DIY ASHP Defrost Control

[launboy]

I'm reaally kicking myself for letting our old dehumidifier get thrown away. It would be useful for so many projects. My latest project is a Hot Tub Heat Pump, which hopefully I'll be starting tomorrow. I'm turning a window A/C into a air-water Heatpump to reduce the cost to heat our hottub.

Frosting will definitely be a problem I think, considering I'll be running between 30*-40*F. (Whether or not I'll even get enough heat from the unit at this temperature to matter is a whole nother issue ). My thought was though, that cold temp dehumidifiers, aka ones that are able to run below 65*F have a sensor on the front of the evap coil that detects frost and will start a defrost cycle(shut it off in our case). Do these just operate based on temp? When it gets below xx* they initiate a defrost? Could this be used on my hot tub heat pump?

**EDIT** I think it could. a little googling led me to an eBay seller with a Frigidaire Defrost Tstat, and in his description he says:
"This control clips on the suction line of the refrigeration circuit.

I usually mount it about 4" to 6" away from the compressor. The suction line is the larger line. This control will sence the temperature in the line and if the dehumidifer beings to ice up this control will stop the compressor and allow the system to automatically defrost. When the line warms back up, the compressor will start up again."

Leading me to believe this could be added to a relay to control the compressor to avoid frosting too badly, and more importantly, liquid to the compressor.

Adam