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Heat pump controls demystified


When it comes to heat pumps there are three types of technicians:

  1. Technicians who break out in a cold sweat at the thought of working on one.
  2. Technicians that think they know how they work, but don't.
  3. Technicians who know heat pumps like the back of their hand.

Heat Pump PCB

Here's what we're going to do; for the fellows in category 1, we are going to make you so comfortable with heat pumps, you may actually start to like them. Category 2 guys, you know a lot, but some of it's wrong, we're going to get you running at 100%. For you guys in category 3, you can go grab a cup of coffee and chill for awhile. Just kidding, read along, and if you have something to add, send us an Email, if we use your suggestion we will give you kudos as a contributor.

Most technicians see heat pumps as a mysterious collection of circuit boards, sensors, strange valves, and thermostat terminals that make no sense like "O" or "B".

We used to be right there with you, bewildered and confused, but over the years we've not only come to understand them, we've come to like them.

Let's talk about the biggest cause of heat pump induced anxiety, defrost controls. There are two types of defrost control. The really old electro mechanical style that's made up of line voltage relays, timer motors, and various devises used to guess if there is frost on the coil. We're not going to talk about these because this stuff is old, really old, Pittsburgh Steelers winning super Bowl 14 old. If you run across one of these, do everyone a favor, replace it with a new high efficiency heat pump. They may still run, but the efficiency of these dinosaurs is lousy. Heat pumps manufactured since the late 1980's use electronic boards to control their operation.

I know, we said the "E" word, Electronic. If electronic controls make you nervous, you may want to stop here and check out our article "Circuit board troubleshooting tips", then come back and read on.

There are basic defrost control boards, and more recently, defrost control boards that have some intelligence. Both controls do the same thing, they initiate defrost mode, the only difference is how they decide when to initiate it, and how they decide when to stop it. Before we go any further let's make sure everyone is clear on what the defrost cycle is. The defrost cycle is used to melt frost from the outdoor coil, this is done by running the unit in the cooling mode with the outdoor fan shut off, and the indoor supplemental heat tuned on. The outdoor fan is shut off to allow the coil to warm up to melt the frost faster. The supplemental heat is turned on to reheat the supply air that is being cooled while in defrost mode. Using defrost mode only when necessary is critical to energy savings and comfort for obvious reasons. If the reasons aren't obvious let me explain; it's cold outside and you're running in the cooling mode with electric heaters energized! It's like driving your truck with the gas pedal mashed to the floor while stepping on the break to keep from speeding.

This is where we run into a problem; defrost boards don't know if there is frost on the coil, they assume there is frost on the coil. They base the assumption on coil temperature and time, in that order. The thinking goes like this; water can freeze below 32 degrees, so if the coil has been below 32 degrees for over 60 minutes there must be frost there, right? Well, no. But heat pumps don't have eyes, so the best they can do is guess if the coil is frosted. You know that jumper on the board where you select 30, 60, 90, or 120 minutes? That's the amount of time the unit must run in heat mode after the coil temperature drops below28 degrees before the unit will shift into defrost mode.

Here's the sequence-

  1. The indoor thermostat calls for heat.
  2. The outdoor coil temperature starts dropping as the unit runs. When the coil temperature drops below the defrost thermostat set-point (typically around 28 degrees) the timer starts counting down from the time selection you made with the jumper.
  3. When the minutes run out, the control board shifts the reversing valve into the cooling mode, de-energizes the condenser fan and energizes the indoor electric heat with its "w" terminal, then starts a fixed 10 minute termination timer.
  4. The defrost control now waits for one of two things to happen; ten minutes to lapse, or the coil temperature sensor to open because the coil is warm (typically around 80 degrees) at which time the control puts the unit back to heat mode by shifting the reversing valve back to the heat position, shutting off the electric heat, and turning on the condenser fan.

The process then starts all over again.

The mechanical defrost function is basically the same for all controls. The difference is what temperatures and times they use to initiate and terminate defrost.

