Central heating boiled

[tonyjeffs]

My mom's central heating is 25 years old, but has been working well. Yesterday I turned the hall thermostat down from 30 to 15. I left. When I revisited later in the day, she told me that the heating had gone incredibbly hot, started making worrying noises, and the water in the immersion cylender had sounded like it was boiling and about to explode. She turned it all off.

What could the fault be? If the stat in the hall failed, which way would it fail? would the heating go off, or would it go to maximum output? Could it be a relatively simple matter of replacing that thermostatic unit?

...a pity because a friend, a retired heating engineer, had visited an hour before the event!

Thanks

Tony

[John]

Could it be that the thermostat is only connected to the pump. What was the setting on the boiler thermostat - had someone turned that up high?

I question why you would ever put a room stat at 30 - as such a temp will be unachievable. Surely 22 should be the max. Likewise 15 could be a bit chilly.

tell us more about the boiler

[Harry Bloomfield]

tonyjeffs wrote on 21/02/2009 :

Two suggestions....

Boiler thermostat turned up to max and max is higher than boiling. Boiler thermostat is shorted out.

[Dave Plowman (News)]

Sounds a bit strange to me. On older systems it was common to set the boiler temp to max.

Certainly something faulty.

[John]

If the DHW is gravity operated then it is possible that the primary pipes are furred up and not letting the heat from the boiler get to the cylinder fast enough - causing kettling (maybe not boiling)

[tonyjeffs]

A helpful development.. My Father in law tells me he had a similar problem with a similar boiler some years ago. It turned out that the pump, which is outside the boiler on a readily accessible run of 22mm copper pipe had jammed, so the water wasn't circulating, so what was in the boiler itself was bubbling. Could be that. Easy to access my mom's pump. nudge it with a screwdriver, or replce it if necessary. It looks about 5 years old to me; not original.

I think I might be getting somewhere - time to get some tools out.

Tony

[Dave Plowman (News)]

Assuming an old basic system, the wall stat simply switches the pump - so the pump stopping shouldn't cause the boiler any problems.

[geoff]

In message , tonyjeffs writes

there's your problem ...

p.s. he's a fitter, not as engineer

[tonyjeffs]

Hi Dave mainly

I don't yet understand the circuit. There is what looks like a thermostat linked to the immersion heater, and this is wired to a pump. This is all in a first floor cupboard I don't understand how switching this pump on and off would protect the boiler (which is downstairs). gravity and convection would help, but I would've thought that for adequate protection agsinst overheating, there must be forced circulation while the gas is burning. ...Not doubting you at all... I just need to understand it for myself before upping tools.

I'm away until Thursday, so will mull it over till then. My mom is ok, since she has electric hot water from the immersion, and has a decent gas fire. Central heating - who needs it anyway!

Thanx again

Tony

[The Natural Philosopher]

I think you need to work out what type of circulation system you have.

Thermostas are almost NEVER connected to pumps directly.

The normal way things are, is that timers switch to thermostats, and via the to motorised VALVES which open and close water circuits, and having opened them switch on a circulation pump for the whole boiler system. That runs as long as there is call for heat for anything the boiler needs to control. Then the boiler will fire up until the RETURN temperature of the water, as seen by the boilers own thermostat in conjunction with its heat setting, gets to whatever its set to.

There is no way a boiler should boil as such. The only way it ca is if the circulation stops, and so there is no return flow at all: then it will fire until the heat exchanger nearly boils, at which time the return flow sensing will get got by heat travelling backwards and cut it.

TBH what the user is reporting makes little sense..

[ARWadsworth]

But that is only normal for systems installed in the last 25 years. Older systems used a gravity loop for the HW and a pump for the CH. The thermostat was connected directly to the pump in these systems.

It will boil if the boiler stat if shorted out or faulty.

More info on his system is needed. An immersion heater linked to a pump and to a thermostat seems very odd and most unlikely.

Adam

[John]

Is it a Back Boiler? What is the Boiler thermostat set to?

[Dave Plowman (News)]

That's bollocks. On the most basic system - with 'gravity' hot water - that's exactly how it works. With Y plan it still does - but you switch between two stats - one for the heating and one for the hot water.

[Dave Plowman (News)]

The immersion heater is entirely separate from the heating system and can be ignored. It has its own internal thermostat. Or rather that's the case in 99.9% of setups.

If there is an electrical thermostat fitted to the cylinder you'll likely have an S or Y plan system. The FAQ gives details.

No. On older systems the boiler can be totally self contained. Ie it cycles away to itself maintaining a set water temperature. But you appear to have a more sophisticated system which should only fire the boiler when there is a call for heat from either the heating or hot water. However, the boiler thermostat is still needed to prevent the water in that overheating. In other words the boiler can't dump all its output into the heating or hot water circuit at all times so needs a way of controlling that. A modern boiler achieves this by turning down the flame - older ones by switching it on and off.

Anyone not sat in front of the gas fire? ;-)

[Tim Downie]

Sounds like a gravity HW system with a pumped CH ring (controlled by the wall stat).

The boiler stat is either stuck of set way too high but that's been masked by the CH system carrying away heat fast enough to stop the boiler overheating. By turning the wall stat down, the boiler is now being controlled (or not as the case may be) by the boiler stat.

Check the boiler stat (which will probably be inside the boiler cabinet). Does it click when turned? Does the boiler shut down when you turn it down?

If it doesn't click or the boiler doesn't shut down then the boiler stat is faulty. It could just be set way too high but I would have though that aby boiler stat ought to shut off a boiler before it boils (literally).

Tim

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "ARWadsworth" saying something like:

Probably the stat/pump feed is taken from the immersion supply.

[RobertL]

People are discussing the boiler overheating, but it's the hot water cylinder that is making the boiling noises not the boiler.

I would suspect a faulty immersion heater thermostat.

Robert

[Tim Downie]

The immersion heater wasn't in use at that time. He's only been using it since the boiler boiled.

Tim

[John]

From what I remember when we had a coke fired Aga type of thing the boiling sounds did come from the cylinder if it got a bit too hot - however - it wasn't a proper indirect cylinder and we got brown water if it got too hot.

[The Wanderer]

It's an old system so you can pretty much forget how modern systems are wired up.

What exactly happened? Is the boiler turned on all day? Were the rads getting very hot? Was it just the cylinder getting very hot? Without that info it's not possible to make a sensible diagnosis of the problem.