Boiler big enough?

Discussion in 'Boiler Forum' started by ArcticCircle, Jan 13, 2010.

  1. ArcticCircle

    ArcticCircle New Member

    Messages:
    4
    Location:
    Anchorage AK
    Is a 80K BTU Lochinvar Knight a big enough boiler to heat an 1800 sq ft, 2x4 framed house? Average winter temps are about 10F

    This is with all new windows and doors, properly insulated, and 100 cumulative feet of high out put baseboard.

    Any thoughts or ideas? Any experience with the Lochinvar products? Reliability? Thanks!
  2. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    Lochnivar is good stuff. should be ok but without a heat loss who knows.
  3. Dana

    Dana In the trades

    Messages:
    2,919
    Location:
    01609
    What he said- unless you've measured or calculated the heat loss of the structure it's impossible to say for sure if it'll keep up. If the glazed area isn't outrageous, the foundation is insulated, and there's something better than cheez-whiz & R-11 batts in the 2x4 cavities, odds are your design-day heat load isn't over 50K on a house that size in Anchorage. But I'm sure there are exceptions to prove the rule...

    "properly insulated" isn't a specification, and 2x4" construction is a bit on the thin side for that climate unless there's extensive use of insulating foam sheathing or the cavities are filled with 2lb density closed-cell foam, etc. But if it's tight and not over-glazed, even with R13 batts or similar & no insulating sheathing it could still be well under the ~70-75K full-fire output of the thing at an outdoor design temp of -15F or whatever you typically need. (Boilers need to be sized for the peak heat load- the average outdoor temp of 10F means little when it's 25-30 degrees cooler than that and you want to stay cozy.)

    If you need the full 75K, and the radiation needs to run at 180F to get that much into the space it could be marginal, since the boiler's output will be ~65K with 160F+ return water from radiation- it'll be well out of the condensing region and running at best ~85% efficiency. "high output" baseboard is also not a spec- no telling what that really means relative to your heat load and operating temperatures. Let's say it's specified at 750BTUs/foot at 180F- your 100' of baseboard still won't deliver that 75K because it'll never achieve that for an average temp if driven by a boiler that's maxing out at ~65K at those temps. But it can easily deliver ~50K to the room at 150-ish temps, and the boiler would clearly keep up, and run a higher (if still not condensing) efficiency. What your actual peak temps & BTU-delivery can be with the radiation & boiler you have requires a more detailed analysis, well beyond what can/should be done in web-forum.

    But it all starts with calculation of what the real heat load is. From that and the baseboard specs you can determine what water temps the radiation will need to support that heat load, then you have to verify that the boiler can supply the requisite BTUs at that temp. You're probably good, but...
  4. ArcticCircle

    ArcticCircle New Member

    Messages:
    4
    Location:
    Anchorage AK
    So are you saying I should go to the 105K BTU unit just to be on the safe side? Its like an extra 400 bucks.
    Last edited: Jan 14, 2010
  5. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
    What they are saying is that someone needs to do a heat loss calculation for your house in order to know what size you need. This depends on the location of your house, number/size of windows, amount of insulation, which direction your house faces, size of house, etc. With this calculation, then you KNOW what size you need. Just guessing doesn't work well. If it is too small, it won't keep up in the cold. Too large will cost you more upfront and likely more in operating costs.
  6. Dana

    Dana In the trades

    Messages:
    2,919
    Location:
    01609
    Not at all- if it turns out your actual peak heat load is only 40K going bigger will be measurably less efficient. The practice of oversizing of boilers "just to be on the safe side" is responsible for a HUGE amount of wasted fuel. (You would pay the 400 shekels more up front for the too-large boiler, then 400 more every year in fuel until the thing croaks in 10000 heating-degree-day climates like yours.)

    Do a heat loss calculation on the house! (ACCA Manual-J or similar.)
  7. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    It is already winter and this is an existing home so you should be able to monitor daily fuel usage to determine what your actual heat loss is for a given average daily temp. While an estimation technique would be nice, this should be better than an estimate since you can apply the unit's efficiency to come up with a much more accurate empirical load at a given temp. Then you adjust for the delta T of the design temp vs. the set point to determine the design load.

    I'm doing this myself for a furnace replacement. Our January temps average about 13-14 F higher than yours (although this year you would feel right at home...we have been 15-20 F below normal until the past two days and I literally wore out my snow shovel.) My zero F cross point is 41,400 Btu/hr output so far (81% efficiency rating on the unit.) 2800+ sq. ft.
  8. Dana

    Dana In the trades

    Messages:
    2,919
    Location:
    01609
    40K is about where my house is at about -10F (full-dimension 2x4 construction with dense-packed cellulose insulation, ~2000sf above grade, R20 on the foundation of an unfinished basement with nothing under the slab but ~50F dirt. Tighter than a wreck, but by no means ultra-tight. This makes me think that unless the design temp for Anchorage is -50F and it's a lot leakier or less insulated than my place, an 80K boiler will be enough, even if he can only get at most 60K out of it in combination with the radiation.

    If your existing burner is 2x oversized, don't use it's 81% AFUE as it's efficiency when making the calc- it needs to be derated by several percent when using monthly or annual fuel use against the average temp as a reference. If it's 4x oversized, derate it;s efficiency by at least 10% to find the true heat-load.
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