This is one from buildhub. I have left my house figures in to give you a starter for 10.

UFH design - see attached.

Lots of insulation otherwise your downwards losses will make you cry. So 150mm PIR and above. Not the 40mm UFH outlets want to sell you. It works best with loops in concrete or screed. If you can't do that you may be better with radiators or fan coils.

@johnmo Hey thanks for that. It'll give me a good head start. I'm assuming we have plenty of insulation under the chipboard floor but I can't check until we actually own the place and I can lift some floors.

Hi Ash, 

when was your soon to be new home built? Because it will only be as good as building regs were at the time.

When you get into details lots of floor covering say suitable for ufh but then they say nothing above 29C temperature on the glue. Only ceramic tile can go higher in temperature. 
To keep temperatures low and hence efficiency high you probably need spacing between pipes of 100mm. A couple of plumbers would have put in 150-250mm spacing which coupled with the floor covering temperature limit would have left us cold. They basically didn’t do a room heat loss at all.

Thanks Judith. It was built in 2008 so should have some floor insulation. It would appear to be suspended timber with chipboard but it could be beam and block. I'm trying to find out what plans were submitted to Building Control.
We're going to be using Marmoleum Click flooring which does have the temperature limit so thanks for the spacing advice. We'll be doing the pipework ourselves so heat loss calcs will be essential.

Posted by: @ash2020

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We'll be doing the pipework ourselves so heat loss calcs will be essential.

You can download a free copy of LoopCad on a trial basis for a month. Well worth the effort.

@johnmo Thank you. That'll be useful.

Heatpunk is pretty good for a house heat loss, which you need to start. https://heatpunk.co.uk/home It’s easier than many tools and with reasonable output displays. It allows ufh to be entered in W/m^2

@judith Thank you again. I've downloaded it and played around but I can't see anywhere where you can design UFH.

@ash2020 sorry to mislead you. You can’t design your ufh in it but once your house is entered you can check your ufh design matches your room heat loss.

It is is the heat pump section of task pull down and looks like this

Personally I would  start with the house heat loss so you know the scale of the ufh design needed. You enter the ufh in W/m^2 in heatpunk for the room and iterate until you have enough heat and it is cost effective.

You could design a system using the smallest pipe inter loop gap, lots of length etc but it would cost more to install but allow a lower operating temperature. It’s a trade off between up-front costs and on-going operation cost.

@judith Thanks again. I'll stick with it.

Posted by: @ash2020

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design UFH

Download a free copy (trial) of LoopCad.