a couple of things that come to mind.. the ground temp should stay within a few degrees in a properly designed system.. so even during the winter.. should not get much below 50 degrees.. which is considerably warmer than outside air on a freezing day. so the amount of time the system should run is much reduced...

The other thing is that normally gshp are sized a lot more correctly than a lot of air source hp..

Do you know this to actually be true, or is this a guess?

Both GSHP and ASHP are classified as low temperature heating. They are more efficient than oil, gas or electrical resistance, but their duty cycle (the ratio of on-time to off-time) is higher than heating with fossil fuels.

My ASHP runs pretty much all the time, but it is extraordinarily efficient.

-AC

an air source heatpump has a thermostat that cuts the unit off at 37 degrees. beyond that as the temp drops.. the return of heat drops off dramatically.. at this point back up heat comes on.. which if you have electric strip.. you can watch your meter spin and spin and spin.. if you are fortunate to have dual fuel.. where the back up is gas.. you will come out far ahead of strip heating.. This is why gshp is so much more efficient.. some of the newer units are claiming over 27 SEER .. general rule is that one can suck enough heat out of the ground that it provides 4 times the BTU for the draw on the power to run the pump.

the systems have to be sized correctly.. often multiple smaller units.. which are closer to actual heat loads as figured by "j" loading schedule.

But as I am fond of saying.. Conservation is alway a better alternative to running the meter.. Insulation over 20 years is cheap cheap cheap.. and power bills will only be going up up up in Obama keeps up his carbon cr*p which is nothing more than a giant power and money grab enforced by the EPA

My little unit is good to below 17F... I have watched it. Some of the newer ones are good to 5F.

I believe this is referred to as COP (Coefficient Of Performance).

COP = (energy-out) divided by (energy-in)

My homemade unit had a COP of between 3 and 7, depending on the source temperture (where the heat was coming from) and the sink temperature (where the heat was going to).

Regarding energy issues, you might want to check out a web site called The Oil Drum

Very useful information about what's going on behind the scenes.

-AC

ASHP at low ave temps are pretty good
 

My Sanyo 24KHS72 units work down to about 8F before sucking down excessive KWs.

During the heating season, on the average day, both of the systems go into idle mode a lot.
Of course, I over-sized them. (Just in case I move to a really big house). ;)

Here in MA, north of Boston, the weather isn't very severe. Must be too close to the sea..
http://pics2.city-data.com/w1q/lhaq11355.png
http://pics2.city-data.com/w2q/humq11355.png

This heating season was so mild (so far), we never had to use back-up.
I had it set to come on a couple of times, but the low temps never
effected the indoor temp enough to hit the set-point.
When it was down around 4F, I was asleep, but I'll bet each Sanyo
was using at least 1kw for a few hours.

an air source heat pump using 410a may continue pulling heat but you will find that it is greatly diminished beyond 37 degrees. There is always latent heat in the air.. how much power it takes to gather it.. is another thing. manufacturers are always trying to push that envelope. With todays prices on NG falling.. to $2.4 or less per million btu it really does not make much sense to run the compressor..

There are 1000 Cubic Feet per MCF or about 1,020,000 BTU's

Do you have any numbers to back this up?

It is just a guess?

Have you actually logged the performance to come up with this statement?

-AC

It is even worse. your wondering about my comments.. I used to sell HVAC equipment including GSHP.. ( fhp) so I spent a lot of time reading manufacturers literature. Cannot sell it to the professional if you cant tell the pros and cons

Ashp they do play some serious games.. with tiny compressors that dont have much capacity (did you ever wonder why there are so many different compressors all rated the same? Then they put in huge condenser coils and huge evap coils. If one goes to big.. the ability to remove humidity from the air.. drops dramatically.. but you will have cold air. but the dry bulb will be well over 55%

How long ago was this?

-AC