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

 

[Riley]

Very hard to say without seeing but if the discharge is run in 15mm it ain’t right

 

[John.g]

Even if it is piped perfectly then the TPRV will discharge water every time the cylinder heats as there is no place for the expansion volume to go in the event of a punctured E.vessel bladder or no airend air (as in this case) except that it can expand back through the cold feed?

 

[Paul Langley]

Riley - Noted and thanks. John.g - I suspect you may have hit the nail on the head. I imagine, from what has been said, that if I either turn off the boiler and let the DHW go cold or draw off a significant amount of hot water and then let the boiler reheat the cool/cold cylinder, I should see signs in the tundish of water discharging from theTPRV as the water in the cylinder expands and has nowhere to go as the airless EV is not functioning?

 

[SJB060685]

When the water is heated and there is no room for expansion (for whatever reason) then the 6bar expansion relief valve will discharge through D1 to tundish and then through D2 and out. Water should not expand back into the system through the double check valve

 

[Riley]

Hi Paul I’m going to have to stop it there mate as you are fishing for info we will not give. Sorry my friend please take the advice given before and get an engineer in with the relevant qualifications.

 

[Harvest Fields]

Surprised it went on this long mate.

 

[Riley]

Was trying to be helpful