Immersion heater tragedy

[Penners]

I imagine most people have read the horrific press reports of the baby scalded to death due to a failed immersion heater thermostat. A terrible business.

I thought it was worth raising here, because a substantial number of period properties have probably got the type of water heating system that caused this disaster. That is, a vented system with a header tank in the attic, and a hot water cylinder with an immersion heater.

If you have this type of system, then the potential (however remote) for this same problem to occur may exist.

The immersion heater is controlled by a thermostat. If this fails, and if the immersion heater has no overheat cutout, then the water in the cylinder can carry on being heated until it boils. The boiling water can then migrate up the vent pipe and empty itself into the header tank, which just goes on getting hotter and hotter. If the header tank eventually fails, then scalding hot water can cascade down into the house.

I don't to be alarmist about this - it's an extremely rare occurrence. And it can only happen if the immersion heater is not fitted with an overheat cutout. But since period properties are possibly more likely to have old-fashioned systems and equipment, I felt it was worth mentioning.

However remote the risk, I reckon it's worth checking your immersion heater (or if you're not familiar with the equipment, having it checked by a plumber) to make sure that yours does have a cutout.

I do have one question for any electrical/plumbing experts who read this: if the immersion heater is not fitted with an overheat cutout, is fitting a new immersion heater the only solution? Or is there such a thing as a fail-safe thermostat that will do the same job?

 

[Cloudscape]

I've heard more than one example of ghastly things that have happened with back boilers with the same effect - the water has boiled out of the tank, up into the header tank. It happened to a neighbour of mine. Scalding water cascaded through her bathroom ceiling, and brought some of the ceiling with it. A thermostat wouldn't prevent this though, would it? As the owner of a back boiler), I wonder about leaving the solid-fuel range burning at night, or if I am out during the day.

 

[Penners]

Do you mean a back boiler to a solid fuel fire? If so, I can't see how a thermostat could be used in such a system. It's an electrical device, and I imagine your back boiler has nothing electrical for the thermostat to control.

 

[Cloudscape]

That's really what I mean. How can you control a back boiler, considering that back boilers (possibly when incorrectly used, or incorrectly installed) have been known to cause tanks to boil over - exactly the same as an unregulated immersion heater. I know I'm clouding the issue here, but I really meant to extend it. Three things act on my tank - an immersion (currently defunct), the central heating boiler, and two back boilers.

 

[Penners]

I do have one question for any electrical/plumbing experts who read this: if the immersion heater is not fitted with an overheat cutout, is fitting a new immersion heater the only solution? Or is there such a thing as a fail-safe thermostat that will do the same job?

I just want to ensure that this question doesn't get lost.

 

[Schoolmarm]

Our immersion heater wasn't working so we replaced it a couple of years ago. The old one was very short and could never have worked very well.

Anyway it was a quick, easy and cheap job to do.

I also would advise people to do it. It cost about 11 quid.

 

[Aidan]

I do have one question for any electrical/plumbing experts who read this: if the immersion heater is not fitted with an overheat cutout, is fitting a new immersion heater the only solution? Or is there such a thing as a fail-safe thermostat that will do the same job?

The best technical description I've seen so far was the one here, No. 36;

http://www.diynot.com/forums/viewtopic.php?t=37170&start=30

I was fairly astounded by how frequently the media got it wrong and how wrong they got it, especially the BBC.

You don't have to change the immersion heater, just the thermostat. £5ish I'd think, DIY. It sits in a pocket (a blind tube) under the immersion heater cover and ( after isolating the power and disconnecting the wires, of course) it can be lifetd straight out. They come in different lengths.

 

[biffvernon]

My experience of immersion heaters is that they only last for a few years. The last time I replaced ours the new one came with a cut out and I was told they were all like that now - so I guess this is a problem that will gradually lessen as old stuff gets replaced with new.

It's those government nannies keeping us safe again.

 

[chuckey]

For the neurotic, fit an external thermostat on the tank (removing any insulation first), wire this in series with the heater. Set the external thermostat to a higher temperature then the heater thermostat. If your hot water then suddenly gets hotter it means that the heater thermostat has failed on.
Frank

 

[joatex]

I am puzzled why the overflow from the hot water cylinder has to go to the header tank. In New Zealand the overflow pipe from the hot water cylinder is taken straight up through the roof so it's common enough to see hot water and steam emerging from the pipe.

 

[Aidan]

For the neurotic, fit an external thermostat on the tank (removing any insulation first).............

They're liable to fall off, or someone will fiddle with the settings.

