calhome
09-17-2005, 06:05 PM
Am replacing a old (1957) Holland oil furnace in an old (105 years) house to improve efficiency. The old furnace has a 1gph nozzle and is presumably running at an input of 140kBTU (there is no documentation). Given the age of the furnace,I can’t imagine that it’s putting out more than 85kBTU - before duct loss. It’s oversized -- not extremely so, but I wouldn’t want greater output for reasons of
comfort and economy. Quandry is, 2 contractors have recommended a new furnace size of 119 input/97output. Frankly, I’d rather be slightly undersized than oversized,
and most sizing literature I read points to oversizing being a predominate error. A few mitigating factors: (1) this is mild climate coastal valley in California, winter days in 50’s w/ little exception and coldest clear winter night can touch down to 25 degrees, (2) there are 7 total registers downstairs and none upstairs, just a large 12
in square at the foot of the stairs (house is 1,200 sq. ft. down and 1,000 sq. ft up); we don’t really want to heat the whole upstairs to full temp, (3) will be replacing all
the old leaky ductwork with new insulated ducts. Seems that output of 85kBTU would be safe, given old furnace evidence and 74kBTU would be plausible what with new ductwork and the old furnace seeming oversized. Am I crazy to be questioning the 97kBTU output size? That seems a big unit by any standards.
comfort and economy. Quandry is, 2 contractors have recommended a new furnace size of 119 input/97output. Frankly, I’d rather be slightly undersized than oversized,
and most sizing literature I read points to oversizing being a predominate error. A few mitigating factors: (1) this is mild climate coastal valley in California, winter days in 50’s w/ little exception and coldest clear winter night can touch down to 25 degrees, (2) there are 7 total registers downstairs and none upstairs, just a large 12
in square at the foot of the stairs (house is 1,200 sq. ft. down and 1,000 sq. ft up); we don’t really want to heat the whole upstairs to full temp, (3) will be replacing all
the old leaky ductwork with new insulated ducts. Seems that output of 85kBTU would be safe, given old furnace evidence and 74kBTU would be plausible what with new ductwork and the old furnace seeming oversized. Am I crazy to be questioning the 97kBTU output size? That seems a big unit by any standards.