Are you sure that hot water is the way you want to go? Maybe you might want to look at hot air with a heat sink under the slab to release heat at night through the floor?? I'm just tossing things our there.
Water holds heat better than air. Higher density. Concrete (insulated from the ground) is the same. Heat it up and it stores the heat quite nicely, releasing it slowly to the conditioned space. Air heats up quickly but sheds heat just as fast.
Hydro solar collectors are the way to go. Use water with a high ratio of propylene glycol and no drain down is needed.
You have to have some pressure in the system or it will not pump. Not a problem with a closed loop. Pressure it up with a garden hose and a pressure regulator set to 15 psi and you are set. Need an expansion tank, too, to deal with the variance in water temperature between day and night.
Worst case is you need a hot water backup. Use a propane water heater where the gas comes on when the tank temp fall below, say, 80F. Sunlight does the lion's share of the work and the gas would kick in during the middle of the night for a few minutes. Or invest in more storage tanks for the solar HW. Might have to add space to accommodate them but that's always the problem with solar: storage. Hydro or PV, it's always storage that's the problem.
Thanks.So it would make sense to put a couple of feet of stone under the slab and run the water through there also?What I'm saying is if you has a hole a couple of feet deep and added pipes to the bottom and covered them with 4 inches of sand to protect them and filled the rest of the hole with processed or gravel then run the pipes through the floor it would add a heat sink and it wouldn't cost much at all.That should extend the heat release time by a lot.
Actually you only need between 4 and 5 inches of concrete to make a proper radiant panel. Any more than that and you reach a point of diminishing returns. Adding rock under the slab is not necessary. What is necessary is the installation of 1.5 to 2 inches of insulation (the blue or pink styrene sheeting works well for this) to prevent the earth from pulling the heat out of the slab. Heat always flows from where there is more of it to where there is less of it until it reaches an equilibrium. That's one of those laws of thermodynamics. The earth will always be substantially cooler than any radiant panel (in this case the heated slab) so you want to prevent that from happening. Hence, the insulation board that will retard the natural flow of heat into the earth.
You wouldn't want to bury radiant tubing in sand. Sand (silica) is the primary ingredient in fiberglass and is highly insulative. Tubing should only be embedded in concrete. I can't imagine wanting a heat sink because it would be drawing the heat away from the conditioned space. A heat sink would only be useful when shedding heat (as in cooling), not in heating.
The tubing used in radiant panels needs no protection. The generic term for it is PEX which is short for cross linked polyethylene. PEX comes in two distinct varieties. The cheaper type is what is used for domestic water (i.e. potable water). The more expensive PEX incorporates an oxygen barrier and is used exclusively for heating systems (radiant panels, connecting radiators and convectors or forced air coils). The oxygen barrier prevents O2 from diffusing from the atmosphere, through the tube wall into the water. Without and O2 barrier the oxygen attacks any iron based material in the system like, say, cast iron pumps. You can use the non-O2 barrier PEX tubing if you build your system without any steel or iron components. Brass, copper or stainless steel fittings are not affected by O2.
Designing a proper hot water heating system is not easy or self evident. Fortunately there are zillions of online sources of information on how to do it correctly. A good place to start is found
here.
