Boiler overheating the house

[Paul E Collins]

My old combination boiler is misbehaving. It's a Vaillant Combi Heater VCW T3W, and it was here before I moved in, so I have no user manual.

The boiler seems to be on at full blast (many flames visible) when the house is already hot, though it's not *permanently* on and some radiators are cold while others are very hot. The thermostat, previously positioned at around 10 C, doesn't click until I take it almost to 30.

Could this be caused by air trapped in a radiator? What else might it be?

Also, if I turn off the boiler, are there likely to be any problems getting it started again later? I have basically zero knowledge of it.

Thanks in advance.

Eq.

[Osprey]

Take a look at the FAQ section on Balancing your system ... will give you a guide, proably need to open up the lockshield valaves on the cold rads.

Assume you have done teh obvious and bled any air out of them ?

[The Natural Philosopher]

Stuck zone valve most likely. The boiler will still switch off when its inlet temp reaches whatever the internal stat is set to - maybe 50-85C.

No. As to why some rads are hot - that's down to balancing. Or sludge in the system or a failing pump

No. It should be OK.

The fact that the stat - which SHOULD be stopping it all - isn't, points to the motorized valve being shagged. Happened to me once. UFH on continuously for 24 hours - it was like a ruddy sauna. The motor in the valve had failed due to a stuck valve - possibly crud - anyway took the motor bit off, worked on the valve a little with a wrench - it freed up

- got a new motorize bit and Robert was a relative.

Sort that FIRST then look at the balancing on the rads. It may be that you have TRVs on the cold ones and they are simply refusing the water as they are too hot anyway.

[The Natural Philosopher]

You have missed the main point - the thermostat is not shutting the system off.

[marvelus]

Isn't that because hes set his thermostat to almost 30?

[The Natural Philosopher]

I didn't read it that way. I read it that the thermostat which was set to 10C actually showed the house to be near 30C.

[Roger]

The message from The Natural Philosopher contains these words:

Which might point to a long standing problem. I would think even hardy types would balk at temperature as low as 10C within a house.

[Ed Sirett]

The thermostat should make break terminal 3 and 4 on the boiler, it probably isn't so replace it.

Then work out why some rads are hot and others aren't, which might be just because they are not switched on!

[The Natural Philosopher]

Normally it wont. It will normally go to a motorised valve surely..or perhaps not, on a combi.

[Ed Sirett]

On a combi the external controls are often just a t/stat and/or a timer. And the OP has stated this is a combi. All Vaillant combis and system boilers I've seen so far have a pair of terminals which when connected swithc the boiler on (and if a combi, into CH mode aswell).

One of the failure modes of the PCBs in VCW 242's which is a later model than the OPs is for the heating to run even when not demanded. I dunno if this earlier model also has this feature.

[Osprey]

If the stat is set to 30 degree, then it's unlikely to switch off !

What could be happening instead is that the boiler stat is kicking in as there is insufficient heat being taken out of the water. hence my comments to check rads have no air and could need balancing.

If it were me on a new place I would put in some descaler, open all lockshields .. get as hot as you can . switch off boiler and drain fully , refill and drain again, then refill - use inhibitor if you want to on refill.

Do you have any zone valves in this system ? ... or all lll rads on single circuit to combi ?

[The Natural Philosopher]

No, he said that it switched ON at 30 degrees, that was where it 'clicked' Whuich I took to men that teh house was at 30 degrees