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Thread: Boiler Decision: Oil or Natural Gas? 80%? 90%? Condensing? Tankless?

  1. #16
    In the trades Dana's Avatar
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    When I first moved into my house (similar in size to yours) it had ZERO wall insulation, 6-7" of rock wool in the attic, it leaked like a sieve, and it's heat load at 0F outdoor temps was still under 50KBTU/hr.

    Given your comparatively modest outdoor design temps I'd be truly shocked if your design condition heat load was anything like 74KBTU/hour with the windows closed. If you put in a mod-con with a minimum firing rate of 35K, max of 110K it'll almost certainly be oversized, but if you enough thermal mass in the radiation it won't cut into efficiency much.

    Short cycling isn't a function of how often the boiler runs, but how long it runs for each burn. The on-time to off-time duty cycle will vary with your actual heat load, but to be very efficient the burns need to be long enough to not be blowing away heat in startup cycles, and the total duty cycle high enough to not be blowing away a lot of heat instantby. In a high mass boiler any burn less than about 5 minutes, and a duty cycle under 25% (15 minutes out of every 60) means it's not running at peak efficiency, but in a mod-con or low mass combi half that can still be fine. With modulating-condensing burners the ideal efficiency is when it runs nearly continuous burns at low fire. But if you're heat load at design condition is only 35-40K (likely) and your burner's lowest fire is 35K, that means that MOST of the heating season it will be running at less than 10% duty cycle. Going to a burner with min-mod of ~15K would be measurably more efficient in most systems.

    Doing all of that caulking and insulating and going with a "right sized" 82-85% non-modulating burner would likely lower your fuel use more than going with a condensing burner, for less money, at which point the cost-delta of going to a condensing burner won't be as cost effective. As a general rule on older homes like yours it's ALWAYS a better investment to tighten up and insulate the BUILDING to a high efficiency than going to ever higher efficiency on the heating systems. The first $2000 in air-sealing & insulation buys you a lotbigger reduction in fuel use (and increase in comfort) than the next $2000 in boiler efficiency, and that should be the priority.

  2. #17
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    The boiler pictures are of low-efficiency non-condensing boilers. The Burnham is a Series3 with outdoor reset, but still a hot stack.

    As stated; a proper heat load is first, then the condensing boiler with indirect (about three times the installed cost - and life - of a conventional tank-tip water heater - and the right guy to install it.

    http://www.badgerboilerservice.com/boilerchoice.html

  3. #18
    In the trades Dana's Avatar
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    True, the cast-iron-beasties are all hot-stack non-condensing but the heading of the title of the thread is still

    "Boiler Decision: Oil or Natural Gas? 80%? 90%? Condensing? Tankless?"

    Unless budgets are unbounded, there's room for splitting it between higher-efficiency envelope and high-efficiency boiler, and trying to figure a good/better/best investment strategy. Going with a right-sized 80-something efficiency boiler and spending the difference on dead-obvious heat leaks that can probably get the heat load down to within range of a smallest-in-class cast iron boiler seems like a better investment than a mod-con.

    The efficiency of an indirect for a very low-volume user is pretty miserable, and the rationale for the upfront cost delta between his already-installed (but unused) electric tank and an indirect isn't likely to be justifiable. He's been there for 18 years- a single guy either already in retirement or will be within the lifespan of the boiler replacement. Turning the valves and flipping the switch on the electric tank seems like the right thing to do, worry about the replacement later. It's not clear that an indirect is going to have lower operating costs for him than an electric tank at his volume of use, and even if somewhat lower cost it CLEARLY isn't going to pay for itself even in 25 years unless he marries somebody with kids. Getting off the miserable embedded-coil oil-fired inefficiency should have happened back when oil was $2/gallon, given the piss-poor efficiency outside of the heating season combined with the fact that the electric tank has been there all along and has no installation cost.

  4. #19
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Always with the ROI! No room for the modcon Zealot?

    The man that builds with stone, is a true philanthropist.

  5. #20
    In the trades Dana's Avatar
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    Quote Originally Posted by BadgerBoilerMN View Post
    Always with the ROI! No room for the modcon Zealot?

    The man that builds with stone, is a true philanthropist.
    I like mod-cons too, but the nature of the question seemed to be more akin to "what makes the most sense for me" (==ROI) rather than "what's the coolest and most-efficient solution for me".


    At least I didn't try to sell him on a mini-split, eh?

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