Defrost in Cool Mode

This article is in reference to typical Carrier, Bryant, Payne, and WeatherMaker Heat Pumps.

Beware of the defrost cycle in the cooling mode!

Sounds bizarre right? It is more common than you may think, in fact you may have already seen it without knowing so.

What does it look like?

1. The unit will be running in cool mode.
2. The outdoor fan will turn off.
3. The unit will continue to run the compressor IN COOL MODE
   1. Yes, it will stay in cool mode the entire time, NOT heat mode.
   2. If you check, the heat strips circuit (W) will have 24vac.
4. The compressor will run until it turns off on the high-pressure switch (if equipped)
5. The compressor will continue to repeatedly trip on high pressure (if equipped) until it runs hot and opens on internal overload.
6. After 10 minutes, the outdoor fan will come back on
7. When the compressor cools down, it will crank back up.

So, what causes this strange phenomenon? Please see the wire tracing below for reference. 

On these units, the R (24vac) wire comes into the control board of the unit, it comes right back out of the control board to feed the outdoor coil defrost thermostat (DFT). These DFT thermostats contain contacts that should remain OPEN if the coil temperature is above ~65F. When the DFT closes its internal contacts, it returns R (24vac) to the control board to power the logic center.

Once the logic center has power, it starts monitoring the compressor run time. When the time has elapsed for the defrost timer (30, 60, 90 minutes), the defrost control board will initiate the defrost cycle. If the DFT fails in the closed position, then the defrost cycle will max out at 10 minutes and then continue back in the normal cool mode.

How do you troubleshoot this?

If you witness this or suspect this issue, all you need to do is:

1. remove the defrost thermostat (DFT) wires from the control board.
   1. It is a white plug with two pink wires (see image below)
   2. If the unit is actively in Defrost Mode, doing this will immediately terminate the defrost cycle.
2. use an ohm meter to ohm the two pink wires at the white plug.
   1. ensure to make good connections to the wires.
3. If the coil is warmer than ~65F AND your meter shows resistance:
   1. The DFT is bad.
   2. Your meter should show OL (Open Line).
   3. Replace the failed DFT.