Query re. domestic unvented electric water heater

[ristretto]

All,

In my third floor London flat, I have a water heater that is about 20 years old. The tank does not have a separate cold water storage tank.

The plumbing is complex (illustrated in the diagram below) and is displaying some anomalies.

The tank is fed from the cold mains supply via a 22mm pipe, that joins the upper third of the tank and connects also with a thermostatic regulator. In addition to this, however, there is a 15mm pipe that joines the cold water mains, and leads, via a stopcock, to the very top of the tank. At the top of the tank is a void, accessible by a plastic inspection cover. Unfortunately, due to the way the tank is installed, it isn't possible to see what is inside. However, putting my hand inside shows there is a considerable amount of water.

Water is constantly flowing out from the plastic waste pipe, which also joins the very top of the tank, at a rate of about half a litre per minute. If I open the stopcock (which is currently leaking), the void in the upper portion of the tank fills up, and a very substantial amount of water flows out from the waste pipe.

No pressure relief valve is visible. I'd be very grateful if any of you could advise what is going on with this tank.

 

[snowhead]

That's not unvented
The tank at the top is the header /storage tank.
It has a ball valve in it, which is passing , hence the overflow.

Similar to picture, but yours is covered in I assume.
Combination Cylinders | Hot and Cold Water Storage Cylinders

 

[Chalked]

Hi
Looks like you have a thermal store. Your immersion heaters heat a cylinder that is topped up by the header tank, on the top. Your msins cold water goes into the cylinder and is heated indirectly through a coil. Hot water comes out and is then blended back with cold water, through the mixing valve.

Unfortunately, with the symptoms you are showing. It means one of two things. Either the ball valve is passing ( you should be able to see and hear this. Other than that, the coil has split! And will need the complete cylinder replacing.
I'm afraid it's a job for a professional.

 

[steadyon]

From what you describe it looks like a thermal store. The small tank on top is just to replenish the main body of hot water in the tank below. Many of these do not have float operated valves to regulate the addition of water to the small tank, but rely on you looking through the inspection hatch, filling to the level of the overflow and then manually turning off the filling tap. If the tap leaks, then water will continue to enter and flow away through the overflow.

Turn the water off before it gets to the stopcock, replace the stopcock and all should be OK. You're actually one of the luck ones, many don't even have a working overflow!

If you can't turn the water off, then, if there's space, but another stopcock between the leaky one and where the water enders the small tank.

 

[ristretto]

All,

Thanks very much for your quick responses - all incredibly helpful and educational. To be honest, I was completely unaware of the existence of devices like this, coming from Australia, where unvented cylinders have been the norm for the last 40 years or so.

That said, it's reassuring that this is not an unvented cylinder without a pressure relief valve.

It sounds, from the descriptions, that even if there is a ball valve which has split, I can solve the problem for now by replacing the stopcock, and simply ensuring the water level is kept topped up, manually.

As to the suggestion that the coil itself may have split, hopefully this isn't the case, although this isn't beyond the realms of possibility - there is a very small drip from the hot output fitting, which has accumulated some very large deposits of lime - I wonder if the coil could really survive hard water for 20 years.

That said, I imagine the test for the split coil will be when I replace the stopcock - if the coil is split, the water should continue flowing out of the waste - if not, then it won't.

Thanks again for the replies - I'll update this when I've replaced the stopcock.

 

[ristretto]

All,

Thanks very much for your quick responses - all incredibly helpful and educational. To be honest, I was completely unaware of the existence of devices like this, coming from Australia, where unvented cylinders have been the norm for the last 40 years or so.

That said, it's reassuring that this is not an unvented cylinder without a pressure relief valve.

It sounds, from the descriptions, that even if there is a ball valve which has split, I can solve the problem for now by replacing the stopcock, and simply ensuring the water level is kept topped up, manually.

As to the suggestion that the coil itself may have split, hopefully this isn't the case, although this isn't beyond the realms of possibility - there is a very small drip from the hot output fitting, which has accumulated some very large deposits of lime - I wonder if the coil could really survive hard water for 20 years.

That said, I imagine the test for the split coil will be when I replace the stopcock - if the coil is split, the water should continue flowing out of the waste - if not, then it won't.

Thanks again for the replies - I'll update this when I've replaced the stopcock.

 

[ristretto]

Thanks again all. I replaced the stopcock and everything is now working fine. The olives on the pipes were stuck tight. I was hoping to replace them, as the new stopcock is a bit shorter than the old, but ended up simply screwing it in with the old olives and nuts. Surpringly, it was leak free.

The hot water supply now lasts all day - result.

 

[ristretto]

Thanks again all. I replaced the stopcock and everything is now working fine. The olives on the pipes were stuck tight. I was hoping to replace them, as the new stopcock is a bit shorter than the old, but ended up simply screwing it in with the old olives and nuts. Surpringly, it was leak free.

The hot water supply now lasts all day - result.