Furnace thermostat help

[Bill]

Have you experienced this annoyance?

It's a bit chilly in the camper, maybe 65 degrees. You nudge up the thermostat just a touch. The furnace comes on - and runs and runs and runs. As the inside temp rises past 80 with no sign of letup, you drop the thermostat back a bit until the furnace shuts off. And now the furnace doesn't come on again until the temp drops to 60.

This is really annoying. It is caused by the fact that, as the room heats up, the warmer air doesn't get inside the thermostat housing very well, so the thermostat doesn't realize the room has heated up, and it keeps calling for heat. There is a fix for this built into the thermostat, but it may need adjustment - mine did.

There is actually a tiny adjustable heater built into the thermostat. When the thermostat calls for heat, this tiny heater turns on and heats up the thermostat's sensor. If the heater is adjusted correctly, it heats the sensor at the same rate that the room warms up. At the same time that the room gets to the right temp, the sensor is warm enough and turns off the furnace.

To adjust the tiny heater, first pull the cover straight off the thermostat - it comes off easily. Everything inside is low voltage, so you can't get a shock, but work carefully since some of the components are delicate. The adjustment for the preheater is the small copper arm shown at the tip of the arrow in this PHOTO. As you can see, the arm can swing from about the 7 o'clock position to the 10 o'clock position. In my TM, the arm was set at the fully down position (7 o'clock), which corresponds to no preheat. If the arm is fully up (10 o'clock), there is too much preheat - the furnace shuts off before the room warms up. The position shown in the photo is about 9 o'clock, but this was still too high in my rig. The correct position turned out to be about 7:30 - 8 o'clock.

You need to determine the right position by trial-and-error. It is time-consuming, but not hard. Replace the cover during each trial, of course.

Bill

Bill,

Thanks for the sage advice. Our seems to get awfully hot in the front of the unit where our kids sleep, because of how long it seems to run before shutting off. (This makes it a little tough for me if I'm watching TV in the front when they sleep, 'cuz it can get pretty warm.) I'll take a look at ours to see how to adjust it just right!

Thanks for the information!

I had no idea that there was actually a little thermal circuit inside the thermostat. I knew that little thingy adjusted run time, but I thougt it just changed the senstivity of the bimetal coil and distance to the magnets.

Plus, I have to check mine this weekend; but I'm pretty sure that mine has a mercury switch... guess mine was built before the mercury free days...

I imagine the adjustment would be similar though. During the last outing I made the anecdotal observations that set on 80F it would warm to somewhere in the low 70's and then cycle on and off. Although mine cycled okay, I wanted the set temp to agree with the actual temp... if you know what I mean.

We'll see what happens this weekend.

[Civil_War_Buff]

OK, so I had the TM out this weekend, and I figured it would be a perfect opportunity to check this out.

I opened the thermostat, and the lever was parked in the full down (7 o'clock) position. I moved it up to the 8:30 position, and when I started the furnace, it blew cool air.....I waited several minutes, it was still cool air...then I moved the lever back to where it was and the air warmed up. Was this coincident? Any thoughts?

Larry

[Bill]

Larry, I have no idea what that means. Certainly not coincidence.

A couple questions. What was the temp in the trailer when you tried it? And how high did you set the thermostat when you tried this? As it turns out, the furnace has a built-in ignition delay - first the fan starts, and then the flame comes on several seconds later. If the air was almost warm enough to satisfy the thermostat, it is possible that the tiny heater in the thermostat warmed up the sensor during this delay. The result would be that by the time the ignition delay expires, and it is time for the flame to light, the thermostat thinks that the trailer is already warmed up. For example, if the temp in the trailer was 65 degrees, and you set the thermostat to 67 degrees, it might not light the burner. This would not indicate a fault or problem, just a too-high setting on the adjustment. Just for the fun of it, set the thermostat 10 degrees higher than the room temp, and see if it comes on and blows warm.

Second, did you try a lower position on the adjustment? Say, 7:30? As I mentioned, 8:30 was too high on mine - I ended up at 7:30-8:00. You may have experienced an exaggerated version of what I described, when I said the furnace turned off too soon at the 9 o'clock position.

In the end, if you don't experience the overheating problem I described, then turn the adjustment back to the bottom and let it be.

Let us hear more.

Bill

[Civil_War_Buff]

Bill,

You may be on to something. I do believe that the conditions were as you suggest..ie the inside temp was around 64 and I think I set the thermostat to about 67-68ish. I will try it 1) with a higher temp on the thermostat, and 2) I will set the preheater to the 7:30ish setting. Thanks for the insight!

Larry

Hi Bill

Thanks sooo very much for this information. My wife and I came back on Sunday from a trip to Boston. It had gotten down as low as 22 degrees. our only complaint was that it was either too hot or too cold due to the thermostat. I was going to replace the whole unit with a residential digital type I have in my home....didnt know if it would work or not.

We supplemented the cold times with a little space heater....then I thought maybe the space heater had some affect of the performance of the thermostat.

Ill have to check and adjust this the next time I go out.

Again....thanks

George
West Palm Beach

Wow, Bill......great info........explains a LOT when it comes to why my furnace never worked before, I never knew about that thing in the thermostat. Thank you so much for taking the time to explain all of that......After finally getting my furnace to come on after all this time, I thought it was because of a purge cycle or something I wasn't doing before. (Mine would just run and run, and never heat or light).


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