DuoTherm Heat Pump Icing Up

Got it. Thanks

One thing. The CONDENSOR fins MUST be clean. If you have NOT cleaned them, then the dirt/debris will hold the moisture on the fins and they will ice up faster. The 10 button CCC and the 5 button CCC boards have different red plastic 2 wire connectors. They will not interchange. 24 inch would be for a Penguin model with on board control board. 48 inch would be for evap area control board mounting. All 5 button CCC boards use the same design ambient sensor. 24 or 48 will work. Doug

One of the campgrounds I stay at in the winter has Heat Pumps for the office/internet lounge/Bath-Rest rooms and laundry. They ice up when running in HEAT mode (outside coils) and then when done heating reverse long enough to melt the ice.

I suspect an installation error. (Frost sensor misplaced) but.. Can not guarantee Make sure the coils are clean.. check the owner's install manual(s) to insure it is properly installed including any frost sensors. then call for warranty repair/advice if needed.

NOTE: heat pumps are NOT designed to be used below a set temp. like (but not necessarly) 40 degrees. F

It ices up pretty frequently, but I have noticed that sometimes it is when humidity is high.

One thing that also effects how quickly and how much the outside ices up is how much humidity is in the air. This could vary day to day and in different regions. That is, it will ice up much quicker in high outside humidity combined with freezing conditions.

Thank you, Doug. I was hoping you'd show up. I've got the older 5 button CCC. I finally did get the heat pump to go into defrost yesterday when it was 33 degrees out. I did notice that it appears as though the word "defrost" only appears on the CCC when the unit is actually defrosting, that is, when the fan is off and the freon reversed. I thought it would stay on from 30 to 42 degrees to let you know it was in the defrost state. If that is what it's supposed to do, that is, stay on the entire time that its between 30 and 42 (approximately), then the sensor is definitely bad. I'm pretty well convinced that is what is wrong. So I'll order one. They list two, one that is 24 inches and the other 48 inches, but from the pictures it looks like only the 48 inch one has the red plug that's correct for my control box. I can't tell for sure from the picture about the 24' one.

howardwheeler wrote:
Well I finally got a solid reading on the Ambient Sensor, and it was way off and in the wrong direction. The chart provided by Duo Therm to check it starts at 55 degrees and goes up. I don't know why they don't give figures for 40 degrees and lower since that's where the defrost cycle is supposed to cut in. Anyway at 55 degrees there's supposed to be a reading of 11,667 ohms and it goes down significantly from there. At 65 degrees it's 8959 and at 75 it's 6939. Well it was around 45 outside and I got a reading of 7180. I think I'm going up on the roof and take the shroud off, find the sensor end and put it in ice water, 32 degrees. Then I'll see what it says. I watched the CCC thermostat and while it was in the 30s outside the thermostat never read "defrost" as its supposed to. I may have a bad sensor, but it also could be hooked up to the wrong place and getting abnormally warm temp readings.

The Ambient sensor is connected by Dometic on the outside, so will be in the correct location unless someone has moved it. OHM readings on Sensors and Thermisters do NOT give accurate results, and BAD sensors will OHM out correctly. Doug

You need to state WHICH system you have. The older 5 button CCC or the new 10 button CCC. The 10 button, the Defrost is activated below 42 degrees ambient and down to 30 degrees, When in this temp range, the compressor is limited to 25 minutes running, after this 25 minutes, the fan shuts OFF for 4.5 minutes, while the compressor reverses flow for the 4.5 minutes. After the 4.5 minutes, the compressor reverses flow for HEAT operation, and after 30 seconds more the fan comes on. This cycle will continue while in the 30 to 42 degree ambient range. BELOW 30 the HP will shut down and start the AUX heat system. This is the same operation for the 5 button EXCEPT, the 5 button the Defrost will stay ON. The 10 button the Defrost and HP will toggle back and forth. Soooooo, I would replace your Ambient sensor. Doug

Well I finally got a solid reading on the Ambient Sensor, and it was way off and in the wrong direction. The chart provided by Duo Therm to check it starts at 55 degrees and goes up. I don't know why they don't give figures for 40 degrees and lower since that's where the defrost cycle is supposed to cut in. Anyway at 55 degrees there's supposed to be a reading of 11,667 ohms and it goes down significantly from there. At 65 degrees it's 8959 and at 75 it's 6939. Well it was around 45 outside and I got a reading of 7180. I think I'm going up on the roof and take the shroud off, find the sensor end and put it in ice water, 32 degrees. Then I'll see what it says. I watched the CCC thermostat and while it was in the 30s outside the thermostat never read "defrost" as its supposed to. I may have a bad sensor, but it also could be hooked up to the wrong place and getting abnormally warm temp readings.

I have found a service manual online for an older model, but the main components seem similar. There is an ambient sensor that is supposed to send the heat pump into defrost mode once outside temps go below 42. It has a red plug and goes right into the control box. My preliminary testing shows its ohm reading to be within the service manual's specs, around 13480 ohms and it was around 48 outside. But I'm going to check it again tomorrow when I have two extra hands to hold my meter. I'll let you know.