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

  1. #16
    DIY Senior Member dlarrivee's Avatar
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    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 by dlarrivee; 06-23-2012 at 09:35 PM.

  2. #17
    In the trades Dana's Avatar
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    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.

  3. #18
    Plumber jimbo's Avatar
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    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.

  4. #19
    In the trades Dana's Avatar
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    Quote Originally Posted by Denver Dave View Post
    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.
    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.)

  5. #20
    DIY Junior Member BrianK's Avatar
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    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.

  6. #21
    In the trades Dana's Avatar
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    "Well insulated" means different things to different people (and in different climates.) A mere 11" of fiberglass in the attic wouldn't even make current code-min in much of New England. The 5.5" batts on 2x6" construction would meet current code if they're a high density type, but the typical R19s wouldn't make the grade. To me "very well insulated" means "better than code", not code-minimum, not that there's anything wrong with the current code minimums. I suspect the code-min for Calgary is about your level of insulation, which is fine, but it doesn't come with braggin' rights. ;-)

    Your attic venting scheme is probably doing more for cooling the conditioned space than the furnace-fan (which is still behaving as a 600-1000W space heater, after all.)

  7. #22
    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.

  8. #23
    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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