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Old 01-21-13, 05:52 PM   #1
petehouk
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Default Timer for Furnace Fan

Hi Everybody. I am trying to find a way to more effectively circulate air in my house. I have been heating more with the wood stove lately, and there are times when the temperature differential throughout the house gets pretty large.

when this happens, I usually turn on the fan on our forced air furnace. It does a pretty good job of evening things out. The problem is that the fan tends to get left on longer than it probably should. And I can't leave it running if I am going to be out for a long time since there won't be anyone home to turn it off.

The same thing used to happen with our bathroom vent fans. Someone else who lives in my house (who happens to be married to me, but I'm not singling anyone out here or naming names or anything) had a tendency to leave the bathroom fan running long after any offending smells had been evacuated. I fixed that problem with one of those spring-wound timers.

Can I wire one those spring-wound things in parallel with the switch on my thermostat? Then if the temperature gets too far out of whack, I can twist that timer to 30 or 60 minutes and forget about it.

I have also seen a few thermostats that will automatically cycle the fan on. But those are pretty expensive and they run the fan more than I would like. (Braeburn 5300--$85--will run the fan as little as 12 minutes out of 120; Lux CAG1500--$55--will run the fan as little as 12 minutes out of 60.)

Thanks for your help!

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Old 01-22-13, 09:23 AM   #2
workaholic
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I would get a low priced thermostat, mount it somewhere near the wood burner, and wire the A/C function of the thermostat in parallel to the fan switch.(you can probably do this at the furnace). When the air temp near the wood burner gets high enough the new thermostat will think it's time for some a/c and turn on your fan, then turn off when the air temp goes down.
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Old 01-22-13, 09:50 AM   #3
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That sounds like a pretty good idea.
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Old 01-23-13, 06:04 PM   #4
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I agree...that dual thermostat DOES sound like a pretty good idea.

My only concern (also a concern with the spring wound timer) is that I don't want to mess up the control board on my furnace. I don't have a very sophisticated furnace, but it has a pretty complicated looking circuit board on it. I'll admit that I am a little gun shy about trying something unusual because I don't want to break my furnace.

Having the fan circuit with two parallel switches seems like it would work...but I am not eager to find out the hard way why it doesn't work.

Has anyone else out there tried a setup like this before?
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Old 01-25-13, 08:39 AM   #5
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The "brain" in your furnace won't know the difference, a closing contact, whether a manual selector switch or a thermostat, will send the same message; Turn Fan On.

If your not 100% confident about your wiring skills then my advice to you is to take your time, sketch your idea out, add wire colours, terminal numbers and any other information you find. Most furnaces have a wiring or connection diagram pasted somewhere in the cabinet, you should be able to cipher out the fan switch connections. Use a muli meter to verify the operation of your thermostat, experiment with the components. And probably the most important thing, disconnect the power from the furnace before you start wiring, and don't repower until all your connections are complete and safety covers are back in place. Most furnace control circuits are low voltage 24VAC, and most suitable thermostats are designed for this voltage- do not attempt to run the fan power circuit through the thermostat.

Hope this is helpful!

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