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  1. #1

    Determining if a furnace is undersized?

    We had a Carrier 58MVC 3 stage furnace installed last winter. At the time we had several bids; all were for a 100k btu unit except for this one, which spec'd 80k btu.

    This was to replace an older 100k unit in a 2800 sq ft house in Portland, OR. I was actually skeptical about all of the bids, considering no one was willing to do a calc. But the company guaranteed our satisfaction so we went with their assessment - 80k btu.

    Basically, the unit is great in conjunction with the programmable thermostat- normally in the lowest stage. However, the problem is when we want "heat on demand" - when someone gets up earlier than expected, or we come home earlier than expected, it just seems to take much longer than the old furnace to come up a couple of degrees. The heat and velocity coming from the registers is just not very satisfying.

    Before getting into the technical issues, I just wanted to ask what kind of performance one should expect in terms of bring a room up to 68 degrees, from say 62 degrees when the weather is 35 degrees outside.

    For some context - 2 stories (over a small basement) 2800 square foot house, built in 1988. 2 x 6 construction, average tightness but lots of windows. In Portland with relatively mild winters.


  2. #2
    Join Date
    Jun 2006
    SW MO.
    Still not enough info to go on. Your best bet would be to get a load calc. done and probably a blower door test.
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  3. #3
    Join Date
    Sep 2002
    I don't know
    A furnace is only undersized if it can't keep up on the coldest day of the year.

    Having a blast of hot air every few minutes is not desirable; fast heat on demand can only be achieved at the expense of comfort and energy efficiency.

    The solution is to not let the temp get too low; try a more modest setback.

  4. #4
    The solution is to not let the temp get too low; try a more modest setback.
    This is the approach we took when feasible- basically we would program it for 62 at night, then, another set point at 65 or so at 6:00 a.m., in case one of the kids woke up early.

    However, bringing it up manually from 65 to 68-69 would take an hour or longer. I guess it is just less responsive than other systems we've lived with.

    Another perspective is, imagine you have a perfectly accurate load calc. Now, imagine you installed equipment to match it optimally. At this point, would you be able to predict how long it would take to increase the indoor temperature from 65 to 68 at a given outdoor temperature? If not, what variables are preventing this prediction?


  5. #5
    Join Date
    Jan 2004
    If you had a perfect load calc, yes you could predict the amount of time it would take.

    Your new furnace, is less then 5000 BTU's output smaller then your old 80% efficient furnace.
    But, I don't think it goes to third stage right away, I think it only goes to second stage at first.

    If it does go to third stage right away, it should only take a few minutes longer then your old furnace. Call the installing contractor, and have him check it out for proper operation.
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  6. #6
    Join Date
    Mar 2005
    burlington county n.j.
    Quote Originally Posted by amd View Post
    A furnace is only undersized if it can't keep up on the coldest day of the year.

    this says it all right here...

    does not matter how long it takes to recover from a setback as long as it will keep the temperature you want on the coldest days.

    it will be running longer than the old one did but burning less gas when it is running.

    the air from a new properly sized heater is always cooler than the old beast it replaced due to larger blowers and fan operation being controlled by timers on the board instead of thermostats in the heater.

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