The main feature of the 2-stage immersion thermostats is that they're manual reset, you have to push the button when they trip, so that should alert someone to the fact that there's a ffault.

The system you suggest is auto-reset, it could be turned off and will boil the water again when it's turned on again.

Besides which, new immersion thermostat, £4.50 (contains both primary control and secondary manual reset thermostats) ; new clamp-on thermostat £8.50.

 

[Cloudscape]

I was wondering about that. Maybe there does need to be a failsafe, such as a valve or vent where hot water and steam can escape.

 

[Aidan]

I am puzzled why the overflow from the hot water cylinder has to go to the header tank.

Liable to freeze.

Run-away vented hot water system = hot water in the cold tank usually.

Run-away unvented system (or vented system where the vent is blocked) = big explosion, 4oz of HE equivalent it is reckoned, capable of taking the side out of a house.

See http://www.waterheaterblast.com/

That is a small unvented 12 gallon heater, deliberately rigged to burst.

Modern plastic cold water storage tanks will tolerate boiling water if properly installed. The recent case involved a 30 year old tank. In the fatal accident in 2002, the tank was only partially supported and failed when the hot water softened it.

 

[Aidan]

I am puzzled why the overflow from the hot water cylinder has to go to the header tank.

Liable to freeze.

Run-away vented hot water system = hot water in the cold tank usually.

Run-away unvented system (or vented system where the vent is blocked) = big explosion, 4oz of HE equivalent it is reckoned, capable of taking the side out of a house.

See http://www.waterheaterblast.com/

That is a small unvented 12 gallon heater, deliberately rigged to burst.

Modern plastic cold water storage tanks will tolerate boiling water if properly installed. The recent case involved a 30 year old tank. In the fatal accident in 2002, the tank was only partially supported and failed when the hot water softened it.

 

[Ian Bond]

How likely is it that the vent would get blocked - mine's fitted with a tundish just after the temp and pressure relief valves.

In the highly unlikely event of the external vent opening getting blocked, the hot water would pour out of the tundish. Of course this would be very bad, but not an explosion?

I'm worried now!!!!

 

[Aidan]

How likely is it that the vent would get blocked - mine's fitted with a tundish just after the temp and pressure relief valves.

In the highly unlikely event of the external vent opening getting blocked, the hot water would pour out of the tundish. Of course this would be very bad, but not an explosion?

I'm worried now!!!!

You have an unvented system, there is no vent pipe. Don't confuse it with the vented systems being discussed here.

There are two safety devices (thermostat and manual reset thermostat) on the immersion heater supply to stop the water boiling.

If the water boils, there is a pressure relief valve and a temperature & pressure relief valve which would discharge the water/steam long before the cylinder burst. The tundish is on a drain from the two safety valves.

The chances of all those devices failing is infinitesimal. It's never happened in the Uk. It's happened abroad, usually after someone's been a-tampering. The only cylinder explosions (I know of about 3, one 40+ years ago) in the UK have been vented systems where the vent was blocked.

 

[Ian Bond]

Thanks Aidan, I am reassured.

Apologies for confusion with the vent - I meant of course the pipe leading from the relief valves and tundish to the drain.

 

[Penners]

You don't have to change the immersion heater, just the thermostat.

Thanks, Aidan, that's just what I wanted to know - your link was very informative.

It's probably worth adding, though, that some people may have immersion heaters that have an overheat cutout, so they will be safe, even if their thermostat doesn't have one.

 

[Penners]

Our immersion heater wasn't working so we replaced it a couple of years ago. The old one was very short and could never have worked very well.

Anyway it was a quick, easy and cheap job to do.

I also would advise people to do it. It cost about 11 quid.

"Quick, easy and cheap"? well, yes, if you've got a well-designed plumbing system.

Ours has been added to and modified numerous times - sometimes well, sometimes badly. As a result, whenever you have to drain down the system, it's a b*stard of a job to get it refilled without airlocks. It usually takes me the best part of a day, sore knees (from crawling around in the attic), wet arms (from stuffing a mains-fed hosepipe down the feed from the header tank to blow out airlocks) and a sore head (as in "bear with a ...").

 

[Schoolmarm]

Oh dear.

Well ours has been messed with/modified a lot too. Most alterations were to improve things but it is now such a jumble of pipes and stoptaps that I find it hard to remember what does what.

The latest problem we have is that when the heating comes on, red hot water splurts out of the overflow pipe onto a porch roof then onto the lawn. Not ideal and I am not sure what to do.