John Molyneux
Member
In an earlier thread you guys helped me get my head around how to arm myself with enough good data to argue against what seems to be a common inclination for heating contractors to want to install oversized equipment. I just wanted to follow up for the benefit of others to reinforce that using actual energy use versus seasonal heating degree days seems to be a pretty good way of getting to a heat loss number that is at least accurate enough for a decision about whether you need a 50-, 80-, or 100,000 btu/hr boiler.
Our 1400 sq/ft New England cape is currently heated with a 125,000 (DOE) btu/hr hot water boiler feeding cast iron radiators. Existing radiator capacity is approx. 80,000 btu/hr at 180F AWT. The boiler's obviously way too big based on how much it cycles.
Dana's initial estimate based on my actual fuel use per heating-degree-day this past winter was 25,000 btu/hr. We adjusted this up to 32,000 when he found out I had kept the house in the low- to mid-60's during the measurement period. I subsequently re-ran the calc with a range of assumptions and got very comfortable that the btu/hdd formula put me in the 30,000 btu/hr range. Then I ran the SlantFin software (which I'm led to believe tends to overstate), using some very conservative assumptions, and came up with 36,000, just to give myself an upper bookend.
I just got the results of an independent energy audit and their professional opinion is .... 29,000 btu/hr. (They think they can drive that down to 20,000 btu/hr with a variety of air sealing and insulation, much of which won't cost me anything thanks to the existing state and utility rebate programs here in Maine.)
So at my implied heating loss of 450 btu/degree-hour, I only needed 27,000 btu/hr on our coldest winter day this year, and only needed around 20,000 btu/hr on average over the period December-March. Armed with that info, I can now have an informed discussion with my heating contractor why he was recommending a Solo 110 that can only modulate down to 30,000 btu/hr rather than the Solo 60 that modulates from 16,000 up to 60,000!
So thanks to all.
Our 1400 sq/ft New England cape is currently heated with a 125,000 (DOE) btu/hr hot water boiler feeding cast iron radiators. Existing radiator capacity is approx. 80,000 btu/hr at 180F AWT. The boiler's obviously way too big based on how much it cycles.
Dana's initial estimate based on my actual fuel use per heating-degree-day this past winter was 25,000 btu/hr. We adjusted this up to 32,000 when he found out I had kept the house in the low- to mid-60's during the measurement period. I subsequently re-ran the calc with a range of assumptions and got very comfortable that the btu/hdd formula put me in the 30,000 btu/hr range. Then I ran the SlantFin software (which I'm led to believe tends to overstate), using some very conservative assumptions, and came up with 36,000, just to give myself an upper bookend.
I just got the results of an independent energy audit and their professional opinion is .... 29,000 btu/hr. (They think they can drive that down to 20,000 btu/hr with a variety of air sealing and insulation, much of which won't cost me anything thanks to the existing state and utility rebate programs here in Maine.)
So at my implied heating loss of 450 btu/degree-hour, I only needed 27,000 btu/hr on our coldest winter day this year, and only needed around 20,000 btu/hr on average over the period December-March. Armed with that info, I can now have an informed discussion with my heating contractor why he was recommending a Solo 110 that can only modulate down to 30,000 btu/hr rather than the Solo 60 that modulates from 16,000 up to 60,000!
So thanks to all.
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