Overlay UFH with Heat Pumps

Hi all just looking for some experienced responses.

We have a project where we are installing 2 ASHP's to run a 750m Penthouse suite on top of an apartment block.

Initially it was thought that we would be able to use an anhydrite screed, but the architect has now decided that there is insufficient structural strength to do this.

The client still wants to use UFH and is considering either a floating floor set up with pre-formed sheets or an overlay system.

My concern is with heat output because we will have no influence over centres and no thermal mass, and with the overlay system the pipework is 10-12mm so I have concerns over flow rate, but we could shorten the loops to counter this.

We cannot install from underneath and anything we do will now be a compromise, has anyone had experience of the lesser used UFH methods or specifically overlay with a heat pump?

Cheers

Jon

That is used over here a lot and i don't like it much. The people want the comfort but don't care about the system efficiency, typical. Can't they put an self leveling overpour of 30mm or something