I’d have a look at the pressure gauge on the boiler, may indicate a water loss resulting in air.

aha. I get it now.

Still cant help bu thtink it’s rather less intuitive than a return mixing system…

This is mine.

Honeywell CM927 controlled relay on the left controls the pump on/off.
6 ground floor zones balanced for comfort levels manually.
Mixer valve set to 40deg ish. Hot water from centre of thermal store (anywhere between 40deg and 70deg) is blended down with return water from the UFH, the rest goes back to the bottom of the thermal store.

The thing I think is strange on the OP system is having a 2 port valve (TRV) on any system that is a constant volume system. i.e. fixed speed boiler pump. What if all the TRV’s are closed it’ll be pumping against a dead head. Still it appears to be a standard set up, maybe they have a pressure regulating valve in the system……or I’m looking too hard at it in a large commercial system point of view, I’d have inverter speed controlled twin pumpset controlled on a DP sensor for that system :D!!

constant volume system

Wouldnt the pump be on the same circuit as the room stats? so that pump will only run if at least one valve is open. As long as one UFH circuit is open, the inline TRV on the flow side can still be closed and there would be no dead end.

Yeah I suppose the pumps are small and don’t mind if there’s a large flow (all underfloor legs open) or only a small flow from 1 underfloor leg (worst case). On large 2 port systems the pump speed matches the system flow, as the valves shut the pump slows down, older systems were 3 port with a bypass leg with a balance valve that matched the heater coil resistance!

EDIT:

As long as one UFH circuit is open, the inline TRV on the flow side can still be closed and there would be no dead end.

I was kind of thinking from the boiler primary pump if all the TRV’s are closed.

Good u/floor systems will only call for heat when the water temp is demanding heat, therefore pump only comes on with demand.

Also better systems use variable speed pumps, usually Alpha + on them.

bloody expensive those alpha +

was toying with using them in the barn (I have 3x 15-50s) but at that price …no..