Here is why I was asking, bare with me the post will be a little long however it makes for an interesting debate.
Heat-loss calculated to 56,300 Btu at -14.8F (Montreal)
Heat-gain calculated to 24284Btu at 86F (it's actually 19427 true gain they then add 25% for a HP)
I am on dual rate electricity where I pay a low rate down to 10F and a high rate below that. My furnace is propane, my electric company puts in relay that switches my heat pump to my furnace at and below 10F.
Low rate = 4.33 cents kw/h
High rate = 18.14 cents kw/h
Propane = 67 cents per litre or 9.44 cents per Kw/h
Units that will be purchased are Carrier Infinity 25VNA (greenspeed) HP which can go down to 40% of max output and a 58MVC that goes down to also approx. 40% of max output (this will be imporatant further down)
It is unlikely that if my heat-loss is 56,300Btu at -14.8F that it would be as little as 24284Btu at 10F when I switch over to propane. Therefore somewhere between say 32F and 10F my HP would need the furnace to help.
At 17F my COP is 2.46, my furnace is 96.6% efficient so my rates are:
electricity = 1.76 cents per Kw/h
Propane = 9.77 cents per Kw/h
You can see why I would like my HP to be able to handle the full load down to 10F. It makes no sense to spend all this money on ultra efficient equipment to throw it out the window because I use aux heat earlier than needed.
The engineering company can't tell me what my heat-loss is at 10F for some reason however I believe it to be more than the 2T HP. Since the equipment I am buying is variable speed multistage I think it makes sense to move up to the 3T HP since it can go down to 40% of 36000 Btu or so for summer AC. The extra Ton should produce some good cost savings in winter. Keep in mind also that we only AC for about 2 weeks in summer. It would be nice to know the actual heat-loss at 10F however I am not sure I would be comfortable oversizing my HP by more than 1T if it were more than 36KBtu anyway.
Comments / advice??