I have a Hydroheat heat pump. It is 69,000 btu's of heat. This handles heating my house for 90% of the time. A high efficiency propane furnace (110,000 btu's) supplements the hydro on the very coldest days. This is about the most efficient sizing of the heat pump and aux heat. The propane furnace sits ontop of the heat pump and the fan is in the propane furnace. I use a AprilAire air filter unit and two Homeseer controlled humidifiers on the furnace (1 wire humidity sensing) in the winter.
Quick overview of heat pumps:
In my system, my fresh water well (3/4 hp pump w/ 80 gallon expansion tank) pumps 43 degree ground water (Michigan) to the heat pump. Inside the heat pump is a 5 ton compressor - similar to what you would find in a large A/C outside unit. Basically, the heat pump works like A/C in reverse: the BTUs from the water are extracted into the freon which when the freon is passed into a low pressure zone, gives off the BTUs into the forced air stream, heating the house. The furnace is adjusted to drop the water to 32.5 degrees (just above freezing) which is then pumped out underground and deposited in my lake. The fish love the cold water coming out in the summer!
All the in and out plumbing is monitored by Homeseer using 1 wire temp devices. If the water gets too cold (freeze alert), Homeseer shuts down the heat plant. I have mechanical thermostats running in parallel as safety devices if the auto system goes haywire.
On some heat pumps, you can "run them in reverse" and air condition the house in the summer. For mine, I chose the "domestic hot water" option instead. This is designed to cycle the hot water through the heat pump continually circulating it until it warms the hot water as the domestic hot water supply. I plan on using this second circuit to heat water for a sauna instead. Sauna heat pumps are about $2,000 or so by themselves.
When I purchased my heat pump system, Detroit Edison was running a trial program in which they gave $'s back for trying heat pump instead of gas heaters. For my case I got back a check of $5,000 as I qualified for all awards. The qualifications were based on building the house with new energy saving methods.
Methods used in my house include: 9" foam form concrete filled basement walls, cellulose insulation in 2x6 outside walls and foam filled 2x4 walls, 12" cellulose filled ceiling (R40 from memory), electric fans for air distribution, heat exchanger for replacing stale inside air with fresh outside air while passing heat from the stale air to the fresh air, control system (Homeseer) that times peak usage times and vacant times to drop heat, Argon filled tinted windows, Tyvek house wrap, 2" foam under concrete floor, radiant floor heat in concrete, foam filled wall blocking, and on and on.... ps - the house is pretty energy efficient.
All these energy saving methods really add up. The 69,000 BTU heat pump heats a 6,500 sq ft house that the main floor has 37 ft high ceilings - lots of cubic ft to heat. There is also 65 windows with lots of sq footage of glass (not good for energy, but look great). The heat ducts are insulated to keep heat lose to a minimum. With conventional build methods, I would have had to have multiple propane furnaces.
Back to the Hydroheat - has broken down quite a few times and the company support and Michigan rep are pathetic (mostly the rep co.). If I was to do it again, I would look into Water Furnace brand. My recollection is that the heat pump unit was around $4,500 uninstalled.
In the summer, the cold well water is switched by Homeseer to a coil mounted in the top of the propane furnace which chills the summer air. The warmed well water is then sent to the lake. For really hot days, I have a more conventional 5 ton A/C unit that kicks on.
The heat pump also qualifies the house for a second electric utility meter which is priced per killowat much less than the household electricity (Michigan: .02336 off peak and .06276 on peak vs. .08286 residential off and .09696 residential on peak). Only the heating / domestic hot water heating is connected to this seperate power panel.
Dec electric bill: $210 for air heat / water heat
Jan electric bill: $140 for air heat / water heat
Feb electric bill: $105 for air heat / water heat
Running on propane alone would be $400 / month plus in the winter. I had to run the house the first winter on propane until the hydro setup was finished.
This winter I used only 200 gallons of propane (1/2 tank ~$225). I might have to pay a lease penalty on the tank since I didn't use a full tank
EDIT: Oh, and I forgot that the first year there was a federal income energy savings tax credit that I was able to deduct the system price. Not sure if this is still around or not.