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Old 07-30-2019, 11:46 AM   #1
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Roof A/C issues

My front AC is okay, however the rear unit is blowing hot air. It's as if the heat pump is running rather than the AC. It doesn't respond to the thermostat temperature settings, it's just full force heat. What could be happening? TIA
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Old 07-30-2019, 11:51 AM   #2
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I would pull the power to the thermostat and control boards (if you have them) for about 15 minutes and see what happens.
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Old 07-31-2019, 10:28 PM   #3
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Roof A/C issues

It sounds like your compressor’s maybe not kicking in, and the unit is blowing ambient air through condenser coils baking in the sun. But let’s see first where the problem lies. Reset the thermostat. You have about the same vintage Dutch Star as I do, likely with a DuoTherm 5-button thermostat. If so, then :
1. Turn the ON/OFF switch to the "OFF" position.
2. Simultaneously depress and hold the MODE and ZONE push-buttons while turning the ON/OFF switch to "ON". FF should appear in LCD display until the mode and zone push-buttons are released.
If you get the FF, the Thermostat is ok, and the problem is in the roof unit, possibly compressor fail, compressor start capacitor, or control circuit board. If you don’t get FF, then there’s maybe something amiss in the thermostat board.
Beyond that, it might be better to have a tech check things out.
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Old 08-01-2019, 04:35 AM   #4
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If your compressor is running producing heat with a heat pump unit then you most likely have a bad coil on the swap valve. In most RV roof heat pump units if the swap over coil goes bad the unit defaults to heat. Simple to check coil with an ohm meter. If coil is bad you have just 2 wires to deal with. You do have to remove the top cover to find the swap valve as that is the area it is hid.

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Old 08-01-2019, 05:48 AM   #5
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Thank you for all the responses. I'll look into them.
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Old 08-01-2019, 05:57 AM   #6
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If it is a Dometic /Duotherm my guess would be the reversing valve solenoid coil, this can easily be replaced as long as the valve itself is not sticking.
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Old 08-01-2019, 06:37 AM   #7
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Assuming your AC is original it could be a failure of the reverser solenoid or the reverser valve. Of course it could be a control board failure.

The older AC models defaulted to the heat pump mode if the reverser solenoid or valve failed. Yes, the valve could get stuck in the AC mode but that I've been told that it is unusual to do that. As far as your system/thermostat is concerned, it doesn't know of this failure. I knows you want cold air but doesn't know you are getting hot air. So it assumes you are not cold enough yet thus keeping the unit running pouring more hot air into the coach.

Not good for pets.
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Old 08-01-2019, 07:34 AM   #8
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If your reversing valve is stuck on heating, it is easy to check out. On the roof you would get cold air off your condenser coils.
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Old 08-01-2019, 11:20 AM   #9
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Great ideas. Thanks for all the help. I'll reply when I find the problem and get it fixed.
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Old 08-03-2019, 11:30 AM   #10
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This same thing happened to me a while back. Ended up just replacing the rear A/C. I upgraded to a 15k BTU and sealed the supply and return “holes” on both units while I was on the roof. It made a noticeable difference in temperature and volume of air, and now the coach can stay at 72 degrees in high-humidity mid-90 degree days, under direct sunlight.
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