Let's assume and enter into any good HR software: 500 SF concrete slab HR heated floor, 5000 BTU/H room total heat loss (400 BTU/H downward floor heat loss), and 0.5” diameter PEX. Required upward heat gain is 4600 BTU/H and floor surface temp will need to be 75.8F in order to provide it.
12” PEX spacing, 89.8F circuit supply temp and 0.34 GPM results in 4600 BTU/H actual heat gain.
6” PEX spacing, 89.8F circuit supply temp and 0.34 GPM results in 7551 BTU/H actual heat gain.
7551 BTU/H is 64% more room heat gain than 4600 BTU/H at the exact same 89.8F circuit supply temp and 0.34 GPM flow rate.
Let's assume and enter into any good HR software: 500 SF above-floor plate system HR heated floor (with R1.0 floor finish), 5000 BTU/H room total heat loss (400 BTU/H downward floor heat loss), and 0.5” diameter PEX. Required upward heat gain is still 4600 BTU/H and floor surface temp will still need to be 75.8F in order to provide it.
12” PEX spacing, 99.5F circuit supply temp and 0.34 GPM results in 4600 BTU/H actual heat gain.
6” PEX spacing, 99.5F circuit supply temp and 0.34 GPM results in 5966 BTU/H actual heat gain.
5966 BTU/H is 29% more room heat gain than 4600 BTU/H at the exact same 99.5F circuit supply temp and 0.34 GPM flow rate. 29% is significantly less than 64%. A 99.5F circuit supply temp is 10% inefficiently higher than 89.8F. There is no floor surface temp difference between the concrete slab HR heated floor and the above-floor plate system HR heated floor so "comfort" is identical.
Moral of the story… People who design HR floor heating systems really need to obtain a copy of John Siegenthaler’s Modern Hydronic Heating and actually study and master it before designing these systems. People who design HR floor heating systems really need to actually know how to use good HR design software when designing these systems and not just blow smoke out their butt...
Borst Hydronic Radiant Floor Heating Design Software