Heat Pump PCB Closeup

Now that you have an idea of how the defrost cycle is controlled let's dig into the nuts and bolts of the basic defrost board. Defrost boards have low voltage terminals with designations like, R-C-Y-W-O-B-DFT. (R) input- it supplies 24vac to the control. It's connected to "R" at the indoor unit and the "R" on the thermostat. (C) input- it supplies 24vac common to the control. It's connected to "C" at the low the indoor air unit. (Y) input- 24vac from the thermostat. It's connected to "Y" on the thermostat. It turns on the compressor for cooling and heating mode. (W) output- 24vac from the defrost control. It's connected to the indoor units electric heat relay coil. It energizes the electric heat elements while in defrost to keep the supply air warm. (O) input- 24vac from the thermostat. It's connected to the "O" terminal on the thermostat. It energizes the reversing valve to shift into cooling mode. On a call for cooling "Y" and "O" are energized together. (B) input- 24vac from the thermostat. It's connected to the "B" terminal on the thermostat. It energizes the reversing valve to shift into heating mode. On a call for heat "Y" and "B" are energized together. (DFT) input- 24vac from coil thermostat. When the outdoor coil temperature drops below its set-point it starts the countdown timer for defrost (the amount of time selected 30, 60, 90,120 minutes) (T1) input- 24vac from the compressor contractor coil. It's energized when the compressor is running. It enables the defrost circuit and also energizes the condenser fan relay that's built into the board. (Test) input- You may have noticed that defrost boards have two pins labeled "speed up" or "test". When these pins are jumped together, the timing for all board functions is sped up by 256 times.

To run a unit through a defrost cycle do the following-

  1. Power the unit down and disable the outdoor fan by Removing and taping off the outdoor fan common wire. Power unit up again.
  2. Make the unit call for heat.
  3. Let the unit run until the coil drops below 30 degrees.
  4. Jump the "test / speed up" pins until the reversing valve shifts (about 15 to 30 seconds), then remove the jumper immediately.
  5. Defrost should terminate after the coil warms to 80 degrees or ten minutes has lapsed.
  6. Power unit down, reinstall the fan wire, and return to operation.

If the unit didn't enter defrost mode try this procedure-

  1. Jumper terminal "DFT" to "R".
  2. Make the unit call for heat.
  3. Jump the "test / speed up" pins.
  4. The system should go into defrost mode, then seconds later cycle back to the cooling mode.
  5. Return to normal operation by removing both jumpers.

If the unit still didn't enter defrost mode check for 24vac at terminal "T1" during a call for heat, this terminal needs to be energized to initiate defrost. If it's not, check the integrity of the wire going to it and verify that you have 24vac between terminals "R" and "C"

Note; a defrost control uses either the "O" or the "B" terminal, NOT both. Most manufactures of residential equipment energize the reversing valve for cooling (the "O" terminal) and consider it fail-safe. If the reversing valve coil were to fail in the winter, the unit would still be able to provide heat. Heat is generally considered more important than cooling.

Things to consider-

We use the term De-Frost, not De-ice for a reason. Heat pump coils should not become iced up, the defrost cycle cannot clear a coil that has turned into a block of ice. If you have a coil that turns into a block of ice, you have a problem. If the charge is correct, and the defrost thermostat is functioning properly and mounted to the coil properly, you should select a shorter time out period on the board. If it's set for 90 minutes and the coil is icing up, try 60 minutes for a few days. Typically the time out period is selected based on humidity in the area. If the heat pump is located in a valley or next to a lake, the defrost time should be lower, say 30 or 60 minutes. If the heat pump is located in a cold dry area, you can select a longer time out period, like 90 or 120 minutes.

Remember, the colder it is out, the less moisture is in the air. The less moisture there is, the less frost will form on the coil.

See, that wasn't so bad, was it?


