When I built the HP Hot water tank for my parents I had concentrated on the temp output, suction and current draw of the compressor. I knew that the high side would vary greatly with the temp. I would very slowly increase the charge and monitor the current draw. With a cheap receptical watt meter I would get an idea to the efficiency. I would check on it every few days and teak the charge looking for the sweat spot. The unit had a cap tube and I had used BBQ propane. The cost to operate the machine was about $20.00/month about the same as the rental fee for there previous propane fired tank.
Unlike the compressors previous life of a de humidifier the temps are fairly steady at room temp. The HP hot water heater with the introduction of cold water could start at 40 Deg.F and limit out at 140 Deg.F. Thats quite a swing and the pressures will reflect this. Thats why I like to concentrate on the current (Watts) as the compressor is moving a near fixed volume of refrigerant.
As you would monitor the temp increase and the power consumed (watt Hours) you can calculate the Coefficient Of Efficiency with that volume of water for that particular refrigerant charge. For the temperatures and energy input there will be a sweat-spot. This is called R & D and this is how the big-guys do it in their labs. I have found that propane consumes much less energy to move an amount of heat compared to R22.
Have fun
Randen
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