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am i misguided in thinking proposed boiler is too big?

ewh Member Posts: 6
I recently bought my first house, a foreclosed 1960's single story 980 sq ft ranch with full basement in maine. the oil boiler is a 20 year old thermo-dynamics s-100 which was submerged in a flood. after that there was a freeze up. the oil tank needs replacing as it was installed touching the concrete wall and rusted out. also needs chimney liner. the only thing good is the oil line from the tank to the boiler.
I got only one viable quote so far. it was for a Biasi b4, which has a 84,000 btu/h output, and an electric hot water heater. I don't think the guy who did the estimate did a heat loss calculation, but he did install the old boiler, so he may have had something on file or something. He also didn't ask about renovation plans (which include a lot of insulation and installing a 20,000 btu woodstove in the 500 sq ft open kitchen, living, eating area) but we did discuss switching to 2 zones, one for bedroom, bath and office, one for living area.
my concern is that when i do a heat loss calculation (i have done 5 or so different ones now, from simple rule of thumb to really detailed ones) i come up with a range from 31,000 to 45,000 btu as existing, and 22,000 -35,000 btus post renovation.
also, i only have 71 ft of baseboard in the house, and i think that is only good for 40,000 btu or so.

so my questions are:
1 - Do my numbers seem the least bit reasonable? Do i have good reason to be concerned?
if so, it seems like oil boilers don't come with outputs that low (the Biasi B-3, however, is smaller than what was specified in the quote) so:
2 - Would a more appropriately sized propane boiler make up for higher cost per btu vs the low efficiency of an oversized oil burner?
3 - How do you think about wood stoves when sizing heating systems? If i need 35,000 btu on the coldest day of the year and the stove puts out 20,000, does that mean i only (when the stove is burning) need a 15,000 btu boiler, making the proposed 84,000 btu one more than 5x too big?

Thanks for taking time to share you expertise. I have learned a lot (and probably just enough to be annoying) from reading the forum for the last few days.


  • JohnNY
    JohnNY Member Posts: 2,761
    I would get the smaller boiler if I were you, but if I were me, which I am, then I'd get a modulating condensing gas/propane boiler instead and let the boiler worry about how many BTUs the house actually needs.
    Contact John "JohnNY" Cataneo, Master Plumber
    in New York
    in New Jersey
    for Consulting Work
    or take his class.
  • Jamie Hall
    Jamie Hall Member Posts: 17,111
    My only comment in addition to @Hatterasguy 's is this: he said "no comment" on wood stoves. I'll comment. Don't depend on that wood stove. It's not the work. It's that if you aren't there, or if for some reason (a touch of the 'flu, for instance) you can't fire the darn thing, then you are out of luck. Go with the B-3 and use the wood stove when you can -- and want to.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Ironman
    Ironman Member Posts: 6,579
    The B4 is definitely too large. What's the sense in having a boiler that has far more output than what the radiators can deliver?

    I'll add something that most folks don't consider when choosing oil vs. propane: the cost of maintenance. Oil requires a lot more than propane. At a minimum, the nozzle and filter need to be changed yearly and any soot needs to removed. 1/16th of an inch of soot can cause an 18% reduction in efficiency and raise the stack temp 100*. Most Americans don't properly maintain heating equipment. The attitude is that if it's running, it doesn't need servicing. That will cost far more in fuel than proper maintenance, but either way, it's the hidden cost of oil that people don't consider when making a decision.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • ewh
    ewh Member Posts: 6
    thanks everyone. this is helpful. any other thoughts on oil vs propane would be helpful. being that i am starting with a clean slate i feel slightly hung up on the decision. i need to get propane anyway, for the kitchen range. also, there is a very slight chance of getting natural gas in my neighborhood in the nest 5-10 years.

    thanks again
  • Brewbeer
    Brewbeer Member Posts: 616
    Some folks like oil, some like gas, it really depends on your circumstances. If you need a lot of BTUs, oil's a higher BTU per dollar ratio may point you in that direction. If you don't need a lot of BTUs, propane's greater flexibility might be a better option.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread:
    System Photo:
  • rick in Alaska
    rick in Alaska Member Posts: 1,243
    Propane is extremely expensive up here, but if it is not where you are, and you might be getting gas in the next few years, then it might be your best option. Assuming it can be converted in the future, which I believe they can. Also you would not have to deal with the fuel tank.
    I would look closer at your heat loss numbers again though. It seems high to me for a 980 square foot house, but I don't know your insulation values. I am heating 2400 feet with if I remember accurately, 32000 btu's. But, I am also well insulated.
    But, get the smallest unit you can.
  • gschallert
    gschallert Member Posts: 170
    @ewh, if I was in your shoes I'd go with a propane solution since you mention you're getting it anyway for cooking. I think it will give you more options for CH & DHW and very well might be convertible to NG should it become available.

    My in-laws just converted from oil to NG and combined their CH with DHW and it's working out great for them. They replaced an oil fired CI boiler and electric HW tank with a combi mod-con boiler. Sounds like with the wood stove as secondary heat that might be an option to explore for you.

    I have LP and just replaced my boiler with one that can be converted to NG if necessary.