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U can get thin floor stuff but it's a shortcut......
25mm of the super insulated celotex equivalent to 100mm of normal, then the thinnest that I know of is: LoPro10 - Nu-Heat - Underfloor heating and renewables the next thinnest is Overlay Lite Underfloor Heating
Managing the flow and head loss using a GSHP will be critical, you will need as a minimum a buffer or low loss header, my preference would always be for a buffer, a good control system (heatmiser) someone needs to do the heat loss calcs to make sure that either of the above solutions can deliver what you need. All ufh works better with a thermal mass, though if you ain't got it then no choice. (Flow rate, flow temperate, head loss, heat loss, emitter output; Simples 🙂 )
Get this: CIBSE - Underfloor Heating: Design & Installation (Domestic Building Services Panel) 2012 it's only £18.50 and will save a fortune in mistakes.
Reply to the thread, titled "absolutely no clue" which is posted in UK Plumbers Forums on Plumbers Forums.
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