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Thread: choosing the right size modulating furnace

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  1. #1

    Default choosing the right size modulating furnace

    We live in a part of the country where it often gets to twenty or thirty below during the winter and sometimes maintains those temps for two or three weeks straight. We live in a fairly drafty 30'x30' two story plus full basement home with a glass-doored fireplace but no attached garage. The house is drafty because the insulation probably needs a boost and the windows need replacing. We will do both, but it will have to be on a 5- or 10-year time frame as we cannot afford to do it all anytime soon.

    We are replacing our 80,000btu 80% efficiency oil burning furnace.

    We would like to replace it with a 100,000btu 95% modulating furnace, specifically a York Affinity with the selling point being that it modulates in 1% increments and does not require a special thermostat.

    Our question is this...
    Since it modulates, does it really matter if the furnace is 80,000 or 100,000btu? We have heard it both ways from furnace people, with some saying that the 80,000btu would be best because it would be more efficient and cheaper to operate. Others say that the 100,000btu model is better because since it modulates, the furnace will only run as much as necessary no matter what the btu's are, and that we will appreciate the higher btu's in the winter cold.


    There is only about a $200 price difference between the 80,000btu model and the 100,000btu model.

    Any thoughts on which would be a sounder choice?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    DOes the 80K unit you have now keep the place warm enough?

    It may be too big. There is a bottom limit on how low the modulating furance can go, so on a mild day, the smaller one could end up being more efficient since it could run longer at lower firing. You get the most efficiency and comfort if the thing runs all the time rather than turning on and off, so modulation really can help. Another option that I really like is a variable speed fan. These ramp up and down with the need and there is no rush of cold air when the furnace first turns on.

    Have you done a heat load analysis? That should really be the first thing to do.

    Keep in mind that an undersized furnace doesn't mean the house will get colder and colder at the same outside temperature. If the furnace was sized to maintain 70-degrees at an outside temp of -20, it won't just fail if it gets to -21, the inside would drop to 69 and stay there. Depending on how much insulation you have and since the coldest temps are usually just before dawn, that would mean it would drop slowly overnight, and may never actually drop below that 70-degree mark.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3

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    I agree with all of what jadnashua said. Its better to run a smaller unit longer then a large unit for a shorter period of time. you may even want to go smaller with the wood stove available to make up the difference. You will also want to do a heating load calc on the home with new windows, better insulation and air sealing. Thats what I would size the unit for

    I also googled the york affinity and it a simple two stage furnace with a 90% afue which is still way better then a 80% single stage but you might want to check your facts before you purchase .

    good luck

    Lou

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