Boiling hot water!

[Giddypants]

My hot water is literally boiling! Noticed it last night and again today whilst running the bath.
The thermostat is normally on 65* so turned it down to 50 but its still boiling, any ideas what is wrong?

[PigletJohn]

have you got a hot water cylinder?

is it heated by a gas boiler?

is there an electric immersion heater?

Can you see one of these near the boiler or cylinder?

[Giddypants]

Yes hot water cylinder, with a gas boiler, and electric immersion heater (which is never on) I can't see one of those valves but there is a big red round tap, at one side of the cylinder. To the other there is a white box, but I can't get the cover off to see what's in it.

[Giddypants]

Boiler about 2 years old

[Giddypants]

And in the garage

[Giddypants]

White box looks like this has power to it

[PigletJohn]

If the boiler is fairly new, the 3-port valve in my link may be inside the casing.

There may be 4 large pipes coming out of the boiler, probably from the top, which are (should be) inside plastic foam lagging.

Two of these pipes will go into the cylinder, a flow and a return. After you have run a bath, these pipes will become hot as the boiler pumps heat into the cylinder. The flow will be hotter than the return.

When you put on the central heating, the other two pipes will heat up as it pumps heat to the radiators.

You should be able to detect which is which by observing when they heat up. You might likje to stick labels on then for next time.

I suspect that when the CH is running, heat is leaking into the cylinder, and when the cylinder is heating, heat is probably leaking into the rads.

It will happen even if you leave the HW set to "off" though it may take a long timne to heat the cylinder.

If you turn the CH off at the timer or room stat, and run a bath, feel if the two CH pipes, or any of the radiators start to warm up at the top.

Because the weather is cold now, the VCH will be running a lot, and I suspect that excess heat is leaking into the cylinder.

In this case it is probably the 3-port valve that is at fault. If as i suspect it is inside the bloiler casing, you will need a gas-safe boiler engineer to replace it.

There is a faint chance that the cylinder stat is at fault, but these go wrong approximately never, and 3-port valves go wrong rather more often.

[PigletJohn]

Oh you've found the 3-port valve. Same as the one I linked to but a different brand. Drayton and Hoineywell are the two best brands.

One of the arms will go to the cylinder, and one to the radiators.

I suspect that the HW arm will get hot when the CH is on, and vice-versa. As it is outside the boiler case it can be replaced by a competent plumber, it does not necessarily need a boiler engineer since the gas train is not involved. But if you have a boiler contract or a reguler service engineer, ask them.

[Giddypants]

Many thanks for your reply, have turned everything off now to let it all cool, I'll do some investigating ing the morning, approximately how much are we looking at paying, have been ripped off before.
In Yorkshire so at least not London prices!

[PigletJohn]

say, an hour's work plus £100 for the part? I expect he will charge the retail price rather than the screwfix price. Specify that if it needs a new one, you want either a Honeywell or a Drayton. Tell him the brand and model number of your old one, because it may be that only the motorised electric head needs changing, which will take less time. If he charges for the whole valve, say you want the brass part as well, since you've paid for it. You can keep it as a spare (they do wear out in time)

[Giddypants]

Brilliant, thank you xx

[Giddypants]

Just to let you know that you were spot on with the problem, Plumber came put today to fix.
the valve which was stuck open had lasted since the house was built in 1989! So was due for a change, was £150 in the end so was quite pleased, thank you again x

[PigletJohn]

glad to know it's fixed

[photoretoucher]

Am I the only one that envies PigletJohn's wife

[PigletJohn]

nah

you'd become habituated.