Thermofin and PEXALPEX

rhl

I am running staple up to my first floor with the thermofin plates and I’ve done half the loops in PEX-AL-PEX.

My first floor is 1100 sqft and has a load of 39K BTU. So that’s 35 BTU/sqft 

Through envelope improvements I (one day) will drop this in half, to 17 BTU/sqft.

I’m using Al Pex to try and increase the floor output (although I’m realizing I should have done copper). 

What is the water temp at which I make it to 35BTU/sqft with the plates ~8” OC? 

I am hoping to one day use a heat pump to heat the floors, so is it possible to do this with 110-120 degree water ? 

hot_rod

here is the quick math, about 2 BTU per sq ft for each degree difference.

So in a 70° space to get a 35 btu output the floor surface needs to be about 87°. That would be uncomfortably warm. Low 80's is considered an acceptable surface for bare feet.

The SWT requirement would depend on the floor coverings.
3/4 plywood or softwood, with 3/4 hardwood would be
about R-2.
You will need supplemental heat for those design days until you get your loads down into the 20's (btu/ sq. ft) or wear sorels in the house

Realistically a comfortable residential radiant floor gets you in the mid to upper 20 btu/ sq. ft output.

Shop floors, ceilings, walls a bit higher output as you can run a higher surface temperature.

A concrete slab outdoors on a 30° day could give you near 200 btu/ sq ft.

It is all about the delta T when transferring heat energy.

Here is an example looking for 35 BTU with r-2 floor. Even if you could deal with 87 ° floor temperature, the woods below, that the plates attach to may not handle the 140° AWT!

Copper in the plates would lower required SWT a few degrees, better conduction transfer, not really increasing the BTU delivered however. The surface temperature (radiant panel) is in charge of that number..

rhl

Thanks @hot_rod this was the info i was looking for!

rhl

Is there a web version of the tool you are showing me?

hot_rod

Cardboard versions are nom longer produced, here is the ap.
https://www.drakeip.com/RadPad/index.html

Alan (California Radiant) Forbes

Thanks for walking us through the math, hot rod. I call myself a professional installer, but I’m weak on the fine points. 

You said, “A concrete slab outdoors on a 30° day could give you near 200 btu/ sq ft.”  Does that mean a surface temperature of 130F?

hot_rod

Alan (California Radiant) Forbes said:

Thanks for walking us through the math, hot rod. I call myself a professional installer, but I’m weak on the fine points. 

You said, “A concrete slab outdoors on a 30° day could give you near 200 btu/ sq ft.”  Does that mean a surface temperature of 130F?

slab surface of 35F is adequate to melt snow and ice, but that could take SWT temperatures upward of 140F depending in the load. And lots of load(s) in play with SIM, back loss, edge loss, wind, evaporation. This older article shows where all the load components are and the BTU required to cover them.

https://www.pmengineer.com/articles/87603-radiant-snowmelt-systems-at-work