On Wed, 2 Jul 2008 20:05:45 -0700 (PDT), "
wrote Re Heat pump in basement?:

I am assuming that the replenishment of heat comes from the basement
walls, they are concrete block with no insulation.

The replenishment heat from the walls won't be able to conduct from
the earth outside the walls fast enough to keep up with the heat pump.
The "heat" flows like this:

Earth (1) == Basement Walls (2) == Basement Air (3) == Heat Pump
(4)

Heat flow from (1) to (3) will be much slower than from (3) to (4).
The only way to compensate for that would be for the volume of (3) to
be much larger than the volume you are heating.