Just charged and pressurized the solar heating system for the greenhouse, at 25 psi, which is good for a small drill pump. I've posted about this before, but here's a quick recap—
I found two flat-plate collectors in a salvage yard, one with cracked glass and another that appeared to have leaks. Swapped the glass and pressure tested the good one: bingo! Mounted it on a pole, angled for low winter sun. Plumbed it with PEX tubing and sharkbite joints. With good sun, the inlet temp can get up to 140°F. A small DC pump is driven by the 20W solar panel, at the same tilt as the collector. When the collector gets enough sunlight to heat up, the pump runs at a corresponding rate.
Because our winters are extremely cold (-40°F) the loop is charged with a 50% glycol solution. The inlet goes through a temp/pressure gage and down into coils through a 400 gal. livestock tank, the primary heatsink.
The water in the heatsink can get up to 120-130°F.
An AC pump (Taco, used for fishponds, etc.) circulates the warm water (more efficient for heat transfer than the antifreeze solution, where freezing isn't a problem) through a maze of PEX tubing under the greenhouse floor. That's two heat-conducting loops. I don't start the AC pump until there's a 10°F difference in the temps of the tank and the sand floor. I try to keep the floor at 55°F or above through the winter.
The insulated foundation goes down about 3 ft. Inside is a layer of 2-inch foamboard, 6 inches of gravel, and a fill of coarse sand. I installed a drain, but no water has ever come out. Having water held in the porous floor increases the heat retention.
There's a chipboard floor over the heatsink, with black rubber matting. The walking area is capped with dark pavers. The outer beds, sand, hold black containers for tomatoes and peppers, while the center bed is filled with soil, for cukes, beans, greens, etc.
Started up in 2010, with very few problems since. Last year, the DC pump failed, the collector boiled and ruptured the PEX tubing, and the antifreeze blew out. But I fixed it and it's working fine, let us hope for another ten years.
