UFH mixing valve not restricting heat to floor

[Dtwone]

Hi, I've installed a 2 zone kit that uses an RWC mixing valve, Thermomix with hot in, return to boiler and heat out to Ufh pump and return from floor manifold. From underfloor heating store but basically the same as a jug speedfit Ufh kit.
I thought I had a duff thermostat inside the reliance mixing valve but they supplied me a service kit and it's still not working.
The thermostat expands and contracts as it should, I tested it in a bowl of warm water, allowing the hot feed from the boiler to bypass the Ufh manifold but the inlet temp on the manifold is rising to basically as high as the boiler feed in, abou 65 degrees and the mixing valve control has no effect on any setting, I have just converted a y plan system to s plan with a separate zone for the Ufh and the return goes back close to the boiler return into the boiler. I've also fitted a bypass valve which you can hear opening when the boiler pump is on overrun and all zones have stopped calling for heat.
I'm really stumped, it's like the main system pump is overwhelming the mixing valve but it's running quietly on minimum setting.
I've no issues with the hot water or radiator zones or the boiler, and the Ufh manifold appears to be working fine and both loops heat ok, it's just there's no control of the temperature down to the 45 degrees needed for the floor.
I even tried restricting the flow to the valve by partially closing a full bore 22mm isolation valve I fitted in the supply to the Ufh zone but you have to almost shut it off completely to make any difference. There's no other parts in the mixing valve other than the spring and wax stat so nothing else I can see is wrong, I've tried the Ufh pump on different speeds, Ufh zones open or restricted, nothing makes any difference.
Any help would be great as I can't see anything wrong with the layout and components I have and I'm pulling my hair out with 50 degree water coming BACK OUT of the floor after nearly 50m of pipe run at the moment!!!

 

[Sharp Point]

Can you put a picture up of your set up, so we can visualise what you have

Thanks

 

[king of pipes]

As above need a few pictures to help you out . cheers kop

 

[Dtwone]

Of course, picture paints 1000 words... In my airing cupboard upstairs is the main boiler pump and the t off to the Ufh and zone valves for HW and CH, Zone valve for Ufh is by the Ufh manifold simply as there's space there.
Boiler return from Ufh pictured, T compression joint, it's the other side of the wall from the Ufh manifold

 

[Gasmk1]

are you sure you have the flow and return right way round

 

[Dtwone]

Well the flow is connected to H with the in arrow and the other one (C) goes back to the boiler return through the wall so yep I'm pretty sure, sort of thing I'd do though
You can feel the flow pipe heat up as the boiler runs, if I hold the back of the 2 port body, and the Ufh gauge rises as that warms, but the return stays relatively cool until the Ufh return starts to come back at 40 degree ish, I was expecting the mixing valve return to boiler pipe to warm pretty quickly as the floor is already at 16 or 17 degrees

 

[SimonG]

Where's central heating pump in relation to flow tee to underfloor?

 

[Sharp Point]

That auto bypass looks like it hasn’t been wound in, so the spring will be loose inside. Wind the black knob until the red marker travels down towards the nuts.

 

[Dtwone]

The central heating main pump is upstairs in the airing cupboard by the HW tank, the flow from the boiler comes straight up to it then tees off to the trv controlled bathroom towel radiator and then a 22mm tee down to the Ufh, two more tees to the HW tank and rads 2 port valves.
Hard to see in the airing cupboard pic, but from the red pump there's 10cm or so then the bathroom rad 10mm T off, then another 15cm then the Ufh T off, which is the 2nd from right lagged pipe going down to the floor, then lastly the 22mm from the pump goes up another 10cm does a right angle bend and then T's to the two 2 port valves for HW and rads.

The bypass valve has been wound in but I wound it right out to see if it would open to reduce pressure to the Ufh, sadly no difference wound in, I had it on 0.3 bar, or fully out, less than 0.1 bar.

 

[Sharp Point]

Can you sketch how you’ve tapped into the heating pipework that serves the flow to the manifold. Was you aware the cylinder return is always the last tee in?