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Thread: Furnace quotes-BTU question

  1. #1

    Default Furnace quotes-BTU question

    I've received three quotes for a new furnace and heat pump system. My house (2100sq. ft.) has plenty of insulation for the region, new windows, new doors. two of the quotes include a 100,000 BTU (80%) furnace, and the third includes an 80,000 (80%) BTU furnace. The third is also the only one who ran any heat load calculations on the house. I grew up in a home without gas heat, so my knowledge of BTUs is lacking. Are 20,000 BTUs enough that I'm losing the efficiency of the furnace by having to large of a furnace for the house? To ask in another way, am I losing money if I choose a 100,000 btu furnace if an 80,000 btu furnace is enough? (other than the 100,000 furnace cost a little more upfront?)

    Question #2: I think the load calc. stated my house needs 62,000 BTUs of heat, an (80%) 80,000 BTU furnace is going to provide 64,000. Is a 2,000 BTU cushion enough that I'm not going to have a cool home in a few years when the furnace begins to lose it's efficiency?

    Thanks for the help.

  2. #2
    DIY Senior Member TedL's Avatar
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    In general, longer run times are more efficient.

    What's your location and the design temp?
    What do you have now? BTU input, estimated efficiency and/or age?

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    First, there are furnaces readily available that are in the mid-90% efficiency, and with today's energy costs, might be a better choice. Some of them are dual stage, so they can run at lower outputs when the full output is not needed.

    What was the design temp for the heat-load? Around here, it is in the order of -10 degrees F. Basically, the heat load indicates how much heat is lost at those worst-case temperatures. If the furnace is sized for that and the outside is at that temperature - the furnace will run continuously and be able to maintain the design temperature. If it is colder than that, it will not be able to maintain that temperature. If it was say -15, the house would stabilize a little cooler than the design temperature. On a warmer day, it will only have to run intermittently. The best comfort happens when the thing runs continuously and it also makes the humidifier and filtration system work better too.

    One factor is if you use a setback thermostat. If you let the house cool off on a -10 degree night, and it doesn't have some reserve, it will raise the temperature very slowly. Conversely, if the thing is oversized, it will only run for a short time, which isn't as efficient.

    Another thing to consider is a variable speed fan. This can make the most use of the heat you pay for. When first starting up, the vents are cold and the heat exchanger isn't hot yet - turning on the fan full force means blowing cold air out. Starting the fan off slowly means you not only don't do that enough to notice, but it is less noticable when it happens, too. It also slowly scales down when the heat goes off, extracting all of the heat you paid for rather than flowing out the flue.

    Last, if the furnace is serviced on a regular schedule (cleaning, checking the air/fuel mix/CO, etc.), it should not lose efficiency.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4

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    Not sure what the design temp was, but I feel confident that the guy knew what he was doing. I'm in Central Indiana, so I doubt it was to low. We have pretty moderate winters. Rarely dipping below zero. All three quotes were for variable speed blowers. It sounds like from you guys that, assuming he ran the calculations right, an 80,000 BTU furnace should be able to keep up 0.K.

    I guess I'm just unsure of whether the difference in BTUs is going to make so much of a difference that it should be a major issue other than up front cost?... if that makes sense. Would 20,000 BTUs to much cause a furnace to shutdown prematurely enough to see a significantly higher gas bill from a properly sized furnace?

    Indiana energy cost are pretty inexpensive... not sure why but electricity is dirt cheap. Probably the easy access to coal from southern Indiana and Kentucky... anyway, with our moderate winters the heat pump will do most of the heating, until the temp lowers enough to kick on the gas furnace. The price difference between 90% and 80% would just take to long to make up for what the more efficient run time savings would add up to.

    Thanks for the responses.

  5. #5

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    currently have about a 15-20 year old single stage gas furnace... about 65% efficient. I could burn money in my fireplace and save on the gas bill at this point... so anything will be an improvement, I'm just trying to keep from overpaying for to large of a furnace, or finding myself chilly because the 80,000 isn't keeping up.

    Thanks again for the replies... you've both given me some things to think about.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Oversizing a furnace is not a great thing to do, nor is oversizing an a/c unit. They work best when running full-time. A 200K unit would be turning on and off way too much, and never reach its optimum when the system only needed 60K or so. WHile the 80K unit would be running 3/4 of the time at the design temp, a 200K unit would only be running around 3/10 of the time. As said, for both heat and a/c, you will be more comfortable running the appliance at as near 100% as you can. Also, the on/off cycles tend to shorten the life of the unit - and, it may not get hot enough to get rid of all condensation, which can mess things up quickly.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7
    DIY Senior Member TedL's Avatar
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    Quote Originally Posted by taylor46952
    currently have about a 15-20 year old single stage gas furnace... about 65% efficient. I could burn money in my fireplace and save on the gas bill at this point... so anything will be an improvement, I'm just trying to keep from overpaying for to large of a furnace, or finding myself chilly because the 80,000 isn't keeping up.

