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Old 01-06-2019, 02:07 PM   #1
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Dometic HVAC: Sudden Changes

We have a 2013 Surveyor SC189, Dometic ducted roof air with Dometic Single Zone LCD Thermostat (Fan, A/C, Heat, OFF). This system has been working flawlessly.

In preparing for a camping trip next week I was working in the camper and decided to turn on the heat. This was when I realized something was wrong.

Previously I would run the fan on Low (typically about 20 seconds before the fan would engage) to help circulate the air, then switch to the furnace setting and within seconds the gas furnace would fire up and cycle on-off as needed to maintain temp. The Dometic HVAC fan would continue to run on Low without stopping...just as it should.

Today, when turning on the fan, nothing happened for several minutes. Then suddenly a click and the fan motor came on for about 2-3 seconds, then shut off. Anywhere from a couple of minutes upwards to 10 minutes or so the familiar click and the fan would turn on for 2-3 seconds then off.

I lowered the A/C temp to it's lowest setting, about 10 degrees below ambient temperature. The A/C nor fan would come on. I then tried for heat only, raising the setting about 10 degrees above ambient temp. Nothing.

I have attempted the reset procedure several times with no success. Holding down + key & power/selection key until the dashes appear and it beeps. That has changed nothing.

OK...not sure where to go. All the thermostat wiring is good as well as the connections in the roof air unit. 12 volts continuous at the thermostat. I have unplugged, repluged and tugged on every wire to ensure none of them are loose.

I am presently trying to find a wiring diagram for the Dometic thermostat control box. Any suggestions would be appreciated. Thanks!
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Old 01-07-2019, 10:30 AM   #2
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Yesterday afternoon I found a wiring diagram and then confirmed the wiring. Also rechecked all connections. Nothing found. I eventually gave up, unplugged the trailer from shore power and disconnected the house battery. It sat like this overnight.

After leaving the trailer unplugged from shore power and the house battery disconnected overnight, I reconnected everything this morning. To my surprise, everything now works just as it did prior to this strange event starting. However, the fan still does NOT come on with the gas furnace if the fan is set to Auto.

I heated up the inside to 75 and then switched to cooling, set at 65. Within a few seconds the blower came on (set to LO) and a few seconds later the compressor kicked in and it started cooling. OH...I also tried cooling with the fan in Auto mode...does not work (just like the furnace). Oh well...I had rather the fan run continuous anyway but it would be nice to have everything working as Dometic originally intended.
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Old 01-07-2019, 10:38 AM   #3
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There have been reports like this and IIRC it turned out to be the control board in the A/C. There have also been a few instances of strange behavior fixed by replacing the t-stat.
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Old 01-07-2019, 11:05 AM   #4
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Not sure what fan you are referring to here. However, the fan still does NOT come on with the gas furnace if the fan is set to Auto.

If it is the AC fan then it is operating correctly. In the heat mode the AC fan only operates in LO and HI setting.
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Old 01-07-2019, 11:08 AM   #5
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Originally Posted by FlyboyTR View Post
Yesterday afternoon I found a wiring diagram and then confirmed the wiring. Also rechecked all connections. Nothing found. I eventually gave up, unplugged the trailer from shore power and disconnected the house battery. It sat like this overnight.

After leaving the trailer unplugged from shore power and the house battery disconnected overnight, I reconnected everything this morning. To my surprise, everything now works just as it did prior to this strange event starting. However, the fan still does NOT come on with the gas furnace if the fan is set to Auto.

I heated up the inside to 75 and then switched to cooling, set at 65. Within a few seconds the blower came on (set to LO) and a few seconds later the compressor kicked in and it started cooling. OH...I also tried cooling with the fan in Auto mode...does not work (just like the furnace). Oh well...I had rather the fan run continuous anyway but it would be nice to have everything working as Dometic originally intended.
The A/C fan is designed to NOT come on in the AUTO (AU) setting when using the furnace. Only if set to Hi or Low.

Did you mis-type or are you saying it worked on AUTO before?
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Old 01-07-2019, 11:29 AM   #6
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I have a different RV than you, but had a similar problem shortly after I bought it used. After talking with a good RV repair place, they recommended disconnecting the heater from the Dometic controller and installing an independent heater controller. Very easy to do and it solved ALL my control issues. Now the wall controller controls just the AC unit and the simple heater controller controls just the heater. It used to be the signals from the Dometic controller had to be routed thru the AC control board before the heater could/would engage and the AC control board would not function correctly. I still have the ability to turn on the fans to circulate air if I desire, but usually I don't need to.
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Old 01-07-2019, 11:36 AM   #7
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Thanks for the replies and info. I have had numerous people tell me the a/c fan works on Auto with the furnace...mine never has. So it is very possible mine is functioning correctly. There sure seems to be a lot of confusion about the operation of this system.

To clarify, before this event yesterday when everything stopped working except for the a/c fan coming on for 2-3 seconds... the a/c fan never worked in Auto mode with the gas furnace. ...but that is now sounding like this is normal!

Any ideas why disconnecting everything overnight seemed to fix the problem I was having as described in my original post?

Thanks again for the replies and helpful support!
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Old 01-07-2019, 11:52 AM   #8
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Disconnecting the power allowed the computer to reset and that will probably work for a while. Hopefully there was just a power-on glitch and it won't come back.
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Old 01-07-2019, 12:10 PM   #9
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Originally Posted by Bama Rambler View Post
Disconnecting the power allowed the computer to reset and that will probably work for a while. Hopefully there was just a power-on glitch and it won't come back.
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Old 01-07-2019, 12:25 PM   #10
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During a discussion I had with Dometic support some time ago, The tech said that occasionally if the Rv has not been used for a while it may be necessary to reset the thermostat/controller. Not sure why, but it has worked for me.
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