Troublesome three port valves

[Kumquat]

In Oct08 I had my old Myson boiler (on an open vented system) replaced with a new Vaillant system boiler (on a pressured system). Ever since then I've had problems with the three port valve. Intermittently I find the heating won't come on because the valve won't wind fully into the heating only position- it will only come across as far as the mid position or slightly further and then stops, which means I only get heating if the hot water is on too. I discovered I can temporarily fix the problem by moving the lever across manually into the heating position- it will then work by itself again for days/weeks before the problem repeats. It did this a lot between Oct and Dec but then British Gas (who support the system) changed the whole valve for a new one (head and body) and since then it had been fine, so I put it down to a faulty valve until today it happened again.

Co-incidentally (?) I bled a radiator last weekend and then topped up the system- but because I can't see the pressure gauge when I'm operating the fill taps I managed to end up with the pressure slightly higher after than before (1.4 bar when cold instead of 1.2 bar). Not being at all technical all I can guess is that maybe the valve motor struggles more to go in the opposite direction to the spring if the pressure in the system is higher?

Any advice gratefully received- if I call BG out again they will tell me they only changed the valve four/five months ago and threaten to charge for another new one- and that might not solve the problem either. Please dumb it down for me though- I'm a female IT project manager and I've put everything I know technically about this into this post already!

 

[twgas]

the extra pressure wont stop the valve moving, get BG out again, if the valve is faulty they should change it again under the contract

 

[Kumquat]

the extra pressure wont stop the valve moving, get BG out again, if the valve is faulty they should change it again under the contract

So you think the most likely thing is that I am unlucky enough to have another valve that has developed the same fault after just 5 months? There's just part of me that thinks it can't be a co-incidence that for years I ran the system as open vented and the valve worked fine, but as soon as I get it converted to a pressurised system I get two valves with the exact same fault in 5 months.

 

[gas4you]

if the system water is dirty it could be 'helping' the valve to stick and fail.

Was the system flushed out properly when the new boiler was installed?

 

[Kumquat]

if the system water is dirty it could be 'helping' the valve to stick and fail.

Was the system flushed out properly when the new boiler was installed?

Yup- I watched them do it (for ages) and the water was coming out totally clear at the end.

I did have one valve that the sludge killed off a year before I had the new boiler installed- that one moved from position to position ok, just started letting water by so the rads got hot when only the hot water was on. When it's working, this valve isn't letting water by - it's just like it sometimes doesn't have the energy to get all the way across to the H position.

 

[davebgas]

You wont be charged for the part if its faulty......... but if sludge is causing the problem...... well oh dear