In article op.wlif25ksxafqef@desktop, MarkG writes

Well it's suitable for both hi and low pressure systems, but clearly can
get freaked out by large differentials in pressure. They didn't install
it, they just supplied the valve, and it's not even theirs, it's a stock
concealed thermostatic triple valve that every man and his dog seems to
use these days, they just do their own knobs and faceplates to suit...


Been on a trip to Plumbase and picked up a 15mm pressure reducing valve on
the cold side.

http://www.ebay.co.uk/itm/Honeywell-...Valve-PRV-for-
15mm-or-22mm-1-5bar-6bar-/261101591356?pt=UK_Home_Garden_Hearing_Coolin
g_Air&hash=item3ccade1b3c

or a check valve on the hot side to stop the cold going back down.

http://www.ebay.co.uk/itm/BRASS-SING...MM-NON-RETURN-
COMPRESSION-/250688583906?pt=UK_DIY_Materials_Plumbing_MJ&hash= item
3a5e3438e2

Not sure what's going to be best (opinions???), but have both options
available.. The other option I have considered is that my boiler might
not be pressursing the hot side correctly, and it's only become a problem
now, so I guess I need to pressure check both sides to see what I have...

I am a bit surprised by your problems. One of the main benefits of combi
boilers is that you get balanced pressure of both hot and cold water
(the hot water comes from the mains too, it just passes thro a heat
exchanger).

A quick look at the installation manual for the boiler shows that it is
suitable for use on mains supply of up to 10bar without the use of a
pressure reducing valve so I wouldn't expect one to be fitted to yours
(it would be on the cold feed to the boiler).

That said, the symptoms you describe do suggest back pressure on the hot
side, reducing or stopping flow and causing the boiler to shut off.

Have a look to see if you can see a pressure reducing valve before the
boiler as that could be an explanation. The manual I looked at did not
show an internal pressure reducing valve.

You also say the mixing valve has a maximum pressure of 5bar so there
may be an issue there too but the bad news is that if you have over 5bar
on both hot and cold then you may need to pressure reduce both.

I don't think a check valve on the hot side will be of benefit as if the
fault is a cold to hot pressure differential then the valve will be held
closed by the pressure diff, no water will flow end the boiler wont
fire.

As an experiment, try opening a hot tap in the bathroom a bit then turn
on the shower valve. If the flow from the hot tap increases then you
have back flow cold to hot.

HTH
--
fred
it's a ba-na-na . . . .