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Thread: Heat Loss

  1. #1
    DIY Senior Member Nate R's Avatar
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    Default Heat Loss

    Did a heat loss calculation in ResCheck (Free Program for checking whether a houses' insulation and such will meet code)

    Also did the same calculation in another program.

    I come up with about 32000 BTU of heat loss on my home at the recommended design temperature of -10 F.

    My Natural gas forced air furnace is 80% AFUE, and rated at 70000 BTU in, so it's 56000 BTU out.

    This seems to line up with our experiences last year. Never was the furnace anywhere near being on even 45 minutes out of an hour. I'd bet it was much less. I wonder how much money I'm throwing away by shorter cycles?

    Interestingly enough, the next model down is 45000 BTU in, and 36000 out with 2 burners instead of 3. That appears to be the only difference on the spec sheets. Out of curiosity, is there any danger in plugging the orifice for one burner? (Which would turn my 56000 BTU into about 37000) I'm assuming that would increase the gas pressure at the other 2 burners? Or is there a regulator in the system that prevents this? I don't think I would actually do this, but the thought crossed my mind.

    Also: I'll be adding an addition to the home, as I've discussed elsewhere. With MANY improvements to the existing home (New windows, floor insulation, more attic insulation, Tyvek, etc) and with the addition, my total heat load is around 37000 BTU.

    I was thinking of heating the addition independently. But, IF I could find a way to get a duct to the addition, I start to wonder. With 56000 BTU out, I would still have 50% more than I need. With 36000 BTU out, I'm a bit short. What would you do?

    If I DO heat the addition separately, I can get a new furnace for $650. I was going to spend about that much on a heater for the garage. I could get the smaller furnace for the main part of the house and move the current one into the garage.

  2. #2
    Plumber jimbo's Avatar
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    Modifying a gas appliance in any way is a big NO-NO for many reasons, legal and otherwise. Don't even think about it.

    I would check with a heating contractor, or maybe with the manufacturer, to see if your current experience would be considered "short cycling". It seems somewhere in excess of 50% run time is not short cycle, but I am not an expert on that .

  3. #3
    Engineer Furd's Avatar
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    Jimbo is correct, don't even think about it.

    IF your furnace was an older unit without an induced draft fan I would perhaps modify my statement about not trying it but the newer furnaces (any having an induced draft fan) are much more sensitive to the orifice size of the burners. The end result of plugging the orifice of one burner would be to have far too much air being drawn through the furnace and that would upset the combustion process which in turn would change the composition of the exhaust gases.

    Often times a furnace manufacturer will use the same induced draft fan on several sizes of furnaces but they also install a calibrated orifice between the combustion chamber(s) and the fan inlet to control the amount of draft and thereby combustion. Even if you wanted to permanently downsize your furnace by putting smaller jets in the individual burners you would also have to change the draft fan orifice and that is beyond a DIY capability.

    Plus...any modifications, even if done within sound engineering principles, would void the manufacturer's warranty and IF anything happened (like a fire) would possibly cause you grief with your homeowner's insurance.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    The last boiler I had could be adjusted in output by just changing the gas orifice. instructions came in the manual. Check your manual, you might be lucky. It was a closed combustion system. No idea if any furnaces do similar, but it is worth a look.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Senior Member Nate R's Avatar
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    Thanks for the info. I don't see anything that indicates I can change the gas flow for my furnace.

    So, when the house updates are complete, and the new garage built, I'll move my furnace into the garage and replace it with a smaller capacity two-stage unit. (Current furnace is single stage.) I can find this for about the same price as I was going to spend on a garage heater anyway. I don't care how much money I do or don't save with the two-stage unit, the fact that it will be quieter when running most of the time is worth the extra $40 alone. (Our furnace is in a "box" in the Living room of our home, so quieter is better.)

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