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Old 11-12-2019, 08:43 PM   #1
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Thermostat problem ?

I replaced my Coleman Mach analog thermostat (7330F3858) with a Coleman Mach digital thermostat (8330-3482). It worked great this summer in the AC mode. Our nighttime temp is now getting down to near freezing, so I put it on heat and set the temp for 50 degrees because I don't want to winterize yet. The heat works, but when I go out in the morning the thermostat has reset itself to a set temperature of 72 degrees. I've reset it a couple of times down to 50 and 55, but it keeps resetting the set temp to 72 degrees. Is there a minimum set temp on the digital thermostat or do I just have a thermostat that is malfunctioning in the heat mode, but working great in the cool mode? Thanks for any experience you might have with this problem and your input.
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Old 11-13-2019, 09:43 AM   #2
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Have you checked and/or replaced the backup batteries? Mine will reset if you don't have batteries every time the power blips or is turned off.

Also, check the wires on the back, loose wire could cause it to reset.
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Old 11-13-2019, 10:37 AM   #3
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Thermostat keeps resetting itself to 72 degrees when on heat
I replaced my Coleman Mach analog thermostat (7330F3858) with a Coleman Mach digital thermostat (8330-3482). It worked great this summer in the AC mode. Our nighttime temp is now getting down to near freezing, so I put it on heat and set the temp for 50 degrees because I don't want to winterize yet. The heat works, but when I go out in the morning the thermostat has reset itself to a set temperature of 72 degrees. I've reset it a couple of times down to 50 and 55, but it keeps resetting the set temp to 72 degrees. Is there a minimum set temp on the digital thermostat or do I just have a thermostat that is malfunctioning in the heat mode, but working great in the cool mode? Thanks for any experience you might have with this problem and your input.
72* is the default setting on the CM digital thermostat. If it's resetting itself either the 12V power supply is being interrupted overnight somehow or the thermostat is being shorted out or defective on the heat side. As suggested below, pull the thermostat and check to make sure all connections are solid.

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Have you checked and/or replaced the backup batteries? Mine will reset if you don't have batteries every time the power blips or is turned off.

Also, check the wires on the back, loose wire could cause it to reset.
No batteries in this particular unit. Direct 12V wired.
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Old 11-13-2019, 12:24 PM   #4
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Husky,
Like you we replaced our analog thermostat with the Coleman digital and experienced the same problem. I called Coleman and was told it was due to a lost 12 volt signal or too much of a drop in the 12 volt actual voltage. A thread online suggested wiring in a separate 12 volt positive wire from the power distribution center. I did that and it the thermostat would still reset itself at times. I got frustrated and decided to install a battery operated digital thermostat. There are a few threads posted where people installed a Honeywell RTH 111 thermostat. I purchased a a LUX thermostat for about $15 and it works flawlessly.
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Old 11-13-2019, 03:43 PM   #5
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Thanks for your input. I disconnected from shore power and disconnected from the battery. I then check the wires for security. All the wires are still connected tightly. I then reconnected the battery and shore power. When I checked the thermostat, the set temp was 72 degrees. So, for some reason, my 12VDC power is having momentary interruptions causing the reset. I'll have to dig deeper. Thanks for your input to help. Also, what is the model number of the LUX thermostat that is working for you? Thanks
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Old 11-13-2019, 09:08 PM   #6
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You must be losing power to the thermostat as it resets to 72 whenever it loses power.
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Old 11-14-2019, 07:27 AM   #7
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You must be losing power to the thermostat as it resets to 72 whenever it loses power.
That's my 1st thought losing 12volts. Are you on shore power or just running on house batteries ? If just battery and it gets low on voltage that will make the stat reset . This seems to be a problem also on some other Rv forums so possible a bad run of thermostats.
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Old 11-14-2019, 08:04 AM   #8
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I decided to go with a battery powered thermostat so that I was no longer connected to the RV 12 volt system. I used the a LUX DMH110 to replace the Coleman thermostat. The only thing I don't like about it is that it isn't backlit. When I put a 2nd AC in the RV I used a LUX TX100E with back lighting. I bought mine at a local big box store but I see Amazon has both models.

One thing you must be aware of is that neither of these has connections for the AC 2-speed fan. I didn't care because we never run the AC on low fan so just connected the wire for high speed. One could easily add a two-way switch to have both high and low if that was important.
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Old 11-14-2019, 08:57 AM   #9
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Are you sure it's a power interruption? A bad or open ground can also mimic a power fault issue.
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Old 11-14-2019, 09:33 AM   #10
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Are you sure it's a power interruption? A bad or open ground can also mimic a power fault issue.
Wouldn't an open ground cause a power interruption?
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