DIY Air-Source Water HP

[launboy]

The Goal: Heat my hot tub more efficiently than the resistance heat(1350w) does to bring down running cost.

The Plan(for now): Get window air conditioner, remove condensing fan and bend coil downward so it hangs below unit. Put condensing coil in water in place where it will flow through. Eventual plan is to swap condensing coil for some sort of refrigerant-water HX.

The Unit: 8000 BTU Kenmore found on Craigslist for $20.




A test of it's heat-pumping abilities, I put it in the window backwards. Ignore the ghetto sealing job. (all measurements taken with digital IR thermometer with fan on high and after 15 minutes of operation)
Outside temp was 25*F, air exiting unit was -11*F.
Inside temp was 67*F, air leaving unit was 96*F.
Compressor temp settled around 135*F

The coil never got any frost on it in over an hour of running which was a surprise to me, must be really dry out there right now.




*Note* you can see the canister thing referenced below in this picture. The black thing right next to the compressor.

Then when I began disassembling the condenser side for water testing tomorrow I discovered this...

The coil had a nice blanket on to keep it warm. The lack of airflow probably explains the warmer then expected air inside. The clean parts you see are areas I had already pulled it of, it's almost 1/8" thick in some places.

Question, should I be worried about liquid flood-back to the compressor with such a low ambient outdoor temp? There was no frost indoors but the entire line from the evap back to the compressor indoors had thick frost on it right up to the connection on the compressor. The pipe temp at that connection was about 38*F but the temp of the canister looking thing(what is this btw?) just before the connection was about 14*F. The hot gas entering the condensing coil was around 122*F and leaving it about 86*F if that matters.

Will update with progress tomorrow. Comments, concerns and suggestions welcome!

Adam