Well the way my solar hydronic systems works, the floor warms up, the heat transfers to the air space and the thermostat reaches it's set temp, and turns that zone off. The floor in my bathrooms (tile) may reach 90-95 degrees before the thermostat, set at 75, turns off that zone. Floor Temp needs to be higher than about 90 degrees before it feels warm to the touch, so no problem with 150 degree floors in my house.
Witch
I am going to translate that as saying "I don't have a problem with hot floors since the floor itself does not get that hot"
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I'm playing around with a non UL approved "research" set up as well.
Here's what I've got so far
1. A bronze recirculation pump
2. In floor PEX
3. A plastic non-pressurized tank. (open air system)
4. Plans for a solar trough that will still work at -40F.
5. Two 50 gallon hot water heaters with internal 30sq heat exchangers.
I have to use copper, PEX, or brass for corrosion reasons. (open air)
It is legal to use a hot water heater to heat potable water and then use that to heat the floor via a heat exchanger.
It is also legal to use the hot water to directly heat the floor, but that means there is no way to use antifreeze.
Note: to stay legal, you would have to have some sort of sink to justify having the hot water heater there and to use some water. I would also recommend adding a check valve before such a long and possibly "stale" section of water pipes.
To stay safe you have to have a "safety" expansion tank on the potable water side
AND use non-toxic antifreeze on the heat side.
One reason they don't like hot water heaters being used for in floor heating is that you
can not use antifreeze with a hot water heater since even the fold back elements tend to scorch and burn the antifreeze.
Electric boilers designed for heating are carefully designed to maintain fast water flow past the elements and this keeps the element surface temp below the point where the antifreeze starts to degrade.
I could build my own boiler that would be safe, but is it really worth my time.