Too much hot water

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There has been a fatality in the past, whereby an immersion heater went faulty, boiled the contents of the cylinder, which in turn heated the contents of the plastic cold water cistern above, which eventually deformed, dumping it's scalding contents through the ceiling onto a baby sleeping below. The infant didn't survive.

Plastic cisterns are not designed to withstand hot water, regardless of any DIY strengthening that may have been added, your current setup is a disaster waiting to happen. Pretty sure in saying a metal cistern needs to be used with solid fuel heating, so then should fault conditions occur, it will at least withstand the contents boiling.
 
@ JIM CROW. I'm sure it's a stainless steel boiler. As far as I remember I was told it should be stainless as it's a multi fuel stove. I can't remember the reason for this. I was told that there wouldn't be problems with corrosion because of the fittings used. Does this sound plausible?
 
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Thanks Hugh jaleak, a horrific situation no doubt. No doubt you're right about the plastic tank too but there's been no DIY adaptation. The aluminium bars are integrated into the plastic sides of the tank and it looks very robust at the moment.
 
@ JIM CROW. I'm sure it's a stainless steel boiler. As far as I remember I was told it should be stainless as it's a multi fuel stove. I can't remember the reason for this. I was told that there wouldn't be problems with corrosion because of the fittings used. Does this sound plausible?

S/steel or copper, Yes its to stop your hot water from being contaminated.
 
A vented hot water system's fail safe feature is used when the water boils in the cylinder (which should only happen if there is a fault). It then vents into the cold water cistern. Any HW system that has to continuously use that safety feature simply isn't setup properly. The water shouldn't be allowed to get to boiling point.
 
@Madrab. Thanks again. It seems that the guy who installed the boiler hasn't taken into account the output of the stove and the amount of hot water it would produce. So has ran pipework straight to the cylinder and back with no way of limiting the temperature of the water. The stove is the primary source of heating in the house and at times can be on for 4 or 5 days and nights in a row. I had hoped that installing a permanently open radiator into the system would help reduce the amount of hot water to a point where the cylinder wouldn't need to vent into the cold tank. I also thought that it might be relatively simple and work using gravity in the same way it does just now. I thought if I kept the pipework as short as possible the hot water from the boiler would pass through the radiator on its way to the cylinder. Or placing the radiator between the cylinder and boiler would have the same effect. Is this just not how it works? Would I need a pump to get the water the few extra metres? I also planned to divert the cylinder vent into the overflow to stop it heating the cold water.
 
I know of a run away immersion heater which caused the situation you have - the plastic cold tank leaked and caused damage. The cold tank may not burst explosively but the hot plastic will flex causing leaks where pipes are connected etc. You need to control the heating of the hot water in the first place.
 
Radiator would need plumbing in on the gravity circuit, you cannot rely on a pump, as in the event of a power (or pump) failure, you immediately revert back to a dangerous situation.
 
The other point to be aware of is you're wasting a lot of energy- which for most of us has a significant economic cost.

Your system has not been designed, it has been lashed together using whatever someone had on the van that day.
The header tank for the hot water should be metal with a metal ballcock. All pipework on the system should be metal with soldered or compression fittings (exception- Hep2O is rated for occasional overtemperature incidents)

There should be an over pressure/over temperature valve on the DHW side, usually near the top of the cylinder, with metal pipe dumping output to waste (not via the gutters unless they are metal)- this to prevent pressurisation and subsequent explosion in the DHW pipework.

There should be a heat loss radiator on the gravity circuit to the cylinder to prevent boiling and to maintain the thermal syphon (when cylinder temp is near boiling circulation is currently reducing due to lack of temperature differential so boiler is more prone to boiling)

Longer term you'll have a more useable system if you have the space to install a thermal store- this will capture the heat currently being wasted and store it til you need it (allowing you to reduce your burning hours while maintaining current heating hours).

Find someone local with suitable qualifications (HETAS for solid fuel) who does wet heating systems and ask them to come and have a look at what you've got. Quickly.
 
@JohnD. Thanks, I do recognise all those warning signs. I am going to disconnect and remove the retrofitted back boiler. It looks like whoever installed the original electric immersion system did so properly, the cold tank is on a suitable base and it and the cylinder are above a hallway separated from the bedrooms and living room. So is it still the case that a plastic tank is acceptable in an electric immersion system despite the horror stories.
 
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