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Old 05-24-2012, 09:24 PM   #1
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Thermostat turns on Furnace & AC fan

Last weekend I ran the furnace while the trailer was running on battery power only. No problem.

Then one morning I connected the shore power to my generator, and set the thermostat once again to heat. The furnace came on as it should, but also the AC fan. Why would this happen? I flipped the breaker for the AC to keep the AC fan off. The furnace worked normally.

I don't often run the furnace as we use electric heaters. But I swear this never happened last year.
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Old 05-24-2012, 10:13 PM   #2
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Sounds like something fishy with the thermostat. What thermostat do you have in your trailer?
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Old 05-24-2012, 10:25 PM   #3
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Originally Posted by SCOC View Post
Last weekend I ran the furnace while the trailer was running on battery power only. No problem.

Then one morning I connected the shore power to my generator, and set the thermostat once again to heat. The furnace came on as it should, but also the AC fan. Why would this happen? I flipped the breaker for the AC to keep the AC fan off. The furnace worked normally.

I don't often run the furnace as we use electric heaters. But I swear this never happened last year.
Check to make sure the thermostat fan switch is on Auto and not On
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Old 05-24-2012, 10:33 PM   #4
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sounds like the thermostat is faulty or wired wrong to me. if it otherwise controls, i would check to see if it is wired right or none of the bare parts of the wire are touching terminals that they aren't supposed to.

only other possibility would be if u have a heat pump with furnace backup. even with that, i don't see both of them coming on on initial start up.
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Old 05-24-2012, 10:38 PM   #5
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If the fan switch in the thermostat is in the On position, it will run as soon as 110v is available which is why it didn't run until he hooked his gen to the power cord. The fan switch has to be in the Auto position for it to only work when the AC is on.
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Old 05-24-2012, 10:49 PM   #6
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If the fan switch in the thermostat is in the On position, it will run as soon as 110v is available which is why it didn't run until he hooked his gen to the power cord. The fan switch has to be in the Auto position for it to only work when the AC is on.
i have an older thermostat; my A/C fan doesn't operate when i switch to heat
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Old 05-25-2012, 02:32 AM   #7
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Thanks. I'll go out and check tomorrow and report if I figure it out.
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Old 05-25-2012, 10:23 AM   #8
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Do you have the Dometic Digital thermostat?


If so, then I agree with what dunnnc had to say.

Here's some info that might help you.

Use the On/Off Mode button to select FAN. Then use the +/- button to select either Hi, Lo or Auto.
When using the AC and the fan is in the Hi or Lo setting it will run continuously while the compressor will cycle on and off.
In the Auto mode the fan will cycle on and off as well.

If the fan is set on Hi or Lo while using the furnace the AC fan will run continuously, so be sure to set the fan on Auto when using the furnace.
You can, however, leave the AC fan running while using the furnace to help with condensation problems.

After setting the fan, Use the On/Off Mode button to select COOL for AC use. Use the +/- button to select the thermostat temperature setting.
Use the same procedures for the Furnace.

When using the AC, in the Auto mode the fan will run on high when the room temperature is 5 or higher than the thermostat setting. When the room temperature drops to within 4 or less than the thermostat setting, the fan will stop for a few seconds the start again in the Lo mode.

Hope this helps.
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Old 05-25-2012, 10:42 AM   #9
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I second dunnnc's suggestion. I had the same "problem" and it turned out I just needed to figure out that the fan control and the furnace/AC were 2 different settings in the thermostat. I have a 2011 Surveyor SP260.
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Old 05-25-2012, 10:44 AM   #10
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Here is a picture of our Coleman-Mach thermostat with the Auto-On fan switch.
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