turn furnace into a fan for summer?!?

Discussion in 'HVAC Heating & Cooling' started by rbico, Aug 31, 2010.

  1. rbico

    rbico New Member

    Messages:
    13
    Location:
    seattle
    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
  2. DeweyBeach

    DeweyBeach New Member

    Messages:
    18
    Location:
    District of Columbia
    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: Sep 1, 2010
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,274
    Location:
    New England
    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.
  4. rbico

    rbico New Member

    Messages:
    13
    Location:
    seattle
    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.
  5. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    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.
  6. DeweyBeach

    DeweyBeach New Member

    Messages:
    18
    Location:
    District of Columbia
    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.
  7. DeweyBeach

    DeweyBeach New Member

    Messages:
    18
    Location:
    District of Columbia
    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.
  8. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    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: Sep 2, 2010
  9. DeweyBeach

    DeweyBeach New Member

    Messages:
    18
    Location:
    District of Columbia
    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.
  10. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    That is why I mentioned not to use this method in the heating season.
    note; there are many furnaces out there that the lower blower door can be removed with no problem being created to the rest of the system.
  11. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,062
    Location:
    Cave Creek, Arizona
    Turn the gas control valve OFF, so the burner cannot start no matter what else happens. Normally, a furnace blower motor will NOT run until the fan switch detects a minimum temperature, (which would be much hotter than the temperature in the house). Most thermostats do not have a "fan on/manual" setting unless it is a heating/cooling installation.
  12. Bobelectric

    Bobelectric Electrical Contractor

    Messages:
    39
    Location:
    Eighty Four,Pa. 15330
    Have someone change t-stat.
  13. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,062
    Location:
    Cave Creek, Arizona
    without sufficient wires, and the proper circuitry/controls in the furnace, that would do NOTHING.
  14. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
    Spend the 150 and get it wired properly otherwise using it will be a PITA. Beyond that, you can cut a return grill into the basement ductwork with a damper so you can shut it off in the winter.
  15. Denver Dave

    Denver Dave New Member

    Messages:
    1
    Location:
    Denver, CO
    I'm using the furnace fan approach occasionally this summer. There isn't a fan wire for the thermostat, but I have my pilot light turned off for the summer and the furnace has a push pull switch on the furnace where pushing the switch in, turns on the fan. The temperature of the floor in the basement can be 10 degrees cooler than the main floor in the house. To make sure the air is pulled from the basement rather than returned from the main floor, I took the front panel off the furnace, but left the filter in place on the return vents, thinking the air will take the path of least resistance, from the basement - the flowing air is cool as measured by a infra-red non contact thermometer. I wouldn't recommend leaving the furnace open even with no heat if you have small kids or pets - they might get caught in the pulley, although a screen might work.

    Other things we have done are installed solar screens on the south and east facing windows, open the house up to cool off at night and close the house up when hot outside. So far, we haven't had to run our swamp cooler, even with 104 degrees yesterday in Denver. Although I was thinking about turning the swamp cooler on yesterday, until I remembered to turn on the furnace fan for a few hours in the late afternoon. I'm in a competition on www.NegawattChallenge.org and we've really been trying to reduce our electric usage.

    Thanks for the post.
  16. dlarrivee

    dlarrivee New Member

    Messages:
    1,172
    Location:
    Canada
    Taking the cover off the front of the furnace does NOT change where the air comes from...

    Unless you're just taking the cover off the front of the blower housing, but then you'll just be sucking dust up and spreading it through the entire system.
    Last edited: Jun 23, 2012
  17. Dana

    Dana In the trades

    Messages:
    2,986
    Location:
    01609
    Even assuming that 100% of the return air came from the basement, the coefficient of performance for cooling purposes would be abyssmal. You're essentially using the uninsulated basement slab & foundation walls as a not very efficient or effective heat-exchanger to the not-super-cool sub soil, while burning 600-1200 watts (fully inside the house, no less) to move that air around. The net effect is to even up the basement temps with other rooms, but adding 2000-4000BTU/hour to the house in the process.

