Paul, I am in the same position as yourself renovating groundfloor up and need new floor in the kitchen and lounge and I will be installing wet underfloor heating. I installed a small system in my conservatory in the UK. Absolutely brilliant, we have a 27m2 which is used all year round and no doors into the main house. I used Polyplumb (http://www.polypipe.com/POLYPIPE/pp_html/main.html) as my supplier in the UK. They seem to be the only supplier who will sell individual items as opposed to all the other who want floor plans to "survey" before giving you a costing. I know you need to work out thermal loss heat transfer etc but when was the last time an electrician or plumber worked out lumens or BTU's to install lights or radiators in a room? Its not rocket science keep the pipes at 4" sorry 100mm centres and that will be sufficient for any room. Bottom line do once you have you will never go back to radiators except maybe for towel rails. Anyway good luck John