Propane Boiler Options?

Discussion in 'Boiler Forum' started by csm, Sep 7, 2020.

  1. csm

    csm New Member

    Sep 7, 2020
    Absolutely new to heating systems but have to replace my old oil fired boiler with a propane boiler. I've received several quotes from local companies, ranging from 8k to 22k, and not sure how to proceed. Home is approx. 1700 sq ft bungalow with three separate zones (thermostats). Last quote was for a Bosch Combi Pro 151, with the option of maintaining my electric dhw heater until it dies then turning a switch to utilize the combi for dhw. I've been reading some posts here and am starting to question the Bosch recommendation as too large for my home, and with concerns of short cycling. Any assistance would be much appreciated before I commit to this very expensive venture. Thank you in advance.
  2. JohnCT

    JohnCT Still learning..slowly

    May 16, 2018
    That unit is 151,000 BTU boiler. Pros on this site have told me that my similarly sized oil boiler is considerably oversized for my 2800 sq foot house, but I'm in CT USA not Ontario. Connecticut does get many sub zero (F) nights throughout January and February and many one or two week stretches in the teens.

    You should do a heat loss calculation or you can do a simple size calculation using something like this:

    That calculator doesn't have Canadian cities in the list, but there are a lot of cold U.S. areas similar to Ontario you can use.

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  4. fitter30

    fitter30 Active Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
    Have you been through a winter with the old boiler and how did it perform?
  5. Dana

    Dana In the trades

    Jan 14, 2009

    That online load calculator in that SupplyHouse link is utter crap, guaranteed to overestimate reality by 2x or more.

    If there is a heating history on the place, use last winter's oil bills and a nearby weather station history data to measure the heat load using the oil boiler (and it's nameplate efficiency numbers) as the measuring instrument. An explanation on how to do that lives here.

    If you don't have a heating history, run an online Manual-J (-ish) type load calculation using LoadCalc or CoolCalc, taking care to use the most AGGRESSIVE (not conservative) assumptions about air tightess & R-values etc that might be reasonable. (eg: If there it's 2x4 framing with fiberglass batts, select R13 rather than R11 unless you know with a high degree of certainty that they are R11s.) Being conservative on all the inputs will also overshoot reality by as much as 2x- it adds up. Even with aggressive assumptions it's common to see those tools (LoadCalc in particular) overshoot by as much as 30%.

    It's correct that at 151,000 BTU/hr boiler is RIDICULOUSLY oversized for the heat loads of 95% of the houses in North America, and it's minimum firing rate is high enough to pretty much guarantee short-cycling on zone calls for a 3 zone 1700' house. If there is double-pane glass (or storm windows over single-panes), some fluff in the attic and walls, and reasonably air tight it's unlikely that the design heat load at your 99% outside design temp is more than 45,000 BTU/hr, and if the foundation is insulated it's probably less than 30,000 BTU/hr.

    The minimum input of the Bosch Combi Pro 151 gas version is 36,000 BTU/hr, so at 95% efficiency it's minimum output is about 34,000 BTU/hr. (The propane version is similar). That may be oversized for the heat load of your entire house, and is almost certainly higher than the heat load of any individual zone. It's probably ludicrously oversized for the amount of heat emitted by the zone radiation of the smallest zone, a recipe for short-cycling.

    When selecting a modulating condensing boiler the minimum firing rate is more important than the maximum. When the water temps are in the cooler condensing range (50C or so) the radiation emits heat at a lower rate than when running typical ~80C temps. There is negligible thermal mass in wall hung boilers, and if the boiler can't modulate down to match the low-temp output of the zone regulation it will cycle on/off. Measure the radiation size zone by zone. If the smaller zones can't emit the minimum fire output of the boiler at condensing temps it's going to cycle on/off during continuous calls for heat, possibly at a high enough rate to degrade boiler efficiency and longevity. A primer on the napkin-math for that analysis lives here.
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