What is causing water to come from overflow pipe

[Paul Langley]

Hi everyone,
I am new to this site so I hope I am posting on the correct forum. I just bought a house with u8nderfloor heating serviced with a Vaillant Echotec plus boiler. It is a pressurised system (no tanks in the loft) but it has a stainless hot water cylinder so it is not a combi boiler heating hot water direct from the mains. The house was built 11 years ago so I guess everything is that old. As background, I have just replaced a faulty pressure vessel in the boiler (I was slowly losing pressure and water came out when I checked the shraeder valve) and that problem now seems solved. I am a competent DIYer and not a plumber by trade though I did install a complete central heating system in my previous house.
The problem I have is about 0.5 litres of water comes out of an outside overflow pipe when the shower is used and is running for 5 minutes or so. As I did not have the house built and the majority of the pipework is underfloor or boxed in, I am uncertain where the source of the overflow pipe is. Anyone got any ideas as to what the problem is likely to be. I suspect the overflow pipe also discharges water if a hot tap in a sink is left running for several minutes but I have not yet checked this.
When hot water is drawn off from the system and cold water from the mains replenishes it, I am guessing there is some sort of pressure increase that may activate a pressure release valve somewhere in the system but this is only a guess. Any ideas and help will be greatly appreciated and thanks in advance to anyone who can help

 

[SimonG]

get a g3 unvented registered engineer

 

[Riley]

And you should not be replacing the expansion vessel in your boiler as the cover forms a combustion seal.

I would say likelihood is that your shower mixer is passing and forcing water back up into your loft tank causing it to overflow

 

[SimonG]

And you should not be replacing the expansion vessel in your boiler as the cover forms a combustion seal.

I would say likelihood is that your shower mixer is passing and forcing water back up into your loft tank causing it to overflow

Took as unvented hot water.

 

[Riley]

G3 is technically vented too

 

[SimonG]

G3 is technically vented too

smart rs

 

[Riley]

Well I was backing you up you could’ve got away with it if it wasn’t for those pesky kids

 

[Burger]

Do you know it’s definitely an overflow pipe? You said you don’t know where the source is, do you mean where it’s located or what’s causing it?

Could be as Riley said, mixer passing.

 

[Paul Langley]

CBW1982 - pretty sure it must be some sort of overflow pipe. It is a 15mm copper pipe boxed in on the inside of the house, exiting the exterior wall about 800mm above outside ground level with an elbow and 300mm of vertical 15mm copper pipe. I assume therefore it is to discharge something outside if there is a fault. The problem is it is not easy to trace where it comes from.
Rily - Can you expand on your first paragraph as to why I should not change a defective expansion vessel?
Thanks Paul

 

[Riley]

I gave you the reasoning. If it’s in the boiler you can’t touch it legally. If it’s on an unvented cylinder you can’t touch it legally. Give us a pic of the overflow