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Old 09-22-2017, 01:19 PM   #1
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Improving Domestic Thermostat Accuracy

Hey all --

Just had a thought and wanted to see what others thought. We have a 36' 5th wheel with a Dometic digital 3 button thermostat in the hall just outside the bathroom. The temp swings are crazy when you have the furnace on. Gets too warm, then gets too cold before it kicks on again.

It has a thermistor inside near the bottom right corner. I clipped some of the plastic to expose it to more of the outside air. Then I had a thought. This thing is small and screwed directly to the wall. Temperature is absorbed through the wall into the plastic around the thermostat. I think this is having an impact on the temperature this thing is reading. It's very sensitive. When I blow on it, it changes temp pretty easy. So if you have a temp sensor close to something that is influenced by the temp of the wall it's mounted on, I can now see why the temp swings are so big. It would take a long time to change the temp with it this way.

I attempted to insulate the thermostat from the wall by inserting some bubble wrap in between the thermostat and the wall. We will see how this goes. I think a better insulating foam can be used which I will continue to look for but wanted to see what others thought. If you had the thermostat mounted on an ice cube, it's going to have an impact on the performance of the unit. So now my goal is to fully insulate the thermostat from the wall so the temp of the wall has much less of an influence on the temp that the thermostat is reading. We will see.

Thanks,

-Jaime
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Old 09-22-2017, 01:23 PM   #2
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All is true, best bet is plug the whole where the wires come through. If it is on an interior wall and plugged you shouldn't get wide swings, good luck
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Old 09-22-2017, 01:41 PM   #3
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Great to hear. Yeah, there is a hole the size of a quarter behind the thermostat. I inserted a double thick piece of cardboard behind the thermostat that insulates it from the wall and covers the hole. Excited to see if this ultra cheap fix makes an improvement.

Also on a side note, the thermostat has a setting to change the temp differential of 1 or 2 degrees. I set it to 1 and that caused a problem. When the temp is border lining two temps like 75 and 76 degrees, you hear clicking on and off every time it changed from 75 and 76 degrees. Setting it to 2 degrees which is default solves this problem. Will report back once I get a day or two with this new change.

Thanks again for the quick reply.

-Jaime
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Old 09-22-2017, 01:51 PM   #4
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You will still get some swing in temp because of the way the gas heat works. When the temp drops to the point the heat is required, the thermostat kicks on the heater. When the thermostat reaches the temp to shut off, it sends the signal to cut off. The heater stops but the fan continues to run for a couple mins to remove the heat from the heater. This is where I notice the swing in temp. You can hear the thermostat click when the heat is turned on and off but the fan runs for a little while and then shuts off. The temp is perfect in my trailer and the previous trailer when the thermostat clicks for the heat to turn off. It is 2 or 3 degrees higher when the fan turns off. This is not based on any information from anywhere important. Just my own observations.
Jeff
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Old 09-23-2017, 01:50 PM   #5
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A typical thermostat includes an adjustable "delay" (anticipator). The delay enables the system to cycle less frequently. So the furnace will wait before firing until the temp is a bit below the desired temp, and then it will run longer raising the temp above the desired temp. This gives the furnace (or AC) time to rest between cycles.

In my PUP, the delay was set to an extreme, and we had the same issue...freeze or roast. My simple mechanical thermostat was easy to adjust. I pulled the cover and reduced the delay. Much better, and the furnace still does not cycle on/off too frequently. (In a PUP in cold weather, this has the side benefit of not attempting to raise the temp from about 65 degrees to about 75 degrees...a huge swing in what amounts to a "tent".)

Here's a video for a mechanical thermostat:


So, it definitely makes sense to isolate the thermostat from any undesirable influences from exterior walls, drafts, metal that conducts outside heat/cold and so on. But you described a situation where you're either freezing or roasting, so I suspect adjusting the delay will solve that problem.

P.S. It's not surprising that your 98 degree breath affects the thermostat. Also on an interior surface, the surfaces should be about the same temp as the air. Things tend to stabilize, with cabinet faces and partitions absorbing heat from the air and achieving essentially the same temp as the air....just as in your home. In my PUP, the thermostat is on the front face of a kitchen cabinet...isolated from the outside wall by the depth of the cabinet. I presume your thermostat is situated somewhere well away from an exterior wall, so the temp of the mounting surface should not affect the thermostat in some bizarre way. But you could isolate the thermostat from its mounting surface with some 1/4" closed cell foam:
https://www.amazon.com/dp/B001G8E114...a-309563662940
That was only an illustration. You should be able to find something 6" x 6" x 1/4" or so at a hardware store.
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Old 09-26-2017, 06:10 PM   #6
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If you set the fan to run on low then set the furnace to desired temp the circulating air on low will greatly reduce swings and cold spots been doing that on ours and it works great. Also have been recommending it on homes for years with great results
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