Using Atwood 1H2C Thermostat with Dometic A/C ??

I am attempting to replace a Duo-Therm Thermostat Model# 3107612.008 with an Atwood Model# 1H2C. Neither the furnace, A/C compressor, or A/C fan will operate with the Atwood. All functions are good with the Duo-Therm.

The Atwood documentation claims that it "has been designed to operate standard air conditioning units in conjunction with an RV furnace".

Furnace: Atwood Model# 8535IVDCLP
A/C: Dometic Model# 620426.331

Duo-Therm Thermostat connections:
Terminal Wire
-------- ----
+7.5 Red
Cool Yel
Fur Wht
Fan Hi Blu
Fan Tan
Gnd Grn

Atwood Thermostat connections:
Terminal Wire
-------- ----
1) Gnd Grn
2) n/c
3) +12VDC Org (voltage verified)
4) +7.5VDC Red (voltage verified)
5) Compressor Yel
6) Fan Hi Blu
7) Fan Lo Tan
8) Furnace Wht

1) The Orange wire in the thermostat cable was not used for the Duo-Therm thermostat installation, and was not connected at either end of the thermostat cable run. The Atwood thermostat requires a source of +12VDC. The Orange thermostat wire is now connected to a wire labeled "+12VDC" in the A/C wiring compartment. This "+12VDC" wire appears in a cable from the coach wiring, supplying the A/C unit. Have verified that the Orange thermostat wire is at +13.5VDC with respect to the Green ground wire. The Atwood thermostat display "powers up" upon connection of the Green and Orange wires. The Orange thermostat wire IS NOT connected to "CON 1" of the A/C controller, which apparently would be the case for a Heat-Pump / Heat-Strip application, which I do not have.

2) I note that all of the thermostat connections (with the exception of the added Orange +12VDC wire) connect directly to the A/C controller. The A/C controller provides a pair of terminals which connect to the furnace's thermostat circuit. The furnace does not connect directly to the thermostat.

3) Have experimented with the "heat" function with no thermostat connected to any wiring. The furnace goes "on" when the furnace "White" wire is connected to the ground "Green" wire.

4) Have experimented connecting to the Atwood t-stat all wires except the furnace "White" wire. The A/C compressor and A/C fan do not operate.

5) On the Atwood thermostat, there is an undocumented "dot" in the lower left corner of the display which appears to be a "Call" indicator. This appears to be functional, is "on" in "heat" mode with setpoint above room temperature, is "on" in "cool" mode with setpoint below room temperature, otherwise "off".

Does anyone have any ideas why the Atwood won't work?

clarinet
Explorer
Jun 01, 2015
I'm looking to do the same thing. Did the suggested change to the ground wire (move from position 1 to position 4) work? Help!
Fisherguy
Explorer
Jun 02, 2015
No, Chris' suggestion didn't work, neither did running a new power source, I could hear relays clicking but nothing from the furnace or ac.
My bud gave me the thermostat after he tried it and also couldn't get it to work. He's looking for his receipt, if he finds it I'll see if I can get a new one.

Here's the way the Atwood tech told me to wire it.
Blue from duo therm hi fan to Atwood 6
White from duo therm furnace to Atwood 8
Yellow from duo therm cool to Atwood 5
Brown from duo therm fan to Atwood 7
Red from Atwood 7.5 volt to Atwood 2
Ground to Atwood 2
New 12 volt to Atwood 3

Would like to get it to work but will be leaving for 3 weeks on Thursday, put the old duo therm back and it works fine.
clarinet
Explorer
Jun 02, 2015
How do you install the new 12 volt wire?
Chris_Bryant
Explorer II
Jun 02, 2015
Atwood's instructions say to hook the +7.5 (red) to 2 & 4. I still think Atwood doesn't know how the Dometic analog system works, though- they need to switch the grounds to activate any relays.
It would really help if they used anything close to standard terminology with this.
Fisherguy
Explorer
Jun 02, 2015
clarinet wrote:
How do you install the new 12 volt wire?

To test it I jumpered to a light fixture near the thermostat, if it worked I was going to try fishing the ceiling and the wall by removing the trim around a roof vent to help run the wire.

Chris Bryant wrote:
Atwood's instructions say to hook the +7.5 (red) to 2 & 4. I still think Atwood doesn't know how the Dometic analog system works, though- they need to switch the grounds to activate any relays.

Not from what I can see:

Atwood Instructions

Have you actually installed one of these and been successful? Dometic AC and Suburban furnace?
Chris_Bryant
Explorer II
Jun 02, 2015
Further proof that Atwood is relatively clueless- see page 14 of the service manual, which gives the instructions I gave.
No- I haven't dealt with one of these.
Fisherguy
Explorer
Jun 02, 2015
OK, so would that be why I heard relays clicking but nothing out of the AC or furnace?

Interesting page 14 is dated 6/19/02 and the instructions that came with the unit (and the one I linked to above) are dated 9/21/04, they're newer, you'd think the newer ones were correct...?
Chris_Bryant
Explorer II
Jun 02, 2015
Really, the only relay that could click without activating something would be the high/low fan relay- the compressor and fan relay should run those if they click.
They simply do not give enough info on operation, so I don't know what terminal 2 is on the t-stat.
Fisherguy
Explorer
Jun 02, 2015
We're leaving for a trip in 2 days, I don't think I'll have time to try it with 2 jumpered to 4, maybe I'll take it woth me on the trip and play with it while away.

One of the Atwood techs sent me this when I was trying to get it to work.
Chris_Bryant
Explorer II
Jun 02, 2015
On that diagram they hook up #2, with nothing on #4, they also list #2 (and the black out of the control box) as both +7.5 *and* ground. I wish I knew what terminal 2 actually was- terminal 4 they list as relay common, which makes sense- on a standard t-stat that would be terminal R (C or H).