0 # Gene 2011-05-11 20:33
Not a comment exactly more of a question! Will a normally operating defrost board initiate defrost in the cooling mode if the sensor is stuck closed. If not other than the 'o' terminal how does the board know it is in a/c mode so as not to initiate defrost?
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0 # Patrick Peterson 2011-05-13 20:01
Great question Gene.
Most base-line defrost boards don't have any idea if they are in cooling or heating; they just see 24 volts on 'T1' as compressor run time. If the defrost thermostat were to fail in the closed position, and the unit was in cooling mode, the condenser fan would shut off and the electric heat would energize for ten minutes every time the selected time expired (30,60,90,etc.) Looking at the back of one of these boards tells the story; the 'O' terminal has a trace to the defrost relay only; it's not connected to any other circuitry on the board. The good news is that DFTs normally fail open since the current through the contacts isn't generally enough to cause them to weld.
Keep in mind this is for most base line boards; higher end units generally have more sophisticated controls and may "look" at the units mode of operation as well as DFT contact status.

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0 # Ceaser Oreo 2013-02-13 11:47
Defrosts , regardless of whether its a heat pump or a refrigerator or walk in are .........
Time "AND" Temperature initiated,....and ...
Time "OR" Temperature terminated.

Meaning "both" time & temperature must be calling for a defrost cycle to iinitiate....
Either Time or Temp opening will terminate defrost.
This insures no un needed defrosting nor longer defrosting than required can occur
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0 # patrick peterson 2013-02-23 22:33
Why "Wrong" Ceaser? You just repeated what we said.

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0 # Mike 2011-11-04 08:05
On the test procedure shouldn't the outside fan kick off by itself instead of disconnecting the fan wire?
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0 # ZenMaster 2011-11-06 22:15
Hey Mike, thanks for the comment.
You are right about the fan kicking off during defrost.
I suggest disabling the fan before the defrost cycle so the coil will cool down quicker.That way you don't have to wait as long. You could just jump out the DFT, but then you wouldn't know if the DFT was working correctly.
An alternative would be using ice water to cool the DFT.
Again, thanks for the comment.
Feel free comment on other articles as well.

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0 # gustavo valencia 2012-01-22 14:43
Ibeen experience build ice all the time the temp. drop below 40 deg. then i replaced the defrost control board and chek if the defrost is making the cycle like is and somethigs does but most of the time doesnt' so is a pain on the b. and my thermostact doesnt' have 2nd stage heat. you habe to set that to make the auxiliar heat came out manualy. I thign that one is part of the problem, but i not sure or maybe the defrost control thermostat is not working properly. Is a 2.5 tons woodman heat pump. please help me.
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0 # Patrick Peterson 2012-01-22 17:40
I believe the defrost thermostat contacts on the 2.5 ton Goodman close at 31 degrees, so when you perform the check be sure the temperature at the sensor is below 31 degrees. (use ice)
Now jump the speed up pins while the system is running in heat- it should enter defrost mode.
If it doesn't, check the defrost contacts (they need to be closed)
If everything is good to this point, check the defrost thermostat's location on the coil; it needs to be in the factory specified location.

If this doesn't help tell me exactly what is happening and when (use a timer)
Remember that the key to diagnosing anything is- knowing what is supposed to happen and when.

Let me know what you find!

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0 # Lee 2012-12-23 08:17
Patrick, thanks for the great information.

I have two heat pump units for my house. One is functioning correctly but the other one frosts up heavily when outside temperature drops below 30. I've figured out that the fan keeps on spinning when the defrost cycle kicks on. I was wondering if there is anything I can check further to ensure my finding. I am handy with multimeter and soldering, but I am thinking this might be a bit beyond replacing few resistors.

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0 # patrick peterson 2012-12-23 21:30
if it is going into defrost (you hear the swoosh sound) but the fan keeps running; it could be the fan relay contacts on the defrost board are welded closed.