    And if you're not drawing 100% of the return air from the basement, you're using yet another ineffective inefficient heat exchanger to the mix- the (best case sealed but un-insulated) basement ductwork, which has even less surface area than the basement floors & walls.

    And if the ducts aren't perfectly sealed & balanced, and you house isn't the tigthest house in the state, you'll be driving infiltration of (potentially hotter, more humid) outdoor air into the house at some unquantified rate.

    While moving all that air may even up room to room temps it does next to no cooling, but the moving air may add to occupant comfort by wind-chill on skin. The same effect can be had with much lower power with fans in occupied rooms only.

    Alternatively, expending 600-1200 watts on a room sized air conditioner (or better yet, a mini-split) in the hottest rooms of the house would actually remove heat & humidity from the house rather than adding it.
  18. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    PLEASE NOTE: A furnace should NEVER be operated with the cover off the blower compartment. It will back-draft the flue and send CO throughout the house.
  19. Dana

    Dana In the trades

    Messages:
    2,986
    Location:
    01609
    Exterior solar screens are a big improvement for reducing the load, but since peak loads generally occur after noon, putting them on the west side rather than the east would be a bigger reduction in the peak, even if it's roughly the same total heat. But ADDING exterior solar screens to the west side (and keeping the east facing ones) would be in order.

    Swamp coolers would likely prove more efficient than your whole-house fan approach to comfort. But a room air conditioner on an upper floor and allowed to run continuously might use less power than the swamp cooler, and unlike the swamp cooler, would not be converting sensible load (temperature) into latent load (indoor humidity). Evaporating a pound (about a pint) of water takes 970BTU of heat out of the air with very little power input, but the efficiency varies dramatically with the actual indoor dew points, and those dew points rise with time when it's running, which takes the efficiency down. Moving the same amount of heat out of the air with a typical 5000-8000BTU/hr room air conditioner takes about 0.1kwh. Running the furnace fan ADDS more than 2000BTU/hour to the house. Using a local fan in an occupied room adds less than 1000BTU/hour on "high", and less than 400BTU/hr on "low".

    In high dry locations like Denver where the overnight lows dip to 70F or lower even on 100F days, a lot can be gained from nighttime ventilation strategies, using outdoor air to pre-cool the thermal mass of the house, and powering-down as much of the plug-loads in the house as you can during the day. (Every kilowatt consumed inside the house adds 3412BTU/hr to the cooling load.)
  20. BrianK

    BrianK New Member

    Messages:
    26
    Location:
    Calgary, Alberta, Canada
    I also take the fan compartment door off and run the furnace fan for 'poor mans A/C' and it works well. I also remove the attic access panel and put a fan in that 2'*3' opening. This vents the hot air out of the house by pushing warm/hot air from upstairs out to the attic and helps pull the cool air to the upstairs area while the furnace pulls the cold from the basement. I find by moving the warm upstairs out of the house (could have put the fan in a window blowing out but the fan in the attic access also cools the attic down) and the net result is that house cools down faster. And yes the house is very well insulated - about 11" in the attic of nice fluffy fiberglass and 6" in the walls. Basement is also developed and insulated - but is cooler because of the concrete being colder as discussed. This also lets me have a peek in the attic to see that all is OK. I don't like opening it up in the winter.

    Addressing the CO comments - I have multiple CO monitors throughout the house and have never had a hint of CO alarms. Not sure where the CO would come from because the furnace is not in heating mode - that is the gas valve is not open and there is no combustion going on - ergo - no CO. Of course once heating season returns, the furnace goes back to normal with the access panel back in place. I guess if the hot water tank comes on this is a source of combustion but with the required venting (a 6" line fresh air line into the furnace area and a 6" line into the cold air return) this is most likely where the fresh air is coming from and not back draft down the chimney and the CO monitors confirm that I have no problems with this method. I realize the door switch (which I bypass for the above) has been installed to potenially alleviate the possibility of back venting with the fan running...but the proof is in the pudding.
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