Rod Speed wrote:

Problem is that the floor isnt a very good source of
lower grade heat...

Wrong again Rod. A hydronic floor with lots of surface and a low water-air
thermal resistance in an airtight house with lots of insulation works fine.
Ignoring the R1 radiation conductance, the min floor temp required to keep
a house with a 200 Btu/h-F conductance and a 2400 ft^2 floor 70 F on a 30 F
day with a U1.5 slow-moving airfilm conductance is 70+I/(2400x1.5) = 72.2 F.

Nick