    Thanks again for the replies... you've both given me some things to think about.
    The heat delivered by 65% efficiency on 100,000 btu input (you actually didn't answer current input) almost exactly equals 80% efficiency on 80,000 btu input. How big and how well does current system heat?

    Last fall I put an 80,000 input 96% efficiency unit into 2700 sq ft, plus 1000 sq ft fin basement. Works fine in a -10 design temp area.

  8. #8
    Plumber jimbo's Avatar
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    I would support the guy who made at least some attempt at a manual J.

    If the furnace is too large, you will more quickly heat up the area so the thermostat "feels good" and turns off. Howver, you no longer have the circulation going to move the air around and really evenly heat up the whole space. The short cycling will result in uneven heating, hot and cool spots, and is not good for the unit.

    An appropriately sized unit will have a longer run time , producing a more evenly heated space. The unit installation manual should have some guides on run times.

  9. #9

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    Yeah, you're right I didn't answer the input question... sorry. Current furnace is actually 125,000 BTU, so compared to the quoted 80 and 100 it's somewhere in the middle for output. It does o.k. with keeping the house warm. We run it a little cool just to keep the bills lower, and there is an addition that has pretty poor airflow (something that was included in the quotes as just needing an additional vent or two. None of the quotes thought that would be a problem).

    I've called the guy who did the load calc. to get some more info from his viewpoint, but probably won't here back until Monday.

    Thanks for the help everyone. This is a great forum, that I just sort of stumbled across looking for something else.

  10. #10
    DIY Senior Member TedL's Avatar
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    Have them put a performance clause in the contract ("System shall maintain 70 degrees in outside temperatures of xx degrees.") Replace xx with your local design temp.

  11. #11

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    I would seriously look at a higher efficiency unit. 80% seems low.

    Have them run some numbers for fuel cost for 80% and something in the 90's.

    Tim

  12. #12

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    I had all the same questions a few years ago when AC & furnace went K-put after addinga an addition. I had a 2200 sq ft, 2 story with a 600 sq ft addition and 2 new vent legs off the trunk and a finished basement in Tenn. Originally had a 125K BTU, low efficiency gas furnace. Although my AC failed, when we got the quotes to replace all we got 2 - 80K BTU, 2 100K & 1 125K BTU. ALl were 85 - 90% efficiency. We stayed in the middle with the 100K and had no issues. 125,000 to 80,000 is a 36% swing, which is quite a lot. Even with a 20 - 25% efficiency gain you are not quite breaking even with what you had and after it ages it loses effiociency. I would stay in the middle.

  13. #13

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    That's a good idea TedL, I will have to get that included... I actually need to see if the company offers a "comfort" guarantee. One of the other companies offered one, similar to your thinking.

    Since I'm combining the system with a heat pump the upfront cost becomes an issue with jumping from 80% to 90%. Where I live, the heat pump will actually do a majority of the necessary heat throughout the winter months. Just didn't want to end up cold when temps do dip.

  14. #14
    Retired Defense Industry Engineer jadnashua's Avatar
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    If the calculation was correct, the house won't magically get a lot colder if the outside temperature gets below that if you have at least that amount of BTU's available. As you approach the design temperature, the furnace will run longer and longer until it runs constantly. If the outside temperature continues to drop below that design temperature, the house will gradually cool off. At any outside temperature once it has dropped below the max output, it will stabilize somewhere.

    I disagree about the furnace becoming more and more inefficient as it ages. If it is cleaned and adjusted, it should remain at or near its design efficiency until the heat exchanger or some other major piece dies, then you'd need to replace it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  15. #15

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    Thanks again for the help everyone. I received a message back from the guy who gave me the load calc. He used -5 as a design temp, which I think Central Indiana did hit maybe twice this winter... so It's rare. He also put in that we have very little attic insulation, even though we have roughly 13 inches, so he built in some wiggle room. Also, stated he would be more than happy to put it in writing that the system will keep the house at 70 at the design temp. So, I'm happy, sounds like I will be getting a new 80,000 BTU furnace and some significantly lower heating bills.

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