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Thread: turn furnace into a fan for summer?!?

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    DIY Junior Member rbico's Avatar
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    Default turn furnace into a fan for summer?!?

    I have a furnace in my basement where it is cool. My upstairs gets really hot in the summer. I heard a DIY guru on the radio say that some furnaces offer a "fan-only" option so that the furnace can blow cool air from a lower part of the house up through the vents. I called the company that installed mine bu they said it would be $150 to install a switch. The guy I talked to said he has that option on his house and it works great.
    Keeping that in mind, last year - having nothing to do with the above - a little 6" rubber hose wore out on my furnace and there was a safety mechanism that prevented the heater/flames from going on but the fan kept going blowing cool air through the house. It took a few days for me to figure it out what the problem was - I just bought a rubber hose from an auto store and it works fine now. My thought is that I could just remove that rubber hose which would shut off the heat/flames but allow the blower to blow. Would that be safe? Or is there any other advice. $150 would normally not be a lot, but times are tight for everyone.

    thanks

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    DIY Junior Member DeweyBeach's Avatar
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    Does your existing thermostat have a "fan only" position (i.e., system "off" but fan "on")? If so, I'd just use that. If it doesn't have that option, I'd pull the thermostat from the wall and see which wires it has hooked to it -- my thinking is that if your furnace will run in fan only position but your thermostat can't, you may be able just to change the thermostat and achieve what you're trying to achieve. If all that fails, you can probably wire in a switch to apply 24 volts AC (which is normally operating circuit control voltage) to make your furnace fan run on its own. And if THAT won't work, you can always flip off the gas pilot light on your furnace during the warmer months and then do whatever's needed to make your furnace fan cut on.

    Basically what you're doing is a form of geothermal cooling -- the basement concrete slab is always going to be cold, and moving the warmer air around the house and getting it closer to the slab so the slab can absorb some of the heat (or, put differently, moving the air the slab has already cooled to the warmer parts of the house) is going to make things feel more comfortable.
    Last edited by DeweyBeach; 09-01-2010 at 12:53 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    I'll second looking at the thermostat wiring. IF you have the manual for your furnace, look at that. Often, they wimp out and only run two wires to the thermostat, where the furnace has options for more. If the furnace is wired to control the fan separately, then it's easy, but is even easier if you have it already wired to the thermostat.

    Keep in mind that just because the basement is cooler, doesn't mean that running the fan will bring that cooler aire upstairs...you need to have a cold air return down there to get that cold air to the furnace to then blow around. If the downstairs only has exhausts, and no returns, you'll only be moving the hot air upstairs around.

    If the furnace has the internal wiring to run the fan separately, but it is not wired up to the thermostat, you could either run a new cable or just wire in a jumper. Parts wise, all it would take is a short piece of wire. If you wanted to put in a switch, Radio Shack or HD, or most any hardware store would have one that would work for a couple of dollars.

    If you can post the make and model, someone can probably tell you. The manual should show it. If you run a new cable, install a new thermostat that has the switch to control the fan separately.

    But, if you don't have cold air returns in the basement, without adding some, none of this will do much of anything.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member rbico's Avatar
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    thanks for the replies. The guy I talked yo mentioned the thermostat,asking if it was a 2-wire or a 3-wire. I replaced the thermostat a few years ago and I think it is only 2-wire, but it certainly doesn't have a fan-only option.

    I am not sure about the return. I think it pulls air from the floor the furnace is on, but I am nit sure. I guess I should figure that out first.

    I will look up the model number and post it here. Thanks for the help.

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    Plumber jimbo's Avatar
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    Very unlikely that a furnace in the basement takes its return air from the basement. It would take combustion air locally, but not return. You need to check that out.

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    DIY Junior Member DeweyBeach's Avatar
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    Quote Originally Posted by jimbo View Post
    Very unlikely that a furnace in the basement takes its return air from the basement. It would take combustion air locally, but not return. You need to check that out.
    I'd think that would depend if it's a finished basement or not. If the house was built with any thought at all of anyone spending time down there, then it's possible there's a return duct on that level.

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    DIY Junior Member DeweyBeach's Avatar
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    Quote Originally Posted by jadnashua View Post
    I'll second looking at the thermostat wiring. IF you have the manual for your furnace, look at that. Often, they wimp out and only run two wires to the thermostat, where the furnace has options for more. If the furnace is wired to control the fan separately, then it's easy, but is even easier if you have it already wired to the thermostat.
    In my home, whoever wired the thermostat snipped off the unused wires, but in the furnace, they left the unused wires their original length after they stripped back the wire jacket and then wrapped the unused wires around the jacket. I found that the thermostat wires were not tightly stapled to the studs inside the wall, so I was able to grab the wires with vice grips, tug gently, and expose a couple more inches of usable wire when I needed them at the thermostat end. YMMV.

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    DIY Senior Member Hube's Avatar
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    one easy way to circulate the cool air from the basement(especially if there are no returns in the basement) is to simply remove the blower door of the furnace, install a makeshift filter over this opening, and run the blower by using the T stat setting (fan "on").
    Note; this method only to be used for circulating cooler air from the basement.....not during the heating season. When its time for the heating season make sure you put the blower door back on and re-set the t stat to fan "auto"
    Last edited by Hube; 09-02-2010 at 06:55 AM.

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    DIY Junior Member DeweyBeach's Avatar
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    Quote Originally Posted by Hube View Post
    one easy way to circulate the cool air from the basement(especially if there are no returns in the basement) is to simply remove the blower door of the furnace, install a makeshift filter over this opening, and run the blower by using the T stat setting (fan "on").
    Note; this method only to be used for circulating cooler air from the basement.....not during the heating season. When its time for the heating season make sure you put the blower door back on and re-set the t stat to fan "auto"
    To do this, you may have to bypass the door switch which isn't the best idea in the world. And that switch is there for a good reason -- if the door isn't closed properly when the burners are on, you can vent carbon monoxide in the house, which can definitely kill you.

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    DIY Junior Member Russell Gooch's Avatar
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    Sorry to bring this post back from the dead but it is pretty easy if you have the extra wires just not hooked up on the tstat side and or furnace side turn off power to furnace then hook up the G wire if you do not have the wires or compatable tstat you can literally run two wires to a switch like a toggle or light switch to the exterior of the furnace or even simply jumper the R wire and the G wire.

  11. #11
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by Russell Gooch View Post
    Sorry to bring this post back from the dead...
    I hope you washed your hands after digging up this corpse.

    One thing I did not see mention of is humidity and mold. If not controlled, sometimes it is best to leave the basement air where it is rather than circulate it. Drawing warm humid air down into the basement can create mold issues with the fan spreading the mold spores throughout.

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