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Thread: 60,000 vs 80,000 BTU furnace

  1. #1
    DIY Junior Member CurbYourEnthusiasm's Avatar
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    Default 60,000 vs 80,000 BTU furnace

    One contractor offered a 2 stage 80,000 gas furnace and when I asked him (the other quotations I have are for a 2 stage 60,000 gas furnace) why he said: "The furnace that we are going to install will run most of the time on low fire, which is around 50,000 BTU's. We recommend this furnace over a 60,000 BTU because the 60,000 will have to run a lot longer because it will be running on low fire which is around 35,000 BTU."

    I liked him a lot; he saw the chimney flue loose (and the house inspector had mentioned it in the report) and he fixed it right away (he just pushed it properly in place). I wonder whether the other contractors saw it. So I would have given him the contract even if a few hundred $ more expensive.

    But now, I do not know what to believe. Were all the other contractors wrong by offering a 60,000 furnace?
    Last edited by CurbYourEnthusiasm; 01-08-2013 at 03:41 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    WIthout doing an analysis of the houses energy needs, neither one may be the correct size! Under ideal conditions, on the coldest design day, the furnace would run constantly. This provides more comfort and efficiency along with longevity. You lose energy with each cold start, and cycles wear things out faster than duration of operations. Replacing a furnace with a similar sized unit is usually going to result in oversizing - oversizing affects comfort, purchase price, and cost of operation.

    Not all furnaces are mulitistage, and then it becomes even more important to size it correctly. Each time it starts up, you'll get a shot of cold air as the furnace heats up the ducts and heat exchanger, then when you shut it off, you lose most of that heat which is why running longer is better. Depending on the house, even 35K is likely more than needed. The only time extra capacity is somewhat useful is to rewarm the house after it has cooled off - then, it can recover quicker. Otherwise, excess capacity is just costing money.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Plumber jimbo's Avatar
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    We don't know what is existing....but if you have an older 80K BTU, 80%, that is 64K BTU. A new 60K BTU at 95%....we are in the same ballpark, and the staged burner makes it more efficient. Need to compare apples and oranges, with a dose of a manual J factored in!

  4. #4
    DIY Junior Member CurbYourEnthusiasm's Avatar
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    Quote Originally Posted by jimbo View Post
    We don't know what is existing....but if you have an older 80K BTU, 80%, that is 64K BTU.
    I have a very old oil furnace.

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    Nuclear Engineer nukeman's Avatar
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    Check the plate on the furnace. Somewhere it should say the BTU input and BTU output. That will give you an idea of the size that is needed. However, it is possible, or even likely, that even the old furnace is too big. A furnace that is too big will cost you more money now as well as more money later (less efficient than rated). In addition, larger furances will usually be louder as you are trying to move more air through the ducts.

    You want a contractor that will do a Manual J calculation (this is worth doing even if you have to pay for it). A Manual J will calculate an estimate of the heat load during one of the coldest days in your area. Once you have that info, the furnace could be sized based on that.

    Many HVAC guys will replace a furnace or AC unit with the size that is already there (or go bigger, because they figure that bigger is better). The orignal is often oversized to begin with as contractors are always worried about callbacks. If they put in a furnace that is too small, they get callbacks saying the house won't warm up and the contractor ends up having to come back to replace it. If it is too big, it just costs the customer money (now and in the future), but they don't get callbacks from the house being too cold.

  6. #6
    In the trades Dana's Avatar
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    Quote Originally Posted by jadnashua View Post
    WIthout doing an analysis of the houses energy needs, neither one may be the correct size! Under ideal conditions, on the coldest design day, the furnace would run constantly. This provides more comfort and efficiency along with longevity. You lose energy with each cold start, and cycles wear things out faster than duration of operations. Replacing a furnace with a similar sized unit is usually going to result in oversizing - oversizing affects comfort, purchase price, and cost of operation.

