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  1. #1
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    Furnace capacity sizing based on observed run times

    In another post I have mentioned we will be looking to replace a nat. gas furnace maybe next fall. It's been a cold winter in Northern IL. We had a couple of nights when it hit about 30 below. Our house it about 1,500 sq. feet and our furnace capacity is 60,000. I think our design temp is 4 degrees (zip 61071). I might try to figure out how to supply my run time chart from Ecobee to anyone interested if I can figure that our. Generally run times were about 35-40 minutes while well below zero degrees. Load calculations are a total foreign concept and not available through my preferred contractor. My wife and I are considering requesting a 40,000 btu replacement furnace. We keep our house at 68 degrees and don't do set backs. What say you?

    Thanks! joe

  2. #2
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    what efficiency is the 60k? what is "well below" zero degrees?

  3. #3
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    I would not put in anything that is below/less than design. Just because you want/like how that MAY run, the next owner might not.

    If your preferred contractor does not do load calcs, find another contractor. If he does "seat of the pants" or "rule of thumb" on sizing equipment, what else is he doing it on?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  4. #4
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    Thread Starter
    Hi Mark,
    Our current furnace is 97%. Would like new equipment near that efficiency also. Well, we had several days when it stayed below zero for the highs. An example the night it was -30 I think our daytime high was about -10 or so. Also, our attic insulation and is R50. And two bedrooms are above our attached garage we heat to 60 degrees with a " hot dawg". Two of the adjacent walls are then common to the living space also. Did I make sense?
    Thank you,
    Joe

  5. #5
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    Quote Originally Posted by pacnw View Post
    I would not put in anything that is below/less than design. Just because you want/like how that MAY run, the next owner might not.

    If your preferred contractor does not do load calcs, find another contractor. If he does "seat of the pants" or "rule of thumb" on sizing equipment, what else is he doing it on?
    The next owner doesnt matter. They can change it how they see fit.

    That said, your questions and mine should be answered.

  6. #6
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    Quote Originally Posted by joeselectric View Post
    Hi Mark,
    Our current furnace is 97%. Would like new equipment near that efficiency also. Well, we had several days when it stayed below zero for the highs. An example the night it was -30 I think our daytime high was about -10 or so. Also, our attic insulation and is R50. And two bedrooms are above our attached garage we heat to 60 degrees with a " hot dawg". Two of the adjacent walls are then common to the living space also. Did I make sense?
    Thank you,
    Joe
    Lennox and others make a 98% unit. If the run times are while at night you are only slightly oversized. Also, if the localized temperature averages over the past few years reflect these -10, -20, -30F temperatures on a regular basis then keeping the 60K may not be a bad idea. A 40k would very likely be too small based on this micro-climate data. Only a Manual J heat loss calc can tell you exactly what you need.

    Micro-climate does NOT take into account the 4 degree design temperature. It would use one of the other negative temperatures.

  7. #7
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    Thread Starter
    Marc, thanks for taking the time. The temperatures we have the meteorologists termed "once in a generation" low temperatures. This will probably be our forever home. Thanks for sharing your thoughts.

  8. #8
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    Quote Originally Posted by joeselectric View Post
    Marc, thanks for taking the time. The temperatures we have the meteorologists termed "once in a generation" low temperatures. This will probably be our forever home. Thanks for sharing your thoughts.
    what were the past years' temperatures on average?

    For example, my "area" usually uses 10 or 0 degrees. My area for the last few years has had long stretches of -10 to -20. I try to size the duct and system for -15. Oversized? Perhaps "by the book", but "by the comfort" Ive never had a complaint.

  9. #9
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    Marc,
    I have no doubt you have had no complaints! I would have to dig into some climate data and do some research. If we would end up going the 40,000 route I would expect that we may lose a degree or two below the set point when those unusually low temps occur. Me thinks their would be some warm air circulating at all times without the stop and start.
    Good thoughts, you're a thinker!

  10. #10
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    Quote Originally Posted by joeselectric View Post
    Marc,
    I have no doubt you have had no complaints! I would have to dig into some climate data and do some research. If we would end up going the 40,000 route I would expect that we may lose a degree or two below the set point when those unusually low temps occur. Me thinks their would be some warm air circulating at all times without the stop and start.
    Good thoughts, you're a thinker!
    If undersized, you'll lose a lot more than that. 20,000 btus is a lot of energy.

  11. #11
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    Thread Starter
    Thanks Marc. 40,000 not the way to go then.
    Have a good night.
    Joe

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  13. #12
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    Like I said, only that calculation can tell you what you need at the given design temperature. You'll be able to dial it in easily with that.

    Have a good night.

  14. #13
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    Duct leakage to unoccupied space will undermine a runtime engineering method of sizing.

    If you're losing any significant amount of conditioned air which is ultimately replaced with outside air, that impacts the demand over a given period of time. Increase the runtime by lowering the capacity and you'll unknowingly increase the demand upon your equipment....which could put you in a bind.

    People get hung up on equipment efficiency without thinking in terms of system efficiency. Duct leakage solutions can often lower your utility bills more than simply bumping your burner efficiency. A contractor who doesn't/can't/won't do a load calc probably doesn't know much about what impacts system efficiency and may not be the best outfit to guide you to the best decision. Good luck.

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