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Furnace fan control by second thermostat...

Posted by jb on October 10th, 2001 04:18 PM

Slightly odd thing here. I want to know how to wire a thermostat in a bedroom that would only serve to turn on the furnace fan when it gets too cold in that part of the house. My large two floor addition was made out of 2x4's and my original part of the house was 2x6. So, the new part cools off quicker than the old part. After much thought and research I have determined that if I just turn the fan to "manual on" for 10 or 15 minutes then the air gets mixed from throughout the house and everything is fine.

I want to put a thermostat in the colder area of the house and have it turn on the furnace fan if it gets below a set point, either until it gets warmer or for a pre-set amount of time, whichever is easier. I would even be happy to have it just come on for a few minutes a couple times an hour regardless of the temp. if that was easiest. How do I wire this?

My system is oil fired hot air with a new lennox air conditioner. One existing thermostat with one lever for heat/ac choice and another one for auto/manual on fan. I don't want to just leave the fan running on manual b/c of all the energy it would use and overkill factor and to keep noise level down at night when you can hear it. This isn't a problem with the A/C because the fan runs so much more. It isn't a problem in the middle of the winter either because the furnace cycles more often and equalizes and mixes housewide air more frequently. This time of year the furnace might only come on once every couple hours.

I have considered many options;
- Restricting the internal branch duct dampers to the 2x6 areas = It helped a little but didn't fix it. Plus I think it created too much back pressure and the furnace cycled more and heated less.
- Installing zone controllers and dampers = overkill and expensive. Not a HUGE house and the problem isn't REALLY bad, just annoying this time of year. Plus I think the 2x6 area would get overheated and I would need further Thermal Equalizing.
- Installing duct booster fans = the 2x4 area heats up fine so I don't need more air in there because it would still cool off faster than the rest of the house.
- Manually turn on the furnace fan between furnace cyles to mix air throughout rooms = Works Great! but I don't want to have to do it manually.

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