MarkP1
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Hi,
Wow... so much great information, hard to know where to start thanking people! So after reading more threads than I can count, I think I have done my homework on sizing for a new boiler / water heater.
House is a 1971 built home, in Topsfield MA. 2500sq ft, 2x4 walls (r-11 insulation), double pane windows, R-35 attic (16" blown in plus spray sealed all holes / gaps). There are 4 zones, all standard baseboard heaters (not cast iron). In the winter we keep the house at 69 during the day (little kids) and 62 at night.
I took my natural gas usage from National Grid, and did a daily linear regression analysis outlined on degreedays.net. This resulted in a base temperature finding of 66F, with an R2 of
0.996, so I am fairly confident in the correlation. The coldest period I have data for is 12/31/13 to 1/29/14. 30 days, the home used 285 therms, with 1228 heating day degrees (HDD) for that same period. We use NG for our water heater and stove, which I expect to be the same year-round. During the summer months (when we turn the boiler off completely), we average 20 therms used for 30 days. This leaves me with 265 therms used to heat the house to 66F on 1228 HDD's.
Dividing 265 therms into 1228 HDD's gives me 0.21065 therms per degree day, or 21065 BTU's per degree day. Per hour, this is 878 BTU's per degree hour.
The current boiler is a standard Burnham cast iron boiler, made in 1983. When it came out, the AFUE was 84.6% (164K BTU input, 138K BTU output, 120K BTU net). So assuming I've actually *gained* a bit of efficiency (calling it 85%), that means the boiler put into my home (878 * 0.85) 746 BTU's per degree hour. The 99% design temp is 5F. So 66F - 5F = 61, times 746BTU's per degree hour, gives me 45,510 BTU's. (if I go with 75% which is more likely, it drops to 40k BTUs).
If I understand this all correctly (and did the math above right), that means I need a boiler that will deliver an IBR net output rating of at least 45,510 BTU's.
I am less interested in a super high efficiency boiler than a longer lived / cast iron model, so I am looking at the Weil McLain GV90+ series. The GV90+3 (91.9% efficient) has a BTU input of 70k, DOE of 65k, and IBR net of 56K. I will add to this an 80 gallon indirect water heater (3 showers, large bathtub, and laundry runs all days with my kids...), on a priority zone. The next model bigger is the GV90+4, which is (91.2% efficient) has a BTU input of 105k, DOE of 97k, and IBR net of 84K.
It would seem that the GV90+3 is more appropriately sized (1.25X coldest thermal demand) than the GV90+4 (1.87X coldest thermal demand).
Am I on the right track? Is there something I am missing? Any advice is greatly appreciated.
All the best,
Mark
Wow... so much great information, hard to know where to start thanking people! So after reading more threads than I can count, I think I have done my homework on sizing for a new boiler / water heater.
House is a 1971 built home, in Topsfield MA. 2500sq ft, 2x4 walls (r-11 insulation), double pane windows, R-35 attic (16" blown in plus spray sealed all holes / gaps). There are 4 zones, all standard baseboard heaters (not cast iron). In the winter we keep the house at 69 during the day (little kids) and 62 at night.
I took my natural gas usage from National Grid, and did a daily linear regression analysis outlined on degreedays.net. This resulted in a base temperature finding of 66F, with an R2 of
0.996, so I am fairly confident in the correlation. The coldest period I have data for is 12/31/13 to 1/29/14. 30 days, the home used 285 therms, with 1228 heating day degrees (HDD) for that same period. We use NG for our water heater and stove, which I expect to be the same year-round. During the summer months (when we turn the boiler off completely), we average 20 therms used for 30 days. This leaves me with 265 therms used to heat the house to 66F on 1228 HDD's.
Dividing 265 therms into 1228 HDD's gives me 0.21065 therms per degree day, or 21065 BTU's per degree day. Per hour, this is 878 BTU's per degree hour.
The current boiler is a standard Burnham cast iron boiler, made in 1983. When it came out, the AFUE was 84.6% (164K BTU input, 138K BTU output, 120K BTU net). So assuming I've actually *gained* a bit of efficiency (calling it 85%), that means the boiler put into my home (878 * 0.85) 746 BTU's per degree hour. The 99% design temp is 5F. So 66F - 5F = 61, times 746BTU's per degree hour, gives me 45,510 BTU's. (if I go with 75% which is more likely, it drops to 40k BTUs).
If I understand this all correctly (and did the math above right), that means I need a boiler that will deliver an IBR net output rating of at least 45,510 BTU's.
I am less interested in a super high efficiency boiler than a longer lived / cast iron model, so I am looking at the Weil McLain GV90+ series. The GV90+3 (91.9% efficient) has a BTU input of 70k, DOE of 65k, and IBR net of 56K. I will add to this an 80 gallon indirect water heater (3 showers, large bathtub, and laundry runs all days with my kids...), on a priority zone. The next model bigger is the GV90+4, which is (91.2% efficient) has a BTU input of 105k, DOE of 97k, and IBR net of 84K.
It would seem that the GV90+3 is more appropriately sized (1.25X coldest thermal demand) than the GV90+4 (1.87X coldest thermal demand).
Am I on the right track? Is there something I am missing? Any advice is greatly appreciated.
All the best,
Mark