Duo Therm Heat/AC function question

It seems they tend to be stuck on the heat function when I attempt to start them, however, I will pay closer attention in the future. Other than our trip last fall, the only use they have seen since I purchased the rig has been a few cycle runs to keep the seals in shape. The units were used for only about a year prior to me buying this coach, not under warranty I suspect, but I do have the paperwork and dealer info. Again, thank you all for the extremely useful information. Once I knew what to go after, I used the control box to switch from off to cool without allowing the unit to start. After a few tries, I heard a definite difference in the sound from the unit above me, at that point I allowed it to continue to start the compressor and bingo, she had switched to cooling. Since that time, both units have cycled off and on frequently without issue.....so far.

I never understood the rational for having to energize it to cool, as Doug said, the default operating condition is cool. It's been like that on my marine units since I can remember and even when Dometic bought Marine Air and Cruisair, that stayed the same.

Chris Bryant wrote:
FWIW, new Dometic heat pumps are reversed, and energize the solenoid in heating, not cooling.
Makes it somewhat of a pain to match control systems, and people see the new CCC2 and try to convert- barely possible with a major (warranty voiding) hack.

I think Dometic changed the function for 2 reasons.
1. Since COLD is the primary function, they wanted the fail mode to default to cooling.
2. By changing the design, Dometic HP's now have 3 fan speeds instead of 2. (The Med speed wire was used to engage the Reversing coil on the old units).
Last, it is extremely RARE for the mechanical reversing valve to fail. It is usually the Energizing solenoid that fails, which is a replaceable part. Also, when ordering the reversing solenoid, you usually get 2 types in the box. Dometic used 2 types and they have no way to determine which models used one or the other. Doug

FWIW, new Dometic heat pumps are reversed, and energize the solenoid in heating, not cooling.
Makes it somewhat of a pain to match control systems, and people see the new CCC2 and try to convert- barely possible with a major (warranty voiding) hack.

If it is a new unit, I would look at getting the dealer to replace it under warranty. The usual cause of a sticking solenoid is a small piece of trash or debris that was introduced into the system when it was built. The valve operates by teflon slides moving from one side to the other and it doesn't take much to hang it up. Not common, but it does happen, and it's likely it will stick on you again. Replacing the reversing valve is labor intensive and requires a moderate amount of skill and experience to do it without burning it up with the torch and most shops are not going to want to do it. I've been changing them for 15 years and my pucker factor still goes up when I have to do one on my marine units.

You can give the valve a light tap on the end with a wooden stick if it sticks again, but noting heavier.

Thanks again, both units up and cooling :B

Here we are, Virginia Beach hit 89 degrees at our camp, now the coach is a cool 72 :C

Thank you! I would bet on stuck, they were never used much and the few times I have test run them I had to turn both units off/on several times to get them to switch to cooling. I wonder if the ole hammer tap would un-stick it? One unit is running full time to keep up with the heat now but holding its own. At any rate, I can make the final repair, now seeing what the part looks like (thanks Chris).

If it is indicating cool mode, but heating, odds are the reversing valve solenoid is burned out. Fairly common on those units. Cheap and easy fix, though there were two versions of the coils, one with attached leads, the other with terminals.

If it is a heat pump, the reversing valve solenoid has to energize for it to run in cool mode. You either have a stuck reversing valve, a defective reversing valve solenoid, or the control board for the unit is defective and not causing the solenoid to energize. It's an easy diagnosis for someone familiar with these units, or you can wait and get every kind of armchair diagnosis here and then take it to a shop.