Where are you located, climate? On grid or off grid? In a mild climate like mine,(western Washington state,USA) where the average January temp is 36F it seems the more economic route is to give up a small amount of efficiency for the lower cost of an air source heat pump. I'm heating my 1000sf underground house with a single speed 9000 btu ductless heat pump. I'm looking at upgrading to a variable speed unit like the Sanyo 09khs71 that can work down to 250 watts to better match the amount of power available from my offgrid power system. I use the voltage controlled relay in my Outback inverter to turn the heat pump on when the batteries are full and turn it off when the voltage goes lower(28 to 24.3). Setting the power used by the heat pump to a lower output would allow the elimination of the continuous off and on. And the heatpump can be up to 50% more efficient at the slower speed and partial output. There is a discussion of the use of this on the wind-sun.com forum. There the use was as a small efficient daytime air conditioning on several solar powered offgrid houses. They also used the Mitsubishi heat pump which runs on 240 volts.
I talked with a neighbor on the grid who had a new house built. He had a single compressor to 3 or 4 air handling units to heat various rooms in his house. That eliminates the need for ducts and the losses associated with that. It takes less power to move heat with the higher density of a liquid instead of moving air with blowers.
Water source heat pumps make a lot of sense in colder climates where the ground is much warmer than the air in winter. Coupling that to a radiant heated floor keeps the temperature difference low and the efficiency high. I've just found that the costs are higher and it has been harder to find a small system to fit the lower heating needs of a small well insulated home and can be powered by the surplus power of an offgrid system. If the compressor speed can be varied then there is an advantage to a slightly oversized system as it may be more efficient at lower speeds. but the cost may be higher.
I checked out a solar system yesterday where the owner came home and the batteries were empty and the inverter shut down. The house is still being built and a heating contractor had just powered up the high efficiency gas fired boiler for the radiant floor heating system. Sounds great and economic. But as soon as I restarted the inverter and turned on the breakers I found a 300 watt load. I tracked it down to the boiler. I could hear a noise inside the boiler. I'm not sure what it was. But the continuous 300 watt load, 24 hours a day for the 4 days that the owner was gone was enough to drain the 16 L-16 battery bank.
I'm not sure if it was a defect, just this boiler, installed wrong, or what, but this is a warning to be aware electric power drains from supposedly efficient heating systems. Heating contractors may not even be aware of this problem.
Chris
