Help Wiring Furnace Thermostat

[jor_az]

Atwood 8900 Furnace
Atwood Heat Only Thermostat

I replaced my roof air unit with an Atwood. The new unit has its own thermostat so I removed my original (Duotherm) and installed the provided Atwood T-stat to control the AC. It works fine both for cold and heat from that unit.

The Duotherm also controlled the furnace so I needed to install a direct heat only Atwood t-stat to control my single furnace. I am doing something wrong and am looking for some help. The furnace runs, provides heat and shuts down with the switch but will not shut down automatically when it reaches the target temp. Even on the lowest setting it continues to run. The t-stat is new.

I read Gary Brinck's great furnace document and it indicates my problem has something to do with the Time Delay Relay. I am definitely confused at this point.

Where am I supposed to connect the two wires from the t-stat (red & white) at the furnace?

Thanks.
jor

 

[Henry J Fate]

If you have 2 wires coming from the furnace and at the thermostat, those two wires should operate the furnace if they are connected together.

The thermostat may have the ability to connect the wires on a drop in temperature (for turning on the heat) or a rise in temperature (turning on the Air Conditioner) If the thermostat offers both of those options, be sure it us wired to connect the wires on a drop in temperature.

The function of the thermostat should be able to be tested with a multimeter to verify the wiring.

If none of this makes any sense or doesn't apply to what you are experiencing, I would suggest to post more detailed information about the thermostat and the wires coming from the heater. Pictures always help too.

 

[jor_az]

I'll try to be more clear.
Heater: Atwood 8900 III
T-stat: DOM38453

My original set up was a 4 button Duotherm t-stat that controlled two roof ACs and the furnace. The Duotherm had only a 6 wire data plug. I replaced one of those roof ACs with a different unit that included its own T-stat (the Duotherm could not be used with the new AC). The new t-stat has its own data plug and does not control the furnace so I need to install a heat only t-stat.

I bought an Atwood (Dometic) 2 wire heat only t-stat (DOM38453). I ran two 18 gauge wires from the thermostat to the heater. I'm trying to find out to which terminals I need to connect these two wires.

I am attaching two photos:

Photo 1: The red arrow points to the OE 4 wires coming into the furnace (red, yellow & 2 blues), The green arrow points to where one of those two connect at the circuit breaker.
Photo 2: The other blue wire runs diectly to the motor and the terminal that the blue area is pointing to.

This has got to be simple but...

Thanks.
jor

 

[Gary RV_Wizard]

The "set temperature" is totally a thermostat function and the furnace has no input about it. The furnace runs the fan & burner as long as the thermostat says "heat required", i.e. puts +12v on the wire to the furnace.  It's not the time delay relay - that merely keeps the fan running for a short time after the burner shuts off or before the burner lights at start-up.

My guess is that you have the wrong wire going to the furnace, so that the +12v signal is always on. If not that, then the thermostat is probably defective.
One other possibility: are you sure you have disconnected the furnace from the previous control system? With a combined a/c & furnace thermostat, the furnace control if often routed through the a/c zone control. Could it be that the a/c system is still commanding the furnace?

 

[jor_az]

One other possibility: are you sure you have disconnected the furnace from the previous control system?

Yes. The Duotherm is controlling the rear AC and heat pump so I don't think there is any connection to the furnace.

My guess is that you have the wrong wire going to the furnace, so that the +12v signal is always on.

That is the issue for sure. I ran 2 new wires (r & w) from my new t-stat to the heater compartment. I thought all I needed to do was disconnect the OE wires and connect my new 2 wires to the proper terminals but I can't figure out which ones.

If not that, then the thermostat is probably defective.

The t-stat is new so it's probably good.

Thanks.
jor

 

[Gary RV_Wizard]

The 8900 III manual shows the thermostat wiring on pg 5. See

http://techsupport.pdxrvwholesale.com/wp-content/uploads/2017/09/Atwood-Furnace-Series-8900-III-LD-Model-8940-Installation-Manual.pdf

Also pg 12 of the Atwood furnace service manual:
http://bryantrv.com/docs2/docs/hflamefurn04.pdf
It appears to be pins 2 & 5 on the terminal block.

 

[jor_az]

I see. Thanks for the tip. It's raining today but I'll give that a try and post.
jor