Earlier this year I was wondering if a newly installed space heater was large enough to heat the main room of our uninsulated family cabin. I was given a figure of 45btu/sq ft to heat the place. Using this figure I needed 14,400btu to heat the 320 sq ft. I had 22,000 available. I tested it later during the winter and the place heated up slowly at first when the wind was blowing hard, but when the wind went down over night it heated up fast. I woke up to a room in the 70s.
So fast forward to today when driving home from work. I was thinking about this number and wondered how the numbers worked on my house. The house is just under 1600 square feet. 1600*45=72,000. Using the 45btu figure above a 72,000btu furnace should heat my house if it wasn't insulated. But it is insulated. Not the best, it's a 90 year old home with original windows. But I have R50 in the ceiling and under the siding is a bit of styrofoam. The windows have triple track storms.
The furnace is a 100,000 btu forced hot air. I understand the need to over size to a point since one really doesn't know the temp increase a furnace will have to cope with in the future, but is my furnace grossly oversized?
It was only one year old when we bought the house, so it's only about 5 years old now. If we are still in the house when it finally wears out, should we look at a smaller one? When looking at furnaces, is it better to size one that will fire and stay on for a long time like a wood or pellet stove or one that starts for 10-20 minutes and then shuts back off? Is the on/off expansion/contraction thing something to be avoided if possible? Is metal fatigue an issue in the heat exchanger? Would it be cheaper all the way around to do something like choose a 50,000 btu furnace and then supplement the heat with electric baseboard when there is a real cold snap?
This is all a hypothetical situation since I don't see myself changing my 5 year old furnace. With the stove going it's not used hard so it will probably last longer than most. But if it ever comes time to replace it or we move to a house in need of a new furnace...
Matt
So fast forward to today when driving home from work. I was thinking about this number and wondered how the numbers worked on my house. The house is just under 1600 square feet. 1600*45=72,000. Using the 45btu figure above a 72,000btu furnace should heat my house if it wasn't insulated. But it is insulated. Not the best, it's a 90 year old home with original windows. But I have R50 in the ceiling and under the siding is a bit of styrofoam. The windows have triple track storms.
The furnace is a 100,000 btu forced hot air. I understand the need to over size to a point since one really doesn't know the temp increase a furnace will have to cope with in the future, but is my furnace grossly oversized?
It was only one year old when we bought the house, so it's only about 5 years old now. If we are still in the house when it finally wears out, should we look at a smaller one? When looking at furnaces, is it better to size one that will fire and stay on for a long time like a wood or pellet stove or one that starts for 10-20 minutes and then shuts back off? Is the on/off expansion/contraction thing something to be avoided if possible? Is metal fatigue an issue in the heat exchanger? Would it be cheaper all the way around to do something like choose a 50,000 btu furnace and then supplement the heat with electric baseboard when there is a real cold snap?
This is all a hypothetical situation since I don't see myself changing my 5 year old furnace. With the stove going it's not used hard so it will probably last longer than most. But if it ever comes time to replace it or we move to a house in need of a new furnace...
Matt
