My primary heat source is a 3-ton water-to-air geothermal heat pump. It was factory-filled with R-22 (freon). It was designed for warmer climates, as it had a factory-installed freeze protection switch that shut the unit off when the outgoing water temperature came close to 0C. I bypassed the freeze switch and used a water and antifreeze (windshield washer fluid) mix for the loop.
The optimal amount of refrigerant in heat pumps (and air conditioners) depends on the temperatures of the cold and hot sides. I wanted to tweak the refrigerant charge, but R-22 is bad for the atmosphere and hard to come by. I came across some information on propane (r-290) as a refrigerant which indicated it can be used as a substitute for R-22. I can get it cheap at my local hardware store, and even Greenpeace likes it.
I read about people using propane for DIY computer cooling, and someone that recharged an R-22 system with propane. The first thing I needed was a manifold gauge set. They tend to sell for $100-150, but I found what I thought was a good deal on ebay for about $50. It has plastic handles on the valves, and one was broken on arrival. About a minute after I hooked it up to the high and low side schrader valves I heard a loud pop. It took several more minutes to figure out one of the hoses had burst and was leaking. Now I would HAVE to recharge the heat pump.
I had a gauge set (with 2 of 3 hoses still good), and a couple 16oz canisters of propane. Fuel-grade propane can have moisture in it which is supposedly bad for a heat pump. Instead of trying to buy refrigerant-grade propane (r-290), I decided to run the propane through a drier. I bought a drier with 3/8 copper sweat connections and a shrader valve at Wolseley (about $20 total). I bought a propane torch and unscrewed the tip. Here's my parts:
I cut the 1/4" copper tube off the schrader, then soldered it all together: