Like Michael says the mats and panels are much more efficient but if you have the space, black poly pipe works just fine. In the pic there is 1200' of 1/2" pipe with the manifolds feeding 12 100' lines in parallel. The entire thing is about 8x50' with the pipes layed on black tarpaper. Found that tarpaper doesn't hold up too well so just last week I got some black rolled roofing and put that underneath hoping it will hold up better. Last 04 August I measured about 22800 BTU/Hr from the field heater and about 17000 from a 4x20' Fafco mat heater. I've got a whole series of build pics and descriptions for anyone interested. You can PM me and include an email address and I'll be glad to send them out. About 1 megs worth.
Al