[Xringer]

If ask yourself, where does warm air come from at midnight? (to feed the ASHP)
It's likely warmth from leftover solar warming during the day, (stored energy)
much of it being released from the pre-warmed ground, rocks & man-made junk.

And, we all know what's sitting a few feet below the grass,
it's geothermal (80% nuclear) heat, just trickling up to the surface.

I'll bet that earth tubes would allow an ASHP to work better at low temperatures.
So, on those days when it was 4.2deg F outdoors, a source of warmed up
air coming into the ODU coil, could really help efficiency.

Around here, where the lowest temperatures normally occur just before sun up,
that little boost of warm air might keep the back-up from coming on..

Screens can keep out the squirrels and a little mold wouldn't do much harm.
If the tube was built right, (all down-hill) it could be flushed out with water.
Or, you could leave a line in it, to drag a cleaning brush, end-to-end.

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