Low temperature from bath thermostatic valve

[sparkybob]

Have had a thermostatic valve on my new bath for 6 months without a problem.
Over the last couple of weeks, the water coming out of the tap has been getting steadily colder, even when the thermostatic mixer is turned to its hottest. Its now only lukewarm at best but does get slightly hotter if the flow is slowed.
I have a combi-boiler and mains fed cold system which works fine throughout the house.
Ive stripped down the thermostatic valve and all the seals look OK so whats going wrong ?
Do I need new check-valves on the hot/cold feeds ?
Do I need to reduce cold water flow at mains incomer ?
Does boiler temp need altering (set to 90%) ?

All replies gratefully recieved !

 

[hi-spec plumbing heating]

whats the other hot taps like?

 

[ChrisR]

Is your cold mains pressure extra high?
Is the moixer suitable for high pressure supplies.
If it should be ok, moan at vendor.

 

[sparkybob]

All taps in house have scalding hot water.
Have 2 showers in house and they run at 50-50 hot-cold mix to provide a hot shower.
Mains pressure is very good. Not best Ive seen, but get good pressure at all taps.
Have tried closing cold water valve on bath so that flow is less, but still get only warm water.
This is beginning to sound like a boiler problem but only had that installed last year (Worcester system) !!!

 

[hi-spec plumbing heating]

All taps in house have scalding hot
This is beginning to sound like a boiler problem but only had that installed last year (Worcester system) !!!

it can't be a boiler problem if all other taps are scalding hot.
is the boiler firing up when using the bath mixer ?

 

[sparkybob]

Problem solved (hopefully)
Guts of thermostatic valve perfectly OK but when checked in housing, found all 3 seals need to seat in set positions.
The tolerance on this is very small
A grub screw in the side of the housing ensures correct location of the valve and hence the seals inside.
Although it located the valve in a fashion, the grub screw was initially loose (didnt pick up on it since it appeared to be doing job) and the valve had risen 1-2mm in the housing.
This is enough to allow the cold water (always at higher pressure than hot) to pass its seal and mix with the outgoing flow thus cooling it.
Once the valve was pushed into place and the grub screw tightened, I now get scalding water out of the bath tap.
Result
Sorry for the long reply, but thought someone might benefit, since so many threads seem to come to dead ends.
If youve got a problem, tighten your nuts someone at the factory might not have done it for you