I have radiant heat in one of my houses, and with it not being occupied right now, but me; not wanting to drain the water system, I wanted to see if I could keep the building above freezing without heat.
What happens is, the center of my concrete slab is at 57 degrees, where as the outside of my slab, is far less and losing heat to the cold. But by just running my circulators, I am pumping water through the floor, thereby averaging out the water temperature throughout the floor. So far the lowest it has got down to in ambient temperature is -5 degrees (f), but it was blowing a gale, so it was -35 below zero (f) counting wind chill. Inside, with no heat on an unoccupied 2000 square foot home, it was 50 degrees inside.
I was pretty impressed.
It is too cold to be occupied, but there is enough geothermal heat to have to pay for propane on a home not making income.
Google search: "site:heatinghelp.com btu load calculation"
Lot of info there from guys who do this for a living and are actively trying to do it better. But their search option, IIRC, isn't great, so better to use google with the "site:" argument. Might try a search for "radiant temperature".
I have since turned off the heat in that home, drained all the pipes and am now watching the temperatures on a daily basis. It is interesting in that the temperature is not dropping below freezing despite it being down to -10 degrees below zero (f) outside.
The slab is of sufficient size, and pulling enough 57 degree heat in the center of the room, and keeping the building from freezing. That is pretty amazing considering there is no internal heat being generated. No appliances running. No occupants. Not anything, and this is NOT a passive solar home, There is enough Geothermal Heat to keep it from freezing!
Why should I lose weight? They make bigger overalls. And they sure don't make overalls for tiny ads: