When it comes to heat pumps there are three types of technicians:
- Technicians who break out in a cold sweat at the thought of working on one.
- Technicians that think they know how they work, but don't.
- Technicians who know heat pumps like the back of their hand.
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-
- The indoor thermostat calls for heat.
- 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.
- 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.
- 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.
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-
- Power the unit down and disable the outdoor fan by Removing and taping off the outdoor fan common wire. Power unit up again.
- Make the unit call for heat.
- Let the unit run until the coil drops below 30 degrees.
- Jump the "test / speed up" pins until the reversing valve shifts (about 15 to 30 seconds), then remove the jumper immediately.
- Defrost should terminate after the coil warms to 80 degrees or ten minutes has lapsed.
- Power unit down, reinstall the fan wire, and return to operation.
If the unit didn't enter defrost mode try this procedure-
- Jumper terminal "DFT" to "R".
- Make the unit call for heat.
- Jump the "test / speed up" pins.
- The system should go into defrost mode, then seconds later cycle back to the cooling mode.
- 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?