On Thu, 20 Dec 2007 21:24:00 -0800 (PST), wrote:

Also, it's very slow to change the
temperature in the house. In the morning, our house would warm up at
less than 1f per hour. This really makes a setback at night
impractical.

I may be mis reading what was in your great message, but, why not use
the furnace to warm the house in the morning, then shift to heat pump
once the house makes it to the desired temp, assuming the outside temp
permits.
Many, many years ago I bought this house which had hot-water baseboard
heat, no A/C. I added A/C. 4 years ago when replacing the A/C i
shifted to H.P. since the additional cost was minimal, and I'm a fan
of redundancy. I turn all heat off at night. If interior temp really
drops, I use the boiler to bring the heat up, then shift to the H.P.
I rationalize this (with no facts to back it up) that the short use of
the fossil fuel, is offset by the decreased use of electricity for the
longer period required to reach the same temp.
I do find that even with the installed variable speed air handler,
that the full bore H.P. operation required to warm things up in the
morning can be noisy. However, once there, and the same with the A/C
in the summer, I can't even tell it is running.