you, but you can’t find them after the install because they sell systems so cheap they’re out of business, or they simply won’t pick up the phone, that’s not going to help you when you need some follow-up.

You can buy any system, but if the blower settings, gas pressures, static air pressures, high and low voltage wiring, fuse sizes, a precision refrigerant charge, and airflow, water drainage, gas piping, intake air, exhaust system, thermostats, and other safety codes aren’t set up correctly, you’ll find your new system not lasting nearly as long as it could have.  It can be the difference of your system lasting ten years or lasting 20 years.


Other brand names in the field would be considered middle-of-the-road type systems. These names, in no particular order, include:

  • Rheem (or sister brand Ruud)
  • Amana 
  • Day & Night
  • Heil
  • Bosch  

Why are they mid-tier systems?  As a technician, I seem to repair these systems more than the premium names.  The repair parts are available just like others, and the warranties are just as strong.  That’s never been a problem for me.  But, it’s a fact that they break down at some of the most inopportune times.  So just keep that in mind.


Even more brands perennially end up at the bottom of these lists.  In no particular order they are:

  • Goodman
  • Daikin
  • Payne 
  • Coleman 
  • Tempstar 
  • RunTru 
  • York

These have the most challenging time breaking the stigma attached to them.  They carry this stigma because they are the brands installed on newly built homes in middle America.  HVAC contractors will only win their bid to get a large job like a new pre-planned community if they have the lowest bid.  So, they have to use the cheapest equipment they can get their hands on.

You can find most of these cheaper systems online. They sell to whoever will buy them. And, you’ll see the cheapest contractors, home flippers, and DIY’ers buying this equipment and trying to install them themselves. This comes back around to it mattering WHO installs your equipment and not entirely about WHAT equipment you buy.


If the brand you were thinking of isn’t on this list, it could be that here we are talking about your typical unitary or ducted split systems and package units.  Names like Mitsubishi, Fujitsu, Gree, Midea, and others make ductless mini-split systems.  We’ll talk about those in another discussion.

All of the mid-tier and higher brands typically have three levels of systems they offer.  

  • (Entry-level) A single-stage heating and cooling option
  • (High quality) A two-stage option
  • (Most efficient) A variable speed option


The single-stage option has the simplest form of technology, is the lowest in price but the lowest in value.  While they are UL Listed and safe to put in any home, lower-end models have more vulnerabilities than higher-end equipment.  

I can’t really say whether a Trane, Carrier, or Lennox entry-level system is better than the other.  The technology is the same.  Heck, the compressors, which are the heart of the air conditioner, are virtually the same. I can say, for my home, I would feel a lot better installing one of these three instead of the mid-tier or lower levels.  It’s not because I’m an elitist or anything.  The elite products are the higher-end technology variable speed systems.  


Almost every part of these single-stage systems can be repaired with universal parts.  Meaning you don’t necessarily have to go through the distributor to get the replacement part.  Single-stage motors, compressors, control boards, pressure switches, and gas valves are everywhere and readily available.  Very likely even on your technician’s van right now.


Two-stage systems have better technology.  They run more efficiently and control the temperature in your house without fluctuating as much.  The main feature of a two-stage system is that they all typically run at around 70% capacity in the first stage and 100% capacity in the second stage.  These systems will run the majority of the time in the first stage, which is where you start seeing the money savings.  Two-stage systems are great for two-story homes that have two thermostats or zoning.  These systems can be set up to run in the first stage when only one floor is calling for air.  The second stage will only come on when both zones are calling for air.  This is how I have it set up in my house.

I’ll stay with my single stage theme when I say I couldn’t pick which one is best out of the three premium names.  Trane, Carrier, and Lennox are battle-tested and have been for decades in this technology.  I tell people when it comes to a salesman saying, “oh, but our system is an 18 SEER, not 16 or 17 like the other brand.”  I tell my customers not to get too caught up in SEER ratings and focus on the technology.  Any two-stage system is going to outperform a single-stage system.  The minuscule savings you’ll receive by going with an 18 SEER two-stage over a 17 SEER two-stage is trivial.


Two-stage motors and compressors will have to be ordered from the warehouse near your town that distributes your brand of equipment. There aren’t a lot of universal parts available for two-stage systems.  Capacitors, contactors, and some other parts are universal.  But with higher-end equipment, you see safety components like special pressure switches to protect the furnace or air conditioner from damaging itself.  These parts have to come from the factory.  With Trane, Carrier, Lennox all the way “down to” Goodman and York, I’ve never really had a hard time getting these replacement parts.  At the most, we’ve had to wait for 5 to 10 business days for the part to come directly from the manufacturer.  There are always exceptions to this, but, honestly, it would be the same for any brand.


When you start dipping into the most efficient tier of equipment, the variable speed systems, you’ll start seeing some noticeable differences.  As a Trane dealer, it’s hard for me to say this, but Trane and Carrier have sort of fallen behind Lennox.  Bosch also makes a pretty sweet variable speed system.  They even have the first variable speed package unit.

Remember how two-stage systems have a 70% and 100% capacity? These are the most expensive units, with technology that is less bulletproof than two-stage technology. But, if you’re a techy or just like the premium life, variable speed stands out because of the comfort levels it can produce. Variable speed systems can adjust their capacity levels from about 25% to 100% in less than 1% intervals at a time. They maintain even lower temperature swings in the house.  These systems can keep your home to within a half degree of the temperature you want it. 

These are the quietest systems too.  Because they typically run at a slower speed, they require less energy and create less noise with less vibration.  

Lennox claims the top spot as far as SEER ratings go with a 28 SEER system.  Compare that to Trane’s top unit, which is 20 SEER, and Carrier’s 19 SEER version. 

All of these variable speed systems have WiFi capabilities, are communicating systems, and are ultra-quiet.  Lennox and Carrier variable speed systems work with the Amazon Alexa app. Trane doesn’t have that feature as it only works with its Nexia platform.


When it comes time to repair these variable speed systems, only their proprietary parts will work.  With such intricate technology comes priciness and a higher learning curve for who can actually make the repair for you.  Trane, Lennox, Carrier, and other brands with variable speed lines will usually only make these parts available to respected dealers of those brands.  The skill it takes to handle inverter-type systems is next level.


When it comes to deciding which AC systems are the best, you have three systems perennially at the top of the list.  Trane, Carrier, and Lennox.  While each of their single-stage and two-stage systems all pretty much have the same capabilities, efficiencies, and life spans, it’s the higher-tier variable speed systems where you’ll start seeing the differences.  Lennox has pulled away from the field by offering 28 SEER variable speed systems.  

When you start looking for a vehicle, you pretty much have a brand name in mind.  You might get a higher or lower-end model with fewer bells and whistles, but maybe you’ve always felt comfortable driving a GMC truck over a Toyota truck.

Let me know what you think about this in the comments below?  I see a ton of articles online about this topic – many that someone who isn’t even in the HVAC industry wrote.  Some paid blog writer wrote it.  You’ve got to take it from someone who actually installs them and services the equipment out in the field. 

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Thanks so much for reading, and we’ll see you in the next blog.

Air Source Heat Pump Basics

Air Source Heat Pump Basics

Heat pumps and air conditioners are very similar. I want to share my experience with heat pumps and how they operate to give you cooling in the summer and heating in the winter.

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