    Not all furnaces are mulitistage, and then it becomes even more important to size it correctly. Each time it starts up, you'll get a shot of cold air as the furnace heats up the ducts and heat exchanger, then when you shut it off, you lose most of that heat which is why running longer is better. Depending on the house, even 35K is likely more than needed. The only time extra capacity is somewhat useful is to rewarm the house after it has cooled off - then, it can recover quicker. Otherwise, excess capacity is just costing money.
    No, it would definitely NOT run constantly, even on the coldest day, even if the heat load at the 99% outside design temp was exactly matched to the furnace output at high fire. Only modulating furnaces with 3:1 (or more) turn down ratios would run constantly on design day, not two-stagers. The 99% design condition persists for but a handful of hours (at most) even on design-day, and by mid-afternoon on that day the heat load is usually under 70% of the design condition heat load. Two-stage furnaces typically run for a fixed amount of time at low fire when a call for heat is initiated before kicking up to high fire, and at high-fire the call for heat would be met it it would cylce off, even on design day. The burns may be long in the cool AM, maybe even for a couple of hours or more in the morning on design day if the temperature is set back overnight, but nowhere near constant.


    It takes a very large or leaky house to have a heat load of even 50KBTU/hr in a Vancouver climate. (99% outside design temp = 24F/-5C) A medium-sized ~2500' house built to current code minimums on windows and insulation would usually be under 35K in that climate. (My ~2400' + 1500' of semi-conditioned basement 1923 antique with known insulation gaps and antique double-hung single-panes+ storms has a heat load of ~35K at an outside design temp of -15C.) While there is no real efficiency downside to oversizing a hot air furnace by 3x (which is what you'd be doing if you went with an 80KBTU/hr unit), the wind-chill of the higher velocity air of an oversized furnace and more rapid cycling can cut into comfort.

    Running longer (and fewer) cycles is generally a GOOD thing, for both comfort and for wear & tear/maintenance on the equipment. If anything the 60K unit is likely to be ~2x oversized for the real load, and you should be looking at something in the ~45K range, but don't make that assumption without doing a heat load calculation.

    Even the smallest oil burners out there run ~60-70K, so the sizing of the prior unit won't give much of a clue. But with fuel use measured against weather data between the fill-up dates can put a hard upper limit on what the true heat load is. If your oil supplier stamps a "K-factor" on the billing that's effectively the same information. (I'm not sure if K-factors on billing is customary in Canada, but it is fairly common practice in the US.)

    When in doubt go smaller- 999 times out of 1000 that will be the right decision. Real design-condition heat loads are nowhere near what the installed base of furnaces put out, and the average binned-hourly mid-winter heat load in a Vancouver climate is about half the design condition load. If you should be so lucky to find a 2-stager with output EXACTLY your 99% heat load at high-fire you'll be better off avoiding deep setbacks (the recovery times could be pretty long during cold weather), but it would still only rarely have to kick into high-fire if you keep the thermostat at a fixed temp.

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    DIY Junior Member Failure2Comply's Avatar
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    The correct answer is what the "Heat Load" that they should have run on "YOUR HOUSE" computes the heating and cooling requirements to be. I am in favor of two stage 80+ furnaces, simple, yet efficient. But, you need to make sure it is sized properly so you achieve sufficient stack temperatures to prevent condensation.

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    In the trades Dana's Avatar
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    And in Vancouver's climate in nearly ALL houses under 5000 square feet the heat load will be way under even the 60,000BTU/hr number, and even below the low-fire output of a 60K furnace.

    I'm not sure anyone would favor 80K two-stage furnaces over 60K two-stage furnaces other than never having to run a heat load calc. Most existing houses in VA would not have actual heat loads as high as even the low-fire output of an 80K two-stager, and almost no NEW construction would need anything that big. The only advantage to that level of oversizing is rapid recovery from overnight setback, but none (or at least very few) actually need a burner that big to heat the house.

    Almost almost all newer/smaller ~82-84% units in a retrofit/replacement situation will need a correctly sized flue liner to deal with stack condensation, since older equipment was typically both lower efficiency (= higher stack temp and higher excess combustion air for higher dew point in the exhaust product) and had a larger oversizing factor. Intentionally oversizing a furnace just be able to use an oversized flue without the additional cost of the reducing liner isn't doing anybody any favors. (In MA installing non-condensing furnaces is no longer legal as of this year, rendering the stack condensation issue moot.)

    I personally wish there were more <40K 2-stage options, since those might actually match heat loads of newer-tighter code-min houses. We're currently under IRC 2009 here in MA, and most ~2500' houses built to code have heat loads @ 0F of less than 30K, with many hitting as low as 20K. But most of the market for heating equipment is in replacements, not new housing, so it'll be quite awhile before 30-40KBTU/hr becomes a sweet-spot in the market.

  9. #9
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Never buy a furnace, boiler or air conditioner from anyone without first seeing a Manual 'J' heat load analysis.

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