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0 # Clint 2013-01-11 08:32
I have a problem with a Rheem heat pump. The RV is energized for heating. Seems that the unit goes into the defrost mode every morning around 4:00 AM and runs for 14 minutes (spec according to Rheem info). The outdoor fan never shuts off during the defrost cycle. Is that by design or is my control board bad?
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0 # BigTIM 2014-09-27 07:23
The first thing I would look at is to see if the condenser fan is wired through the defrost board. Some techs., me included, have a tendency to remove it for some reason during an AC S/C, check. So be sure the wiring is proper before diagnosing the board as bad.
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0 # patrick peterson 2013-01-12 13:21
Sounds like the relay that shuts the outdoor fan off during defrost may have failed closed (contacts welded) but there may be other causes.
The best way to diagnose this is to review the sequence of operation for the defrost control and compare to the actual operation.

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0 # Christopher Brown 2013-01-21 22:47
I'm having a problem with my Rheem..package heat pump (RQNJ-A036JK)
The blower motor will only operate when auxilary heat is on..Not in heat or cool, or fan only.
When thermostat calls for run, the compressor and condensor fan come on, but no blower. When temp drops, aux. heat is called for and the blower will then kick on.
Could this be the DFC?
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+1 # Leonard W Buck 2013-01-22 20:43
If the blower fan will not run in (fan on mode) then this has to be resolved Ck to see if fan relay is working it should have 24v to it.(Green to com) if so then turn power off and run ohm test on coil of fan relay. I have had to bypass and add a relay to save the cost of changing the main board.
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0 # Leonard W Buck 2013-01-22 20:52
The blower fan should run in fan mode. start there.
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0 # Patrick Peterson 2013-01-22 21:31
Leonard is correct, Christopher.

(Thank you Leonard! We appreciate your input)

The fan coming on when the supplemental heat is energized means your unit probably uses a set of contacts on the electric heat relay to bring on the fan directly, this bypasses the failed main fan relay that Leonard mentioned; many units use this style of fan control to ensure the fan comes on when the electric heaters are on and also to drive the fan on with the electric heat for thermostats that have a 'emergency heat' switch function.

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+1 # Monte 2013-02-17 17:10

I have a Carrier that recently started behaving a little odd. It still keeps the house warm but the inside fan comes on more often than it used too and it comes on without the compressor two or three times between “real” cycles. I feel sure this is wasting a lot of power.
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-1 # patrick peterson 2013-02-23 21:55
The cause depends on what model unit you have as they have different style control boards. Some will run the outdoor fan without the compressor if there is a high pressure switch trip, others may run the fan depending on outdoor air temp. Reply back with a model number and we'll see what we can find.

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0 # sheila Johnson 2013-03-13 11:35
Bought home with older heat pump and broken old t-stat. Can't find name on outside unit. Main board in unit labled E,G,W2,W3,L,Y,O,C,R. All have t-stat wires connected except, L and W3. W3 has large black wire going into unit but not t-stat. I'm replacing stat with a digital Honeywell pro 4000 stat. Heres the problem, Honeywell says I can't use there stat because they don't know what W3 is. Well me either. Should it be connected? And if so how do I connect the new stat? Please help. Very confused.
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0 # patrick peterson 2013-03-19 22:28
W3 is normally used to stage electric suplemental heat; it is normally tied to an outdoor air thermostat. I say normally but keep in mind that's just an expression. Different manufacturers did things different ways....if you write back, let us know what brand and model you have.

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0 # JChastain 2013-03-26 09:06
I have a Rheem heat pump in a home that was finished in September. During the winter months, the defrost cycle kicks on almost every night and runs for as much as 45 minutes on emergency heat. I read the article and realize why it's using emergency heat, but it is KILLING my electric bill. My contractor and his sub act like I'm an idiot and that the unit is functioning properly. I would love any suggestions on action to take, or even an insight into why this might be happening. Thanks!
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-2 # A MANSFIELD 2013-04-22 15:55
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0 # m regina 2013-10-18 20:31
have a goodman package unit,the defrost board doesn't put 24 volts to the compressor contacter control.new board acts the same.compressor runs if thermostat yellow is jumpered to contactor .pressure switches check ok. is there another input to the defrost board that would cause this ?model is gph1324h41ac. defrost board is dm133 .
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