Thermostatic mixer not mixing

[jashton]

Hi, I'm slowly building an en-suite bathroom. I bought an SL3 2-way thermostatic shower mixer back in March. Once I'd built out false walls etc it was ready to fit so fitted, checked for leaks and checked hot and cold were right way 'round etc. All was ok.
I've finally finished all the remaining work and went to use the shower. the mixer isn't mixing! I have to put it to fully hot to get hot water flowing, then if I move the temp control it just stays hot until the control hits fully cold, then the water (and you can see the valve has switched as there's a quick drop in flow as it goes from hot to cold) switches to cold. If I then move the lever towards hot it stays cold until I reach maximum hot then it swaps to hot etc etc. The valve has a 43 deg antiscalding limit so at very least I'd expect it to mix in some cold on the hottest setting to prevent it going over 43.
I can't think of anything obvious. Flow rate is same for hot and cold both pumped and non-pumped so it's not balancing that's the problem.
The valve was used to check flow etc when I fitted it then sat unused for a few months until the other day. Could it be the internals have seized etc due to build of rust/crud etc?

 

[JohnD]

Just in case...

have you got the hot and cold supplies on the correct sides?

And is it tank-fed cold, not mains pressure?

 

[jashton]

Yep, hot and cold are correct and both flow at equal rate and pressure from the same tank (hot obvioulsy vi athe cyclinder but the header is the same as cold supply). Both through equal lengths of 22mm pipe.
the supply is pumped as well so presseure and flow should be very well matched.
Just spoken to the supplier and he said it could be a calibration fault. Most likely is because I didn't calibrate it! there were no calibration instructions with the valve. failing that it's completely servicable in situ so at least I won't have to rip out any tiling etc