Hey there, I have been a user of greenbuildingtalk.com for a while now but this will be my first post. So let me get right to it. When I build my home in stalled 1000' of 1/2 pex pipe in the basement slab 4 evenly spaced 250' loops into approximately 1000sqft of 6" concrete slab. Insulated underneath with 2' foam sheets. I would really like to get the system up and running but need some help with the details. This is the idea that I have someone please tell me if it will work or not- I was thinking of a closed loop system using my 95% efficient tankless propane condensating hot water heater to heat the floor. I was going to use two pumps circulating through a brazed plate heat exchanger to connect the closed loop floor to the potable water heater. Couple questions- What size pumps do in need both on the heater side and the floor side? What size brazed plate heat exchanger do I need? What kind of thermostate should I use? How much antifreeze water will I need to fill 1000' of pex and the menifold ect. I already have a 4 port manifold from pexuniverse I think. It has the fill ports and the air remover on it already i think. Will I need an expansion tank/air separator on the floor side? What other kinds of special valves/connections will I need? Any other help/design ideas would be greatly appreciated. Thanks so much!

Consider using this:

http://www.blueridgecompany.com/quote

Many tankless hot water heaters will not work for radiant heat. When you recirculate hot/warm water back through them the lowest setting they can run at is usually to hot and will cause the temp to overshoot the target temp. This will cause the heater shut down and may lock itself out and not run till powered off and back on.

Good luck...honestly with the questions you are asking you need to do a lot more reading or do as jonr has suggested and get professional help.

Yes, get a copy of John Sigenthaler's "Modern Hydronic Heating". We have free HR design software and wealth of information on our website that you might find worthwhile. If you are interested in professional HR design software, consider HeatCAD and LoopCAD. Also be wary of the company bots that frequent these forums.

blueridge is the answer.