Gtojohn
Ok your thinking costs. I'm going to take a wild guess concrete in you area $120/cubic yard. and the area of your building 600 sq ft or 20 x 30
A slab of concrete 20 x 30 x 6" is approx. $1333.00 less foam insulation and tube.
Lumber 2 x 12 x 20 pressure treated are $54.00 ea. (Menards)you will require 16 pcs @24" centers plus rims joists so 20 total Minimum $1080.00 you will need plywood 20 sheets @$35.00 so thats another $700.00 for the decking Total so far is $1780.00 additionally you will require insulation underneath not to mention the added cost for piers and saddles. I'm sorry I don't see any cost savings going old school lumber.???
level ground, run perimeter forming boards, put foam on ground, lay steel mesh(or not) put tubing down, pour concrete. First floor completed!! AND half your heating system completed.!
That $100 you were going to spend on those used heat exchangers save it and put it towards the PE tubing. Not to mention those heat exchangers will not work with low temp solar hot water.
I wouldn't steer you wrong concrete slab is the way to go. Ran the numbers, built more than one, enjoyed the FREE heat on my feet.!!
Randen
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