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Thread: Forced hot air furnace too hot up - toocold down.

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    DIY Member Rughead's Avatar
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    Default Forced hot air furnace too hot up - toocold down.

    Hi guys. I'm not there (Scarsdale,NY) but my wife says the upstairs vents are blowin hot and downstairs cool. It's so hot upstairs at nite she's opening the windows (duh?) but hey, like I siad I'm not there. Going home for Xmas and will give it all a good look and report back. Told her to close the upstairs vents and that should divert more heat downstairs. She can't, they're stuck open. Swell. Guess I'll put in new smooth working vents when I get there. It's a one zone system, about 20 years old, oil fired and combined with central AC. Any ideas why there's not equal heat from all vents? Could it be duct blockage? Appreciate any advice before I get there so I can make a more informed analysis. Cheers and best regards, Ric from the UN in Budapest.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    In the summer, I close some of my downstairs vents and make sure upstairs is open to get more cold air upstairs. It sounds like that may have been done there as well. In the winter, you need to reverse that. It's also possible they did this with dampers in the ducts rather than at the registers. Tell her to look for levers maybe down near the furnace. They're open if the handle is along the length of the duct, and closed if they are across it.
    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 Member Rughead's Avatar
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    Default Forced hot air furnace too hot up - too cold down.

    Thanks Jim. Will advise my wife to check for dampers in the ducts in the cellar and if found, ensure they're open. With that and new smoother working registers in the rooms we should solve the problem. Will come back once in NY and have a better idea of the situation. Have a happy holiday. Cheers, Ric.

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    Moderator & Master Plumber hj's Avatar
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    Default heat

    Welcome to the world of thermal dynamics. Heat RISES so it will end up on the second floor first. Even if the downstairs were blowing heat, it would still end up upstairs unless there are doors at all the stairways. The only way to convince it to go downstairs is to have the "cold air returns" at the floor level of the first floor so the heated air has to come down to be recirculated. You normally have the same problem in the summer time, but then it is because the colder air likes to stay downstairs, and let the warm air go up to the second level.

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    In the trades Dana's Avatar
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    Quote Originally Posted by hj View Post
    Welcome to the world of thermal dynamics. Heat RISES so it will end up on the second floor first. Even if the downstairs were blowing heat, it would still end up upstairs unless there are doors at all the stairways. The only way to convince it to go downstairs is to have the "cold air returns" at the floor level of the first floor so the heated air has to come down to be recirculated. You normally have the same problem in the summer time, but then it is because the colder air likes to stay downstairs, and let the warm air go up to the second level.
    Heat doesn't rise- it moves from hot to cold.

    Sure hotter air is less dense than colder air and unobstructed undisturbed air will stratify with the warmer stuff on top, but this buoyancy/convective effect is several orders of magnitude lower than the blower & duct impedance issues. (It takes a heluva hot-air balloon to overpower a 300-600W blower-motor!)

    In the summer upper floors tend to have more direct solar heating plain & simple, they're, less shaded, with a 130F attic instead of a 80F room above it, and an 80F rooms instead of 70F earth-coupled basement below. It has very little to do with the density of the cooler air falling the first floor and the hotter air rising. The direct cooling load on the upper floors is simply higher. With the same duct-impedance issues as during the heating season the first floor may stay cool enough since the load is dramatically smaller. It could be that the duct system was designed to optimize the cooling balance (higher flow to the second floor), which will put it out of balance toward the second floor in heating mode.

    But in the meantime if there's at least some flow out of the first floor ducts you may be able to temporarily balance things somewhat by closing doors restricting flow to the overheated rooms, and leaving the doors to the colder rooms open to minimize the return impedances.

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    DIY Member Rughead's Avatar
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    Hi HJ. Yes I do understand thermal dynamics and our problem is compounded by the fact that our house is actually 3 floors and our bedroom is in the apex to where the heat accumulates. We can close the door to the 3rd floor and close the registers there which I think will considerably reduce the hot air flow into the room and cause the forced hot air in the system to go more into the 2nd and ground floors. At least in principle. But like Jim said, we gotta see if there're any dampers in the ductwork out of the furnace and ensure they're open. That, with better control of the registers should improve our overall heat distribution. Am anxious to get home and sort this out and stop wasting energy. But right now I'm gonna watch the replay of the "Beagles" beat the Giants on ESPN America on cable in Budapest. Born and raised in Lancaster, PA and relocated to NY years ago, but always been and will remain an Eagles fan.

    Meanwhile I told (advised...) my wife to duct tape stiff cardboard over the registers in the bedroom and when I get back there this week I'll hopefully sort this out and if we continue to have a problem I'll be back. I dislike forced hot air heating sytems, having grown up with good old cast iron radiators (DOB 1949). Only 15 months to obligatory retirement and then I can deal with these household maintenance issues myself and not have these stressful nightly Skype talks. We also have sump pump issues, but that's a different category. I've been enjoying all you guys considered and knowledgeable info over the years.

    Thanks for your thoughtful advice and a Happy Holiday to you and your family. Best regards, Ric.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    My home is fairly open. It had the same issues. I ran a return duct to the top of the stairwell and essentially closed off the one on the first floor. I also run the (variable speed motor on low) fan continuously. It pulls the hot air from the top of the stairwell and redistributes it through the house. In the summer, it pulls the hottest air out of the top of the house. After doing this, the delta between the top floors and the lower dropped from probably 7-8 degrees, to about 1-2.

    A second thing that was done was to add a radiant barrier to the attic roof. This lowered the summer attic temperature by about 25-30 degrees, and made the upstairs bedrooms more comfortable. After done, the ceiling temperature was the same as interior walls rather than being a big radiator after being heat soaked the whole afternoon. Pretty good for a sheet of foil faced paper!
    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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