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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. 
diagram of reversing valve

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!

Fox Family Heating & Air serves Sacramento, Rancho Cordova, and all of Northern California. If you’re looking to schedule HVAC service in these areas, give us a call!


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