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Old 11-11-14, 09:07 AM   #3
mejunkhound
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Most commercial HP defrost cycles either are timed or terminate at a preset evap temperature.

Saw this thread, so thought I'd throw in a VERY simple method I use.
Since am a retied old fart, and home most the time, we have a mirror on a tree outside that allows us to see the evap coils from a few different windows we go by during the normal day.
Also have a manual internal push button switch - look out and see frost, push the button and defrost cycle starts, simple as that. This was before we added out DIY GSHP.

If one wanted to get fancy, LED with photosensor to detect frost to activate switch.

The option most in use here is having the GSHP operates when outside air temp is below 42F, above that the air-air HP functions. For the air-air HP, 4T Evap coil with 2T compressor assures that evap coil never drops below 32F, so 'frost-free'.
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