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Thread: Furnace question

  1. #16
    DIY Senior Member Hube's Avatar
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    NMNUNNA;
    That existing return of 18x8 will only provide 144 sq in of air no matter what else you cut into it. Whoever installed this poorly sized return duct in your home did not seem to have any clue as to duct sizing. Do not attempt to add any return in the immediate area of the furnace (it could upset the burner flame and or draw fumes back into the home, etc.)
    The best and safest thing to do is to have a Hvac tech come into your home for a "hands-on" view and then give you a price on what is required to correct the situation. Make sure these quotes are from reputable heating firms, and better still get a 2 or3 qoutes from various firms, if possible.

    Just curious as you claim this is a 175,000 btu furnace (input) with a hoizontal return of 18x8 into a vertical drop of 20x10. At the point of entry into the furnace 's fan compartment what size is it? In other words what size is the air filter at this entry point? .... 20x25 or larger?
    Also, what size are these other return openings thru-out the home?
    Last edited by Hube; 10-28-2009 at 07:24 AM.

  2. #17

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    Filter size is 16 x 20 x 1

    Grill cover size of each return is 11 3/4 by 7 3/4 inches
    I think they used joists for returns.

  3. #18
    DIY Senior Member Hube's Avatar
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    NMNUNNA32

    that 16x20 filter opening is too small for the amount of burner firing (175000) that is why it is tripping off on hi limit from time to time. Get a Tech to come in and he can give a definite solution to the problem.
    Good luck.

  4. #19
    DIY Senior Member Runs with bison's Avatar
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    It is definitely way too big of a furnace for that air handling system.

  5. #20

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    Hube:

    OK. My return duct is way too small and that is why the burners are turning on and off. That is the symptom. What type of issues can I expect because of this? (I am looking for things like waste of energy, cold/hot spots etc). Equipment wise I have to say that I do not have any issues. It has been running this way for 17 years. And also gas use is not really that bad.

  6. #21
    Retired Defense Industry Engineer jadnashua's Avatar
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    Well, the safety circuit is not normally intended to actually need to activate. If it failed, you could have a fire because it overheated. It could warp the heat exchanger and cause leaks. Do you have a good CO detecter? A heating device is more efficient once it is fully warmed up, similar to your car. So, you aren't getting as much efficiency as you could. If you have central air, it wouldn't be as efficient as it should be, either.

    As an aside, probably 5-years ago, I bought my sister a CO detecter. About a month or two later, it sounded off...their furnace had bit the dust and could have killed them all if it had not been there and working...
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #22
    DIY Senior Member Hube's Avatar
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    NMNUNNA32
    If this problem,as you claim has been going on for several (17) years, and you are satisfied with still living with this problem then only YOU are the one to make the decision whether or not to look into any remedy. I,and others have given you some of the hazards that can occur from this furnace operation, but YOU seem to be somewhat satisfied with this problem. You do what YOU think is best
    it's your home so it's your call.
    Good luck in whatever you decide.

  8. #23
    In the trades Dana's Avatar
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    Quote Originally Posted by nmnunna32 View Post
    Hube:

    OK. My return duct is way too small and that is why the burners are turning on and off. That is the symptom. What type of issues can I expect because of this? (I am looking for things like waste of energy, cold/hot spots etc). Equipment wise I have to say that I do not have any issues. It has been running this way for 17 years. And also gas use is not really that bad.
    Actually "...not really that bad" is all relative:

    By running the temp up to the safety high-limit you're likely close to doubling your conducted distribution losses.

    By running undersized ducts the blower motor is burnin' more power, adding to the electricity bill.

    By running undersized ducts you're running higher duct pressures, driving more direct loss from duct leakage.

    By cycling on/off excessively you're wasting a bit of fuel on every ignition cycle.

    Were the furnace right-sized for the heating load, and the ducts right-sized for the air handler, "...not really that bad" in current terms might start looking like " not really that great." (Just a WAG, but I'm guessing right-sizing the world would cut your fuel use by a quarter to a third, if not more.)

    It's possible to keep maintaining furnaces and keeping them going for decades, but ~20-25 years is a typical economic service life, and by cycling the way it does it's had a harder than average life. Stuff wears out, and while it's usually cheaper to keep on fixing it in the short term, it's not always the right thing to do. New units (even high efficiency furnaces) just aren't all that expensive compared to a few service calls over decade. Shopping for a right-sized higher-efficiency furnace might not be an outrageous thing to think about in the next few years, especially where/when there are subsidies for going higher efficiency. It might have has an 78-80% AFUE in 1992, but as-operated I'll bet it didn't beat 65% in year one, and may have slipped a bit in the meantime. Compare that to a right-sized condensing furnace with a 95% AFUE- you may be able to cut your fuel use literally in half, when all of your overtemp/overpressure losses are factored in.

  9. #24

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    Hube:
    Thank you for your recommendation! I agree with you 100% about the return duct undersized. I wanted to make an assessment if this is an emergency situation or wait for few months until I replace my equipment. Since my equiment is kind of old, I am already thinking about replacing it.

    Thanks again for your help!

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