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Worcester 240 - WAter coming out too hot - causing overheat saftefy cutoff?

Worcester 240 Combi RSF.

The Tempereature knob in the front doesnt seem to make any difference, as the water comes out practically boiling. even when knob is down to zero. Having a shower is starting to involve a ritual scream for someone to press the overheat reset switch on the boiler.

I've had corgi around to look at it, although given the age of this thing, it took him a while and a few phonecalls to let me know the problem.

He reckons the Gas Valve may be faulty, instead of opening to the correct amount of gas required, it seems only able to either open fully or close fully. Which means too much gas/heat than is required. This in turn seems to cause the water to overheat especially when using taps (as we need to run them slow to mix with cold water otherwise we burn!).

The Tempereature knob is sending the right voltage to the valve according to corgi.

I've looked at almost every article in this (damn fine) website to see if this has been explained before, but most problems seem to be caused by diverter/heat exchanger, so I'm hoping someone out there can give me a second opinion, I'd be most grateful.

Sorry about the long post!

Regards,

John

John M, Walthamstow
May 2007

Worcester 240 Heating, Home Heating Systems 240 Hot Combi

Hi, answer to ur problem
1) check pump over run & fan over run,
2) hot water censor
as this boiler has only heating control not hot water
After hot water use in which case the burner on full gas operator it should have pump & fan to com in to coolthe heatexchange or the over heat stat pops out

channi
September 2008

The likeley answer to the "strange thing" in your last paragraph is that when you run the cold fast at the same time as the hot you are reducing the flow rate through the diverter valve and it's dropping out of position for bringing the hot water on.
The other respondant could be right about the thermistor being faulty but if your guy is coming back he can check it anyway. Good luck.

T.h.s
May 2007

Before changing gas valve a cheaper option would be to to replaced hot water temperature sensor, after all it is this that tells the electronics to modulate the gas down as the temp rises.

h4gg13
May 2007

THanks for the reply.

1, The central Heating doesnt seem to cause overheat.

2, Doesnt seem to matter how much I open to the hot water tap in kitchen sink, the water just stays hot (I cant keep my hand under it for one second) and doesnt appeat to get colder.

Gas comes on in response to hot water demand, but its either on full burn, or not at all. Overheat only tends to happen when using hot water tap slow. Especially as mixer shower tries to balance cold water with 100c hot supply.

I've noticed another strange thing today as well, when hot water tap is on in Kitchen full and hot water coming out, if I open cold water tap on maximum as well, then the boiler stops heating water and hot and cold taps come out cold? Could the cold water pressure going to boiler cause this? Its not a problem this bit, but I'm passing it on as in case it helps understand. I'm hoping its only Gas Valve that needs replacing which is what Corgi guy said also (and he is trustworthy). I'm just hoping I dont need new boiler.

Thanks again,

John M, Walthamstow
May 2007

The temperature control on the front of a 240 only applies to the heating.
It sounds like the modu-reg on the gas valve may be faulty. The corgi registered guy you got should have been able to tell if this was the case.
Need to know if c/heating behaves normal.
Also if you speed the water flow up does it get cooler.
You should be able to see the main flame through the inspection glass going up and down in response to the water flow increase or decrease.(the correct method of testing involves a manometer and can only be done by qualified gas people... it's the law of the land)
I think you may need to call him back or get a second opinion from someone on site who can go through the correct procedure. Try looking for gas engineers rather than plumbers.
Many of the plumbing companies prefer installation work and don't neccassarily have service engineers on their staff.

T.h.s
May 2007