Deciding on correct BTU's
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  1. #1
    Join Date
    Apr 2013
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    Deciding on correct BTU's

    I have just received bids from 2 contractors to replace my HVAC system. The Lennox rep sized the replacement gas furnace at 88000btu and the Trane rep sized his replacement at 80000btu. Both furnaces are 2-stage variable speed 80% efficiency. My old system is 100000btu but is only about 65% efficient. I understand that my old system only delivered 65000btu. If the Lennox @ 88000 btu is 80% then I will get 72,400btu and the Trane will get me 64000btu. I am concerned because the Trane is actually 1000btu less than the old system. The house is a 1500sq.ft. ranch with a walk-out basement that has been finished after the original house was built. This means additional supplies were added to the trunk after the original system was installed. Am I correct in being concerned? Should I discuss bumping-up the Trane from 80000btu to 100000btu or should I expect the newer 2-stage system to handle this.

    I realize many will ask why only going for a 80% replacement. It is due to the location of the furnace from exterior walls and the need to tear down drywalled ceiling to install a 90+ unit.

    Thanks for assistance.

  2. #2
    Join Date
    Jan 2009
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    Keokuk, IA
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    What's your location? Di the old system run continously on the coldest days (design conditions). By continous, I mean did it literalyl run for more than 60 minutes straight.

    1500sqft with even average insulation shouldn't need more than 40k BTU's in most climate down to 10F. Finished basements, because they are insulated, don't add much load.

    2 stage is for comfort don't oversize because it's 2 stage it defeats the point.

    The 80k Trane should be plenty, but I would have a load calculation doen. Good change a 60k might be enough too.

    How much were you Jan and Feb gas bills? If you actually "need" a 100k furance, you should have gas bills well over $300. Probably close to $400. You would win an award for one of hte least efficeint 1500sqft homes I've ever heard of.

    In most climates, you can heat a 3000sqft 2 story home with average insulation with a 100k furnace.

  3. #3
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    Jan 2009
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    Do you have a gass water heater? Hiow old it it? You might be able to replace that with a power vent model or a HE tankless and run 3 pipes up the existing pipe chase the B vent occupies now. A 60k furnace can be vented in 2" pipe which makes it even easier to install.

    Seriously, see if anyone can do a proper Manual J load calculation. I bet your existing furnace even on the coldest days barely ran more than about 2/3rds of the time once reaching a steady temperature.

    Finally, keep in mind that you probably don't have enough ductwork for a 100k furnace. You old furnace probably only needed 800-900CFM. A new 100k furnace will need 1200-1300CFM!!!! IT will liekly be very noisy on high stage. Even that 80k will need 900-1000... still more than your current furnace.

  4. #4
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    Apr 2013
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    I'm in the St. Louis area. I have 9 windows on main floor and 3 in basement.The old system kept up with our winters and did not run excessively but gas bills did get high during our coldest spells. Out of three contractors none did a Manual J while with me. I became concerned only when I discovered the 80000btu 2 stage Trane would deliver 1000 less btu than the old system. You are right about the 2-stage variable speed. I wanted that so I could get additional savings with the 80% model but the 1000 less btu threw me and I was concerned about the basement. I certainly don't want to increase the size of the new furnace and give up any savings. My current A/C is a 2.5 ton 10SEER that could not handle the summer heat and ran for hours and hours without turning off. I am considering a 15-16 SEER 3 ton. Pairing it with a 2-stage variable speed should really help out in the summer.

  5. #5
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    Jun 2007
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    Greenwood Indiana (Indianapolis)
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    420
    Quote Originally Posted by doug.d View Post
    I'm in the St. Louis area. I have 9 windows on main floor and 3 in basement.The old system kept up with our winters and did not run excessively but gas bills did get high during our coldest spells. Out of three contractors none did a Manual J while with me. I became concerned only when I discovered the 80000btu 2 stage Trane would deliver 1000 less btu than the old system. You are right about the 2-stage variable speed. I wanted that so I could get additional savings with the 80% model but the 1000 less btu threw me and I was concerned about the basement. I certainly don't want to increase the size of the new furnace and give up any savings. My current A/C is a 2.5 ton 10SEER that could not handle the summer heat and ran for hours and hours without turning off. I am considering a 15-16 SEER 3 ton. Pairing it with a 2-stage variable speed should really help out in the summer.
    don't worry about the 1000 btu difference. furnaces only come in certain sizes. Who's to say that the original furnace is the right size. My brand makes a 75,000 btu, then jumps to 100,000. It sounds like a 2.5 ton is plenty of cooling power, but the difference in a 2.5 to a 3 ton is only 6000 btu's. that would be a small window a/c, to put it to perspective. Consider a 2 speed a/c, that way it runs at 2 tons when its not stinking hot out, like at night time, and the ability to kick up to 3 t when its really hot.They de-humidify 15% better also.
    As iron sharpens iron so one man sharpens another Proverbs 27:17 NIV84

