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Discuss Isolate cylinder from system boiler in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Hi plumbing hives

I’m wondering if someone could provide some advice (please!!!!!). I have an issue that my system boiler pressure is constantly over pressurising. The boiler manufacturer have sent an engineer out he says the boiler is fine (it’s only 15mths only). The suggestion from the boiler engineer is is that the cylinder tank (Velux TFF 300) core has likely failed, pushing water bank into the central heating system. It’s not the feed loop into the system. I’ve removed that and the valves are working fine. I have a plumber coming on Monday, but wanted to isolate the tank from the system, so we could at least get some heating on in the meantime. Is this possible? I’ve tried turning the valves off in the cylinder cupboard that I thought controlled the feed/return to/from the boiler, but the pressure keeps rising. Any ideas? Any advice would be gratefully received.
 

Timmy D

Gas Engineer
Messages
464
This is what it looks like to me.

1. Return from cylinder.
2. Primary flow from boiler.
3. Bypass valve from flow to return.
4. Cylinder flow isolation.

So if you close 4 and 1. This stops the flow through the cylinder coil. However, flow/pressure will still be going through the bypass. You will also need to close 3 right down and hope it doesn’t pass.

If all valves work and your coil theory is correct, the pressure should stop going up and you can use the heating.

If it continues to go up, a valve or bypass could be letting by. If this is the case, I’d then close 2. This should then stop it but you won’t be able to get heat to rads.

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Timmy D

Gas Engineer
Messages
464
Hi Timmy. I tried it, but it didn’t work. I’ve now closed number 2 and also the one at the back that wasn’t numbered. The pressure still rises.
Not much else I can suggest without being there I’m afraid.

Does the boiler work at all? If so, turn the boiler off and let the system get cold. Then with the system running, can you feel heat passing through any of the valves/bypass?

Edit: you may not be able to fully close the bypass.
 
Not much else I can suggest without being there I’m afraid.

Does the boiler work at all? If so, turn the boiler off and let the system get cold. Then with the system running, can you feel heat passing through any of the valves/bypass?

Edit: you may not be able to fully close the bypass.
Thanks Timmy. I’ll give it a whirl tomorrow. The boiler works, the only issue is that the pressure rises above 3bar. I’ll let some water out of a nearby radiator, fire it up and see if I can feel anything.
 

moonlight

Plumber
Gas Engineer
Messages
925
Have you tried draining the cylinder? If there is no water in, it can't leak in to the heating. You might find then the pressure drops instead. You will then have your answer.
 
Not that I’m aware of. It was a new build 13 years ago. There’s nothing in the loft. I unscrewed the filling loop and there was no water coming through. There’s three isolating valves on the loop next to the boiler, so no shock really.
There’s not another filling loop somewhere? Maybe in the loft where the f&e tank was.
Hi Timmy. I turned the stopcock of to the house this morning with the valves in the cylinder cupboard reopened. Reset the pressure in the system by letting water out through a radiator.
The pressure seems to rise with the stopcock open and drop with it closed.
Any suggestions what it might be?
 
Last edited:

Timmy D

Gas Engineer
Messages
464
Hi Timmy. I turned the stopcock of to the house this morning with the valves in the cylinder cupboard reopened. Reset the pressure in the system by letting water out through a radiator.
The pressure seems to rise with the stopcock open and drop with it closed.
Any suggestions what it might be?
Sounds like you found your answer. The only mains possible connections to the heating circuit, are the filling loop (proven ok) and the cylinder.

As it’s a mains cylinder, I assume you’re gonna get a g3 qualified plumber in to swap it. Might be worth checking with manufacture if there’s a warranty.

Final test would be to open everything up, let the heating system get over 3 bar. Close stopcock and run a hot tap to see if pressure in heating system drops.
 

king of pipes

Esteemed
Plumber
Gas Engineer
Messages
5,263
Is your hot water discoloured ? Has the expansion vessel or vessels been checked ? failure of these could cause your system to over pressurise ?. Isolate and disconnect the filling loop to check it's not passing water into the system ? . Kop
 

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