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Diplomat Duo Therm Heat pump problem


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The rear AC/heat pump in my '02 Diplomat is blowing hot air only.  Due to covid lock downs by the Commissar here in Michigan it has been more that a year since I have used the coach.  I have read a lot about problems with reversing valve becoming stuck and failing to switch from the default heating to cold air.  I am parked for season and contacted a mobil RV repairman who basically admitted he wasn't knowledgeable with heat pumps.  I appreciate his honesty and saving me the expense of a service call.  He did say he had talked with a Duo Therm tech who admitted that the problem was either a bad solenoid which was repairable, or a stuck faulty reverse valve.  If the solenoid was good, he advised to just bang repeatedly on the valve to possibly free-up the stuck valve.  If that did not solve the problem the only alternative is a replacement.

I would like to hear what experiences others have had with their Duo Therm heatpumps, and what are your thoughts about a defective reversing valve causing a complete replacement of the heat pump.

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I would agree with the tech talking about the valve. If it is really heating the air and not just blowing the same temp in and out. You could check if the solenoid gets power and if so, does the coil have continuity. Go from there. I can hear the valve click when turned on, before the compressor kicks in. Replacing a stuck valve is a relatively big process and you would have to source one from a trashed unit. I fix my ACs just because that's what I do but most people just replace them. Paying someone to replace the valve would hardly be economical. 

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Posted (edited)

Might first check the ambient air sensor. If it is out of range or indicates open  with a multimeter the unit will default to heat. It is 2 wire thermocouple which plugs into the control board and snakes up past the evaporator. Not to be confused with the evaporator temp sensor. Don't recall at the moment what the ohm values should be vs temp. I took the values from my working unit some time ago when I had a similar failure.

Edited by Gary Cole
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Thanks for the suggestion Gary.  I down loaded an older Duo Therm shop manual/installation and trouble guide and I have been looking through the problem list and their solutions.  Not really enjoyable reading, but there are a few good tips in it.

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Different approach but power to solenoid is the easiest check since I don't even have to unscrew the metal cover. If there is power to coil, don't have to even go there.

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Posted (edited)

I think you are right Ivan. The normal position of the 4 way valve is heat mode. I would imagine that would be the case on most all heat pumps.

Edited by Gary Cole
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Yes, the older Dometic heat pumps default to heat mode when the coil fails.  However, the new units now default to cool mode.  

Our heat pump started blowing hot air.  I tried using a rare earth magnet on the valve where the coil attaches but could not get it to reverse.  I ordered a replacement coil and it only took 15 min to install (just two spliced wires).  It now works great.  The replacement coil is about $50.  

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11 minutes ago, vito.a said:

Yes, the older Dometic heat pumps default to heat mode when the coil fails.  However, the new units now default to cool mode.  

I never understood why heat was the default function since it is an add-on option and with my logic, cooling is the main reason for having the unit. Same with residential systems. There is always an alternate source of heat but not an alternative for cooling. Glad to read that they changed it.

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1 hour ago, Ray Davis said:

Dometic is a Swedish company,  those Swedes may have little different idea whether heat or cool should be the default.  🥵🥶

Haha, they might have changed their mind with the heat waves in recent years, good point Ray. I have family there and apparently they have seen the 90's there.

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Back in 2012 (man time flies) we were in N Michigan and my wife was getting ready to go south with the RV and I was getting ready to travel (either to Bolivia or Spain).  The front AC started to blow hot air so I immediately thought reversing valve.  Not knowing what to expect I went on the roof and removed the shroud and found that the wires going to the reversing valve and come in contact with the hot copper tubing for the pump and melted the insulation, shorting out the reversing valve. 

I fixed the shorted wires and resecured all of them and that fixed the problem.  Surprised it didn't blow a fuse but either way it was a cheap fix. 

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I had had a older one go to heat only… coil was good but the 120V relay was bad. Jumped 120V to the coil and used the breaker inside to turn it on/off until I could replace the relay on the board.

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My front AC (duo therm) started doing that before going to the Keys a few years ago. They do default to heat and I replaced the reversing valve ($35?) And no more issues since then.  

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5 minutes ago, Ray Davis said:

deleted

I saw your question before it was deleted.  This was what I used. 

Screenshot_20210714-071927_Chrome.jpg

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2 minutes ago, Steven P said:

I saw your question before it was deleted.  This was what I used. 

Screenshot_20210714-071927_Chrome.jpg

Thanks Steven,   I asked then decided we all knew the answer.   

What I found was you can't just completely delete a message you have to at least put the word delete in before it will accept the delete.

 

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To all who responded to this thread I thank you.  Today I spent some time of the roof of my coach cleaning and checking both AC's.  The rear which was the one that was only blowing hot air got my attention.  I removed the shroud and everything looked to be in good shape.  I used a small leaf blower and cleaned some of the debris, leaves, some acorns and gypsy moths and then tested the solenoid and it was OK.  I then used the technique that was recommended by my mobile RV repairman.  I had my wife turn on the A/C while I used the handle of my screw driver to beat on the reversing valve.  That worked, the unit is working well without any other problems.  Apparently the sticking reversing valve issue is a well know problem with this Duo-Therm Heatpump A/C.  I hope my experience will help others should they have the same problem.

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Now that you mention it that makes sense.  I also thought that the relay might just be to weak to complete the switch over.  But I do not know of a way to test for that.  I assumed that it was either good or bad.

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On 7/14/2021 at 8:33 AM, Ray Davis said:

What I found was you can't just completely delete a message you have to at least put the word delete in before it will accept the delete.  

Moderators can delete a message if you need that done. 😉. You *should* be able to delete…. I’ll check into that. 

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33 minutes ago, Scotty Hutto said:

Moderators can delete a message if you need that done. 😉. You *should* be able to delete…. I’ll check into that. 

Thanks Scotty,   I immediately decided against my question,  didn't want to be called out for digital diarrhea 😁   LOL   Some people have a problem with that

I couldn't delete fast enough, and Steven saw my post.   My plan to never be seen at all, was foiled.  Not a big deal, I didn't use profanity or any other forbidden words. 

 

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  • 4 weeks later...

For what it's worth, I have 2 of the Dometic Reversal Valve Solenoids on order-(Part# 3310714.005) and so far they are on back order. I did find a reversal valve solenoid for a residential unit-(I think) that was the same 120 voltage as the Dometic and it looked the same except the 2 wires that come from the solenoid are 2 flat spades that plug into the solenoid instead of 'made into' the solenoid. The Brand is Ranco and the Part# is LDK-310000-070. I installed it on my Penquin and it works great! Hopefully the other solenoids will come in sometime in the future and I will use one on my front heat pump as it is 16 years old and I'd rather change it out when it's at my convenience instead of waiting until I'm using the A/C when it decides to quit and defaults into heat. Hope this will get someone out of a bind waiting on an original part.

Ranco Reversing Valve Solenoid-works in place of Dometic 3310714.005.jpeg

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