Megaflow problem

[cobley]

I have a Megaflow cylinder which has been installed and working for several years. It has recently been dripping from the overflow so I have followed the instructions and drained it sufficiently to reinstate the air gap which has solved the problem for a while. Today, however, I needed to shut off the hot water (to change a shower valve) and turning off the mains input to the cylinder didn't stop the hot water. I turned off the mains and let the cylinder drain down until the tap stopped which was OK. When I turned on the mains cold water back on (whilst also leaving the cold water to the cylinder closed) the cylinder appears to have refilled! Can anyone explain where this water is coming from? The cylinder also appears to be overfilling as, even when I turn on a nearby hot tap and open the cylinder vent (with the cold input to the cylinder closed) water drips out of the vent as it rushes out (which it didn't used to do). Anyone have any ideas what is going on here, I'd be very grateful...

 

[Deleted member 2053]

If turning off the cock feeding the cylinder didn't stop the hot water and allowed the cylinder to refill when the mains was reinstated then it sounds as if the cock to the cylinder is faulty.

These cylinders need to be serviced annually by a qualified engineer. You can legally carry out your own gas work if you are competent, but you are not allowed to touch your own unvented cylinder unless you have a G3 qualification so in that respect these things are considered more dangerous than gas! If you blow it you will find that you are not insured for the consequential damage.

A proper service in the last twelve months would have not seen you experiencing the problems you are facing today.

 

[justlead1]

Hi, Its probably back filling via a mixer tap/shower on the system.

 

[cobley]

Grahamep, Thanks for you reply. I am certainly not intending to service this megaflow myself but I do want to understand the problem so I can have a clear conversation with whoever I can get to do the job. If I know what is wrong I reduce the chances of being taken advantage of by someone who is charging by the minute for their advice.

I also want to understand the problem so I can properly shut off the hot water so I can do the simple bathroom shower job....

I had thought that the stopcock must the the reason why the cylinder was refilling but I have turned it on and off and one can hear the water start and stop flowing. I had therefore assessed that it was probably OK, hence my confusion.

Justlead1, thanks too for your thought. This sounds pretty likely. We have a couple of shower mixers in the system. Can I ask though would they cause this refilling when the neither of the showers have been used? If it happens when they are both switched off is there anything that can be done to stop it?

Thinking further about this though, I guess it could well be a problem with the valve that I want to replace anyway. It had been sticking in the on position but I guess it could be causing a leak between the hot and cold systems too...

Thanks for your help.

 

[Deleted member 2053]

Had there been that much flow across a valve then I would have expected you to complain that the water coming fom the hot tap was cooler than it should have been. Still it is not impossible that this is the fault. There isn't anything else on the megaflow that should cause this problem apart from the input stop tap.
FYI the service schedule consists of checking the overall installation, checking that no pirate parts have been fitted, checking the operation of the mechanical safety devices, checking electrical devices and settings, cleaning the strainer, and resetting the air gap. I doubt that anything there is complicated enough for you to have the wool pulled over your eyes by an unscrupulous tradesman. As if such people exist!

 

[justlead1]

Hi Why not disconnect the valve supplying the unit, and check if it shutting off visually ?

 

[davetheplumber]

Had the same thing & it was a thermostatic shower valve letting by in the closed position.