So much is detail-dependent, but I will give it a shot. First, some assumptions and background so we are on the same page. I am guessing you are from SD, similar climate to mine in SE Minnesota. My designer shoots for 25 btu per square foot in our climate. A little less than that is OK if it is a shop (not living space where you want it warmer and very consistant). My current shop is about 1100 square feet. Theoretically it should be satisfied with 26000 btu source. I have a 40 gallon LP fired power vent water heater (standard tank type) installed, I believe it is 40,000 btu. That works fine and I run it fairly low all winter (50 degrees), and it is very cheap to operate. There is only 1 inch of extruded polystyrene under the slab, should be 2" really.
I am currently building a new shop, on a farm we bought a few years ago, with the intention of moving there soon, another year or so. The slab is done and the heating system is designed so I can be specific. 2100 square feet total, 3 inches of foam under the slab. Two zones with possibly different temps desired, so two controllers and two pumps. 25 btu per sq foot means 52000 btu input needed. Tubing is always placed on 1 foot spacing, and 1/2" tube should be limited to 300 feet max. Try to keep all runs on the same manifold approx the same length, or within 5% of each other. I purposely left the tubing further from the outside walls and doors, like 3 feet away, in order to use the normal somewhat insulating characteristic of concrete to limit the amount of heat that will be drawn out of the wall at the slab edge. We will see how that works. There should be a slightly colder region along the walls, but typically there is "stuff" setting along the walls anyway, so I don't think I will notice the slight difference. This is probably stuff you already know. I have chosen a tankless water heater (not a boiler) to power this system. It's a Rheem modulating standard venting (not condensing, although if the price was right I would prefer condensing for venting convenience and efficiency). It is modulating between 11,000 btu and 199,000 btu. My system designer says these work out just fantasticaly and they self control very slick. The set-point controller (thermostat-like device) calls for heat, which triggers the pump. When the pump runs, cooler water begins entering the tankless heater, which causes it to ignite and modulate according to the incoming water temp compared to the user-set-desired output. So upon firing up it probably modulates fairly high initially, somewhat depending on flow rate too, and then as the cycle continues and the water coming into the heater is gradually warming up, the heater internal controller will modulate back on the gas flow and reduce the btu to continue to satsify the incoming versus outgoing temp differential. When the set point controller is satisfied and turns off the pump, the heater detects no flow and shuts off. this is how it was explained to me, I have not plumbed much above-slab stuff yeat so have not experienced it yet.
So hopefully some of that does you some good. To specifically try to answer your questions, electric boilers are supposed to be very slick, but I wonder why people use boilers when you are trying to produce only 110-120 degree water. I have seen the boiler based diagrams and they then seem to need a mixing valve to temper the water down from the heated temp to the 110 range. ?????????? why heat it higher than you need to, then temper it back down ???????? In my limited understanding, the water heater is set to a more appropriate temp to begin with, like the 110 range you want to send into the slab. As far as tank typ vs tankless, I am going to be glad to be moving to the tankless type. Less total water in the system. Less space taken up. No standing water to keep hot even if the floor is not calling for heat (much of the Fall and Spring). Modulating feature seems to be made for this application.
People have had bad experiences with tankless heaters due to mineral buildup. In this case, the water goes in, stays in, no new supply of minerals brought in everytime you open the tap, so no buildup.
