Another question about roof-top AC's

[Geocritter]

A few days ago I posted a question about my roof-top AC acting oddly. The response that made the most sense was that it was a thermostat problem. Okay, a simple fix until I looked up a thermostat for my 19 year old AC. It seems I have to purchase some type of new controller that goes on the AC (kaching!), a new thermostat (kaching!), a wiring kit (small kaching), and finally pay $46 shipping (painful kaching) for a total cost of around $340. As frustrated angry thoughts raced through my mind it dawned on me, when push comes to shove this is nothing more than an AC system probably not much different than my home system, especially if you isolate out the furnace part of the system. Why not simply disconnect the current (and complicated) Duo-therm thermostat and go to Home Depot or Lowes and purchase an inexpensive generic AC thermostat and wire that into the system. My question is has anyone taken this approach?
Steve

[dougrainer]

Your problem is this- you have a obsolete Dometic wall tstat system (obsolete since 1996/7). If you pull the tstat, you will see a flat ribbon cable on the back of the tstat that has about 10 wires in it. You will also see the same type cable at the AC unit in the control area. The ONLY way to make that old system work (if tstat was bad) was they had an upgrade wire kit for the OLD 4 button Wall CCC tstat that also was obsolete after 2001/2). The good news is, they ran a regulat tstat type 6 to 8 wire harness from the Roof AC to the wall tstat. So, you can replace the control assbly at the roof AC and then use any aftermarket tstat for home you want, or you can use the electronic type wall tstat from Dometic that uses the standard home tstat wire harness. Doug

PS. Home tstats are 24 volt but they will work on 12 volt.

[dougrainer]

bgholson wrote:
Just had a mobile tech out today for my duo therm thermostat issues. He turned it off, held the top and bottom buttons down while turning it back on. Re-set itself and all is good. Just saw your post so thought it may be worth the 30 seconds it takes to try this with yours.

good luck

Please repost when it fails again. Your Dometic tstat WILL fail again. A reset NEVER fixes operational problems. This is a COMMON response and fix by inexperianced Techs. AND, you get to pay him for that. Neat deal for a mobile tech. The OP does NOT have your tstat system. Doug

[bgholson]

Just had a mobile tech out today for my duo therm thermostat issues. He turned it off, held the top and bottom buttons down while turning it back on. Re-set itself and all is good. Just saw your post so thought it may be worth the 30 seconds it takes to try this with yours.

good luck

[Geocritter]

I'll post again tomorrow but I think I figured out the problem. Yesterday after digging through the pile of paperwork that came with the RV I found the Dometic AC installation booklet. The booklet described in detail how the various controls work. After comparing how the controls on the thermostat worked and how they should work, I determined that my low fan speed circuit isn't working (there are 4 settings Low, Med, High and Auto). The low fan speed is the fall-back speed that Automatic uses during times of low system need requirements (such as in the cool of the morning). I set the system fan to run continuously on medium all day today while I'm gone and this evening when I return I'll check out why the low fan speed isn't working. I'm hoping it's something simple like a dirty or loose connection. I suppose it could also be a fan motor that's wearing out and no longer has the umph needed to start at low power (maybe an inline capacitor is needed?).
Steve

[wa8yxm]

Why not disconnect and go to lowes/home depot....

Well.. In some cases the fan controls also run to the T-Stat, these may or may not be compatable with the standard house type.

Otherwise.. Yes, that would work (And who knows. may even be compatable with fan controls.. I do not know)

[sljkansas]

Here are two common thermostat mods. Honeywell and Hunter

[MountainAir05]

Try this one also.

http://home.roadrunner.com/~morodat/hunter_install.html

[Geocritter]

Golden_HVAC, thanks for the installation tutorial, I'll check my thermostats wiring this afternoon.

rockhillmanor, I remember the time in 1991 when I bought a used home the air didn't work. The service guy said I had a leak, he even found it. But he insisted I needed a brand new system. Instead I had him repair the leak. I used that same AC system until I sold the house in December 2006. The only work it required during that time was a new fan motor which I replaced myself, and about every three to four years I'd have to have it recharged.

It's experiences like that make me loathe to replace things just because they're old.

Of course that's the American way, whether it's machines or people. I'm currently "retired" because being 65 no one will hire me. Apparently, I'm too old and decrepit. However, it wasn't but 5 years ago that I climbed Mt. Rainier, somehow I wasn't too old and decrepit for that, but work in front of a computer searching for oil and gas, better not chance it!

Steve

[Dale_Traveling]

The problem you will run into using a home thermostat is that is runs off of 24VAC not 12VDC. You could wire up the system using a door bell transformer to set down from 120 to 12 VAC but you won't have heat without being plugged in or with the generator running.

[rockhillmanor]

Don't believe all you read on the internet. I thought the same thing too.

Call a mobile RV repair in your area. They WILL have a replacement for you IF that is what you need. They have seen it all worked on them all and IMHO are the best place to go for repairs. They also have 'connections' on where to get parts and fast. They especially excel on older MH's.

I'm here to tell you the Mobile RV Repair guys are way under appreciated. They are my GO TO all the time and they have saved me thousands of dollars just over the past 2 years. The AC unit being the biggest.

Dealerships ALL said I needed an entire new ac unit, new thermostat, new wiring, etc yada yada yada.

Mobile Repair just laughed and in within 20 minutes had my AC up and running for just the service call fee.

[Golden_HVAC]

I agree that $46 shipping is a painful kaching!

Yes you "Should" be able to use a battery powered thermostat. The Honeywell 6000 series comes to mind. It runs on 2 AA batteries. Yet remember that if the battery is dead, the furnace will not work. So keep a extra pair around, or remember to rob the TV remote control when you need to.

Normally there is a yellow wire that energizes the cooling relay.

White is normally the furnace relay.

Another wire or two is used to power the fan on low speed and high speed. You connect the wire you selected to run the A/C fan on the G terminal. You can not run both speeds (it will melt the fan wiring) so I would select high fan speed and hook up that wire to the G terminal, the other fan wire do not connect. The furnace fan runs on it's own.

If you have a heat pump, it will have difficult wiring on it. Good luck! Yet you do not "Have" to run the heat pump, just wire it for cooling, and use the furnace by energizing the white wire with the W terminal.

The thermostat should have a
W - White wire (typically)
Y - Yellow wire cooling
G - A/C fan wire
C - common wire used in a 24 VAC (home) system only
R- input wire to the thermostat. In MY RV, the wire is red.

Have fun camping.

Fred.