Loyal residential customer of HVAC pros here!
When I had my 100,000 BTU 80% oil furnace and 10 SEER, 3-ton scroll split system AC installed in my 1927 3-BR two-story w/ finsihed basement farmhouse 9 years ago, life was good. Now it's not! If I keep this system as-is, I'll spend $18,250 on fuel oil over the next 9 years buying an average of 450 gals of oil per year at $4.50/gal. With electricity at $0.077/kWh where I live, I figure oil would have to be about $2.56/gal to be competitive with pure electric resistance heating. I know I can do even better with the BTU multiplying power of a heat pump to handle most of the work on warmer winter days. Assuming a 50% rise in electric rates over the next 9 years, I figure about $8000 total cost of electric power for heating over the same period, even assuming 100 annual hours of 20 kW resistance heating (7300 kWh heat pump + 2000 kWh resistance per season). I don't know if those figures are reasonable but it's the best I could manage as an estimate. Even if I'm low by 50% it's still $6000 less than the cost of oil heat.
What I want to do is get the oil tank and furnace out of my basement and have an all-electric 15 SEER packaged heat pump installed on the ground outside. I can convert the newly available interior floor space into a small home office as a bonus.
My HVAC contractor told me that heat pumps are sized off the summer heat gain, not the winter heat loss. Is that correct? I was wondering if I could go with a higher tonnage 2-speed scroll unit to get more heating capacity in winter, and rely on the low speed compressor setting to match the lower tonnage cooling requirements in summer. My contractor said the low speed isn't intended to work like that. So, do I take his advice and go with the same tonnage as my current 3-ton AC (which seems to be just right)?
My other concern is how to ensure that the backup electric coils are sized big enough to satisfy the thermostat on subzero days when the heat pump COP is 1.0, but not so big that they cycle constantly when the heat pump output starts to fall (unless that would be normal?). In my area (Cincinnati) there are about 5000 degree days of heating per season. A cold month like Feb 2007 had about 1200 heating degree days. My oil furnace has a 0.85 GPH nozzle so the 100,000 BTU @ 0.90 GPH is actually more like 94,000 BTU @ 0.85 GPH. I wish I had thought to measure its duty cycle on a very cold day but I never have. I think it ran pretty steadily when the outside temp hit -20F.
I'm also prepared to live with the lower heat rise of the heat pump and electric coils compared to oil. My fan thermostat is already set back from the original 135F to 90F to get BTUs into the house faster with fewer going up the flue pipe, so we're accustomed to "cold' air blowing out of the registers.