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Thread: Furnace quotes-BTU question

  1. #16

    Default BTU-Efficiency

    I have a 100,000 BTU Lennox Pulse that is 21 years old. My house is 1700' with a 900' walkout basement that is finished and heated. I live at 7400' altitude in the Denver foothills. This furnace does a good job at heating the house evenly. I keep it set at 72 degrees during the day.
    I am looking to buy a Rheem RGRL 95% 2-stage furnace. The maximum BTUs of this model is 90,000. To get 105,000BTUs I could step down to a Rheem RGRK 92% 2-stage.
    Winters here are usually mild. Most nights are 10-20 degrees but we can get temps of -10 to -20.

    I'd prefer the 95% RGRL but am afraid it won't have enough BTUs for the severe cold. Can anyone offer some insight? I don't like to be cold!
    Thanks

  2. #17
    Retired Defense Industry Engineer jadnashua's Avatar
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    The size of the house is not a direct input to the size of the furnace that is required. WIthout a heat load analysis, it is impossible to say what you need. It was not very uncommon to have builders just put a bigger furnace in that was actually required just to make sure. Energy wasn't too expensive back then, and people didn't really care. A smaller furnace might be more than enough. You know that the 100K job is big enough.

    At the coldest temp, an ideally sized furnace is running constantly. You obtain the greatest comfort when it is on all the time, thus, no ups and downs in the indoor temperature.

    A good heat load analysis requires knowing the size, quantity, and type of windows, doors; the quantity of insulation, and the anticipated temperatures, the length of the exterior walls plus probably a few other things.

    Only when you get this done (and you can probably find a spreadsheet on-line that will help you calculate it) will you know how big of a furnace your house really needs.

    Note, once the furnace can't maintain the set temperature (i.e., it is running all of the time), the house doesn't magically become a refrigerator - it drops a little with each degree the outside temp drops. Eventually, it reaches an equibrium - putting in as much heat as is leaking out. That is why is it required to analyze the house - leaking windows, insulation, etc. Once you know this, you can intelligently choose the best size for your conditions.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #18
    DIY Junior Member chas22's Avatar
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    If it was my house and I were to just ballpark it. I would look at something about 92% 80,000 BTU furnace with a 5 ton drive blower, then add a 13 seer 4 ton condensing unit, and use an expansion valve.

  4. #19
    DIY Junior Member Becka's Avatar
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    Default Which Gas furnance to buy?

    Quote Originally Posted by TedL View Post
    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?
    Curently have a 18-20 year old furnace . Replace heatpump with new Carrier 3-1/2 ton (bigger than what was there), 15 Seer last year. I "thank God" got the maintenance contract on both the furnance and Heatpump. The service people have been to my house 7 plus times for NO HEAT calls since Christmas Day! Everything from door swtich, control panel, ignitor, etc. etc.

    I have Propane GAS backup (when temperature drops below 40 degreees).

    My house is a little over 2,500 sq. feet, 275 yards from the Chesapeake Bay (on sand of course) no basement, crawl space with all pipe underneath the house.

    The current furnace is 100,000 BTU and I am replacing it with the Carrier Infinity Gas furnace. Which should I get 100,000 or 80,000 BTU? The salesman told me either one would work.

    I have has for a load survey.

    What do you think!

    Thanks so much
    Becka

  5. #20
    DIY Junior Member Becka's Avatar
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    Quote Originally Posted by taylor46952 View Post
    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.
    Thanks for the response. The price will be the same whether or not I get the 80,000 or 100,000 BTU Carrier Gas Infinity furnance.
    Which BTU should I choose?

  6. #21
    In the trades Dana's Avatar
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    Quote Originally Posted by Becka View Post
    Thanks for the response. The price will be the same whether or not I get the 80,000 or 100,000 BTU Carrier Gas Infinity furnance.
    Which BTU should I choose?
    When in doubt go smaller. An 80K/80% is 64K out, which is literally twice what I need on a similar-sized house with even less attic insulation at -5F. Knowing the ratings of your existing furnace it's possible to use it's fuel use correlated against heating-degree-day data to come up with a fairly precise estimate (more precise than any heat-loss calc based on construction, since it's a measurement not an estimate.) Most heat loss calcs use "typical" air infiltraion and sloppy R-value/U-value estimates and pad it to protect against undersizing discomfort, which ends up spitting out a number 25-35% over reality, then the contractor goes the next size up from that "for good measure". Odds are your real design-day heat load is more like 40K than 64K, which would make the 100K/80% literally 2x oversized, and the 80K/80% unit 1.6x oversized, which is close to the oversizing assumption in an AFUE test, so it'll actually DELIVER 80% efficiency, whereas the 100K/80% unit will be delivering between 75-78% as-used efficiency. The 80K unit WILL be more efficient, and more comfortable, but not as comfortable (and far less effiient) as a 2-stage condensing furnace.

    Condensing furnaces just aren't all that expensive, and even without subsidy the payback is good. A 95% AFUE furnace, perfectly sized to the heat load will deliver slightly better than 95%. At 1.7x oversizing it'll use 18% less fuel than an 80% unit per year. If you're currently burning through 1500 therms/year with the 65% beast you'd burn 1220 therms/year with the 80K/80% furnace, but you'd burn ~1000 therms with a condensing furnace. At a buck a therm (it may be more, it may be less where you are) the annual difference in operating cost would be $220, or a 5 year cost difference (assuming no fuel price increases) of $1100. If you look at the difference in up front cost, and the after-tax return on that difference it's usually quite favorable compared to other places you might have invested the money. Don't forget to factor in any state/federal/utilility subsidy for going with the higher efficiency unit too.

    Bear in mind that if you lator add insulation to the attic or make other envelope upgrades your heat load goes down, which makes your oversizing factor go up, reducing net efficiency. But with a properly sized 2-stage condensing unit the efficiency extends much better than with a single stage unit, and you'll be taking fuller advantage of the insulation upgrades. (In fact some 2-stage condensers run slighly MORE efficiently when oversized 2x, since the higher stage burner runs at higher temp, reducing the condensing factor, but oversizing does result in reduced comfort.)

    But the short answer: Go with the 80KBTU unit, if those are your only choices.

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