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A. Spruce
Re: Thermostats
A. Spruce
JLMCDANIEL wrote:

It may be possible you have an old thermostat with a mercury switch, no contacts. When the thermostat calls for heat it just connects "R" to "W" on the furnace.

Jack

Yup, it is exactly that (merc switches ). I took some pics of it, but then realized that I don't have any way to post them, so . . . :rolleyes:

Jack
Re: Thermostats
Jack

Some furnaces have 3 speed motors generally low is used for heat and high is used for A/C. You might move the motor jumper from low to medium to see if that helps.

Jack

A. Spruce
Re: Thermostats
A. Spruce

I could be wrong, but I really don't think this system is that sophisticated, this is a 20 year old mobile home with the cheapest of the cheap equipment, as trailers usually are. The existing thermostat is mercury switches on bimetal springs. The controls are one switch for on/off/auto, one switch for heat/ac, then levers to set the hi/low temps. The timer setback is accomplished by a manual timer wheel that you insert pegs into where you want to turn the system on/off.

When the unit runs, I've never heard a difference in motor speed, with exception of when it is manually switched from auto to the on position, other than that it's full speed, regardless of heat or cooling.

I found this pic on the net, it is exactly what I have.

Jack
Re: Thermostats
Jack

Here is a thought that might work, Take the green wire off the thermostat and connect it with the white wire on the thermostat. That way any time it calls for heat it will switch the fan on like switching to on position. The only proble is that it will blow cold air for a while until it heats up.

Jack

A. Spruce
Re: Thermostats
A. Spruce
JLMCDANIEL wrote:

Here is a thought that might work, Take the green wire off the thermostat and connect it with the white wire on the thermostat. That way any time it calls for heat it will switch the fan on like switching to on position. The only proble is that it will blow cold air for a while until it heats up.

Jack

Is this as a test or in lieu of replacement?

I bought a Honeywell RTH2300B today to install. I want to look at the instructions on-line before I open it up to make sure it's going to work with the 5 wires currently in use.

I've never installed a T-stat before, I've heard you need to turn off the power, is this true?

Mastercarpentry
Re: Thermostats
Mastercarpentry

House-type thermostats will work here- there's no difference other than it being cheap stuff to start with.

Phil

Jack
Re: Thermostats
Jack

You will have 24 vac going to the T-stat unless you turn the power off to the furnace.

Jack

A. Spruce
Re: Thermostats
A. Spruce

Got the new thermostat installed. This thing is going to take some getting used to, it only shows the ambient temp, you have to push the +/- buttons for the temp setting to display, then of course, the temp setting changes because you had to push the stupid +/- buttons to see what the setting was. Have I mentioned just how much I HATE stupid electronic gadgetry? It is also WAY too easy for the thing to go into it's programmable mode, rather than staying in manual, which is the only setting that will ever be used.

The fan doesn't seem to be shutting off. I dialed the temp back, the burner shut off, but the fan still runs and is blowing cold air. It didn't turn off until I flicked the fan switch from "auto" to "on" then back to "auto". I am going to wait until morning to mess with it any more because it will be cool enough to run the furnace to see if the cycling issue is resolved, I think that it is, but we'll see.

Another note, I believe I mentioned it above, that in the "auto" setting the fan runs slower than with the switch in the "on" position. I thought this could have been due to the original thermostat being faulty, which was also causing the cycling. Still two fan speeds with the new thermostat.

Question, what does the C wire do? It is not used with the new thermostat. Part of the hi/lo temp setting of the old mercury switches that is now controlled by one wire electronically?

A. Spruce
Re: Thermostats
A. Spruce

The new t-stat may have done the trick. The fan cycled once in probably an hour, and that was about 15 minutes in, turned off for 40 seconds, then went back to work without stopping. Will be turning it on over the next few mornings to take the chill off and see what it's doing.

Jack
Re: Thermostats
Jack

Spruce, the "R" wire is 24vac from the furnace transformer, the "C" is the other side of the transformer similar to the neutral in the house wiring.

Jack

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