UFH to be connected to ASHP ?

[newtoallthis]

Sounds good.

The slab takes a lot of energy to heat it up, but then it takes a long time to cool down - this is going to provide a challenge for optimisation but I'll worry about that when the time comes.

With such a long time constant, as long as/to the extent that the loss is only 1/3 of the capability of the HP then you would be able to run it entirely in the 8 hrs of Cosy cheap periods £££ . Just need to match long term integral of heat input with overall heat loss. I would have thought Vaillant's WC algorithm would do quite a good job of this - even without the fancier options such as active, extended or adaptive.

I am hoping that you are correct.

[newtoallthis]

Update June 2025
It’s all signed up and ready to go, we have a new large radiator in the hallway to give additional volume and flow, the slab is done for the outside unit to be fitted, and the contractor is booked in to install a 5kw ASHP in mid July.
I’ll report back when it’s fitted and we have some real world heating experience - which probably means sometime in November.