first thing i have to say is radiant isn't for everyone, it's a commitment .. you can't flick a switch and be instantly warm, it takes days to raise a slab to temperature, so if you're not spending a LOT of time in your workshop look at something else .. so, here's a very basic radiant overview
electrically heated water is NOT an option .. hot water tanks are too expensive to operate .. so condensing natural gas or propane, geothermal heat pumps, oil or a wood fueled boiler are your best alternatives and you really can't have too much insulation .. ng is the best option if available in your area .. propane is a bit more expensive to operate and has to be trucked in usually and stored in expensive tanks .. oil is expensive because they aren't making dinosaurs anymore .. geothermal is insanely ecomonical to operate, but is a frighteningly high buy in .. wood is cheap, but labor intensive and requires a lot of attention, it's like being married .. i have a wood boiler .. you also can preheat water with a solar panel (actually very easy to build yourself)
the simplest no nonsense FUNCTIONAL way to pipe your system is reverse return, like this diagram borrowed via google
the first loop gets the it's hot water first and returns last .. without getting into the science involved, this works great, period and only requires one pump to do the job
when water is heated it expands, so you have to give it somewhere to go or it's going to pop your t&p valve .. ergo you must have an expansion tank
you will also need to remove air from your system .. all systems have air in them, it's a fact of life .. anyone that's ever farted in the tub knows air rises so you need to put your air eliminator at the highest possible point in the system .. please don't cheap out on your air eliminator; air + water + heat = rust, which equals short boiler life, short pump life etc etc .. if we take one element out of the equation we're golden, air is the one thing we can definitely do without
pumps work by creating a low inlet pressure and a high out pressure, water will move to try and equalize the pressures and so create flow .. air comes out of suspension easiest in low pressure areas, so the best spot is as near as possible to the pump inlet .. this is also the best spot for your expansion tank, which is why air eliminators are tapped at the bottom for expansion tanks
next thing; you CANNOT put regular household pex in a radiant system .. you have to use heat pex, it has an oxygen barrier that stops oxygen molecules from entering your system
you're gonna need to fill your system with water to carry the heat around .. in the trade we have things called boiler drain valves, basically a tap with a garden hose thread on it .. if you tee off 2 of these together somewhere in your piping, with a ball valve between them, you can close the ball valve and force water in one side and flush out all the air from the other without allowing it back in .. air is your enemy
now, to your specific question, i would build 3 loops into the floor, 6 inch spacing around the perimeter and 12 inch spaces filling in with the runs along the longest length to cut down on the amount of bends .. bends are trouble spots where kinks happen and where air and crud can gather .. reverse return with a mixing valve to knock the inlet temp down ..one grundfos 1558 pump (cheap and reliable, but high quality) and a slab sensor hooked up to a 24volt thermostat .. the choice of heater is all yours sir
i'm also a huge believer in cleaning the water you put into the system .. i use a flush cart i built to fill the systems i put together with a cartridge filter on the return line to catch flux and bits of solder and whatever else has found it's way in during construction
by the way, ive built radiant systems with $200.000 price tags .. 5 grand is getting off light
hope that helps jim
