Boiler hiccups boiling water

[DIYPaul]

I have a gas boiler / hot water cylinder / f&e tank setup. Quite frequently now after the boiler's been running for a while, pipes start violently rattling, and soon after boiling hot water starts spewing out the overflow. So, plenty of hot water 🙂, the rads are hot, and everything else seems fine.

Initial thoughts were to give the system a powerflush. Have done this thoroughly, but hasn't resolved the problem. What else is it likely to be? I thought it's unlikely to be the c/h pump if the rads are all hot. Could it be a faulty stat on the boiler? Any other ideas?

 

[Puddle]

Hi Paul

Think you have probably hit the nail on the head with a faulty boiler thermostat,get it sorted asap,not doing your safety or the boiler any good !

 

[bennygas]

quite agree with puddle. just out of interest what boiler have you got, and what've you got the boiler thermostat set at - has it been adjusted recently?

 

[glenno1]

Agree with above posts,first port of call cylinder stat

 

[DIYPaul]

Hi all, the boiler is a Potterton Kingfisher Mf, installed in 2004. I've got the temperature control set about half way up. I did adjust it down when the problems started, should I try putting it down further?
Have checked the cylinder stat by checking it cuts out when up to temperature with a multimeter.

Is changing the thermostat on the boiler a DIY job, i.e. clip the old one out, push the new one in? I've no desire to start tampering with the gas side of things; leave that for the experts.

 

[bennygas]

It was only on the off chance that the boiler stat had been tweaked up that I asked. If it's set pretty low, I think chances are there's a fault there that has its origins in either the production of too much heat or failure to dissipate this heat - as previous posts and your own train of thought have suggested. things like the boiler thermostat, pump or the effects of corrosion debris can manifest themselves as symptoms similar to yours (just while i'm thinking of it, are the downstairs rads hot too?). that said, other appliance faults could also concievably cause similar symptoms (e.g. excessive bp/gas rate, or a gas valve that's slow to close) and in the interests of safety, and to rectify the fault, the source of the problem needs to be established by someone competent.

let's say it was the boiler thermostat (which does sound like a very good place to start looking), well, the physical act of changing the thermostat is pretty straightforward, but its the checks that come after that which require competence - see reg 26(9) of the GSIUR. If you're not competent to do it, don't do it - get someone in. No offence, but if you're thinking "well, am I competent? it should be quite easy, right?", you've kind of answered the question for yourself - and under the regs, competence doesn't just apply to the bits of an appliance that deal with gas, it applies to any work on an appliance. if you get a registered gas engineer in, sure it'll cost you the going rate, but at least you'll know the diagnosis of the fault is probably correct, but moreover that any work is done safely.

 

[mountainman]

Would save time and money too by not faffing about powerflushing when your symptoms pointed elsewhere.

 

[Bernie2]

These boilers seem to have lines of defence in case of overheating? It has or so the 40 manual says an overheat stat. If that is functioning properly it should be cutting out before boiling water spurts out the vent. I assume all the stats and their relationship to each other has been checked out? As is the actually temperature of the water and whether the vent has enough head over the f&e water level to accommodate the certain amount of expansion that occurs in it?

 

[mountainman]

Presumably the system has been working correctly and this is a recent fault so should rule out design issues.

 

[DIYPaul]

It's been happening infrequently for the last couple of years, but more recently started happening quite regularly, hence I decided to start doing something about it.

Since the powerflush (which seemed well overdue looking at the state of the water that came out), it actually seems to happen even more!

 

[JCplumb]

If it's a plastic f/e tank I urge you to turn the boiler off and get an engineer in.
If hot water is coming out of the overflow as you said then you have a potential disaster, this situation has caused serious injury and death to people in the rooms below as a result of the plastic tank softening when hot and spilling its contents through a ceiling.

 

[bennygas]

good point jc - i only scanned the OP and must've assumed diypaul ere was refering to the vent - but if he's on about the overflow proper that does give it a bit more of a sense of urgency. the tank/s that split that time - that was a big tank due to a faulty immersion though wasn't it?

 

[JCplumb]

It was an issue with the stat, but if 'boiling' hot water is coming out of the overflow - who cares what the reason is, turn the thing off and get a plumber in 🙂

 

[Bernie2]

Its simple really. If it is overheating and it is not a design issue then the stats are not working properly. They control the heat input.

If it has been happening on and off for years, then one has to wonder if it is not a design issue. Lets be fair if the water up the vent expands as it should and if the stats had been kept low then the expansion in the vent may stop before water vents out. If the stats are then turned up the water expands further and then vents out.

It is of course dangerous. A plastic tank turns into a lump of melted plastic if kept hot for to long.





If there is an electric immersion heater in the cylinder check that out as well.

 

[DIYPaul]

Thanks everyone for your replies. I'm going to call in a plumber to get this sorted once and for all.

I'd still like to understand a bit more how a faulty stat on the boiler could cause this. I mean, surely if the stat on the hot water cylinder is working, then it tells the boiler to cut out regardless of what the boiler stat is thinking. And the boiler itself has an overheat sensor, which would also cut in before things start boiling over?

Could another explanation be that the c/h pump is not running fast enough, and that it's not pulling the hot water away from the boiler fast enough?

 

[mountainman]

The pump only circulates the water around the system.
If the pump was running slowly then the boiler could overheat but before that the 'stat and overheat device should operate.