  6. #6
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    Quote Originally Posted by doug.d View Post
    I'm in the St. Louis area. I have 9 windows on main floor and 3 in basement.The old system kept up with our winters and did not run excessively but gas bills did get high during our coldest spells. Out of three contractors none did a Manual J while with me. I became concerned only when I discovered the 80000btu 2 stage Trane would deliver 1000 less btu than the old system. You are right about the 2-stage variable speed. I wanted that so I could get additional savings with the 80% model but the 1000 less btu threw me and I was concerned about the basement. I certainly don't want to increase the size of the new furnace and give up any savings. My current A/C is a 2.5 ton 10SEER that could not handle the summer heat and ran for hours and hours without turning off. I am considering a 15-16 SEER 3 ton. Pairing it with a 2-stage variable speed should really help out in the summer.

    Last summer FAR exceeded design condtions on several days. A properly sized system, expecially one 20 years old should have fell behind by 2-4F on those 105F days you had. It should have ran for almost 10-12 hours striaght if sized correctly on those days.

    That being said, a 3 tons 2 stage isn't a bad idea.

    In your climate, if you can cool a home with 2.5 tons, I doubt you even need 48k BTU's that a 60k would deliver. I think a 60k 80% would be enough, but you definitely do NOT need a 100k. No way. I had a poorly insulated 1300sqft 1960's home with 10 windows and full basement that was only 2/3rd finished. Even when it was -5F, 1st stage still held temperature on a 80% 60k furance. That's just 32k BTU's.

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    33,884
    Old furnaces may be 65% efficient but output once warmed up is 80% of input so a 100K furnace would put out 80K when running steadily. They just waste a lot of heat on warmup, shutdown and off cycle.

    As Moto touched on, today's furnaces need to move a lot of air to be efficient and not shut down on limit. Don't oversize! Today's limit switched fail open if they open a few times.

  8. #8
    Join Date
    Apr 2013
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    4
    Thanks for everyone's input. When in doubt...go to the pro's. I will definitely NOT ask to bump up the furnace. The idea about the 2 speed A/C is also something I will ask my Trane rep. Not sure if they have something around a 15 SEER in a 2-speed but I have been told by other manufacturers reps that a variable speed furnace will increase tha A/C efficiency by another SEER or so. They are ready to go when I am comfortable with my choice.

  9. #9
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    Feb 2009
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    Portland OR
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    Have you asked to size down at all? I show your design condition to be 8 degrees and with 1500 sq feet I can't see why a 60K would not be more than enough. Keep in mind if your house only needs 35k BTUs and you buy a two stage 80K, your first stage will be 38,400 BTUs and you will never see the benefits of second stage, you would have been better of buying a properly sized single stage furnace than an oversized two stage furnace.
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  10. #10
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    Dec 2002
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    SouthEast NC ICW & Piedmont Foothills
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    is this natural or LP gas?

    or did I miss that somewhere?
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  11. #11
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    Apr 2013
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    4
    I haven't asked about downsizing but I will mention it. The fuel source is natural gas.

  12. #12
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    Jan 2009
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    Keokuk, IA
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    With 1500sqft, a 45k is probably plenty if you have even average insulation but you'll need a 60k for the 3 ton blower. Dual fuel isn't a bad option in your climate wit ha 80% furnace if your electic is under abotu $0.12/kwhr. Gas Rates are probably about $0.80 where your at. It will also give you 2 more stages of heat so the system runs longer and gives you more even heating all winter. I have it on my downstairs system and love it. IT's runs almost continously on either the heat pump or furnace once it's under about 40-45F. Nice silent even heating in all stages. YRMV based on ductwork layout and equipment size.

    I have a 60k modulating furnace and 2 ton heat pump on my 1700sqft downstairs and the static pressure is really, really low since it used to have a 3.5 ton AC and 100k furance. Yup... oversized by that much.

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