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bdavey543 · Dec 5, 2008, 02:42 PM

I recently "upgraded" to a digital thermostat so I could program heat cycles while out of the house. The HVAC unit is an Intertherm sealed cumbustible furnace model number MGHA-056ABFC-01 that runs on propane. The entire unit sits in the house and it has two wires going to the thermostat (red and white). The digital thermostat I installed is a Honeywell RTH230B, 5-2 programmable. When I installed the new thermostat I connected the red wire from the fan to the Rh terminal on the thermostat and the white heating wire to the W terminal.

The problem is that I can turn the heat on and off at the thermostat but it will stay on constantly. It won't cycle to the temperature I set.

Any suggestions?

hvac1000 · Dec 5, 2008, 04:07 PM

Does this unit have A/C.
Does this unit have a Heat off Cool selector switch on the unit itself.

bdavey543 · Dec 5, 2008, 04:55 PM

No A/C. No switch on the unit.
The Unit has fan settings of off, on, and fan.
Its heat only.

hvac1000 · Dec 5, 2008, 05:02 PM

With only two wires you will have no fan control from the thermostat.

The terminals to use are the R and W on the furnace and the RH and W on the thermostat.

This part I have no idea what you mean

When I installed the new thermostat I connected the red wire from the fan to the Rh terminal on the thermostat and the white heating wire to the W terminal.

The red wire has no business on the fan to start with so this is confusing.

You might try and isloate the problem by removing the wires from the thermostat and then attaching and unattcahing the wires together to see if the furnace turns off and on normally. If so then you have a thermostat problem.

bdavey543 · Dec 5, 2008, 05:06 PM

The thermostat I replaced was a non-digital one but it worked fine. Why would it control the fan and heat just fine but the new one won't?

As for the wires: there were two wires (red and white). THe white one connects to the heater and the redone connects into the limit switches and the wiring and ultimately the fan.

Thanks for your help.

hvac1000 · Dec 5, 2008, 06:07 PM

Did youtry my suggestion to check the thermostat?

You might try and isloate the problem by removing the wires from the thermostat and then attaching and unattcahing the wires together to see if the furnace turns off and on normally. If so then you have a thermostat problem.

letmetellu · Dec 5, 2008, 06:59 PM

The only two wires you need to worry about are the two that are coming out of the wall, Connect one (the red) to the R terminal and the other one (the white) to the W terminal.
Don'tworry about where they go from there unless you have changed some of the wiring.

mygirlsdad77 · Dec 5, 2008, 07:17 PM

I would start by trying hvac's advice. If this seems to make the furnace run(when wires are together) and shut down(when wires are apart), then you need to make sure you have your programmable thermostat set up correctly. You may be turning the heat up when the thermostat is preset not to run at that time of day. There should be a HOLD button on the thermostat. Press Hold, a light should come on that says hold on the digital readout. Now set the thermostat to desires temp and see if it doesn't work. Best of luck, and let us know what you find.