EXPANSION TANK BOILING - PLEASE HELP!

[MATTHEW EYNON]

I moved into a 1980's house a few weeks ago. The house has a Ideal Heat 30 boiler in kitchen with cylinder in airing cupboard and tanks in loft.
Under boiler there are 3 motorized valves, 2 for heating and 1 for HW.
The 2 heating valves are wired together into stat (as there arent seperate zones for heating).
When running heating, once turned off, heated water is bing pushed up vent pipe, into expansion tank, causing it to oveerfill and pour out of overflow. This happens everytime, a few minutes after heating turned off.
This then causes air to be drawn into radiators meaning daily bleeding!
I have checked the wall thermostat (its a new Tado I had fitted by qualified plumber) and that turns boiler on and off effectiovely for hot water and heating.
I have checked cylinder stat and that turns boiler off too as it should.
I have had 3 plumbers / heating engineers out so far and they all have no idea what is going on and are relulctant to commit to anything.
I have 2 children, 4 and 18 months - the 18 month old sleeps under the expansion tank, so I cannot put heating on as it will be a risk! Its going to get cold soon and I need fixing - does anyone have any ideas of where to look or how to start? Any guidance will be appreciated. I have contacted 10+ other trades but they are too busy until November or New Year!

 

[footprints]

Sounds like it might be overheating worth checking if the boiler has a pump over run and if so is it working, or was it wired out when a stat or valve has been changed.
The other possibility is a fault with the pipework set up where the open vent and cold feed enter, but any decent engineer would already have checked that I hope.

 

[MATTHEW EYNON]

Thanks for the reponse - much appreciated. Have 2 more plumbers due today!
Would incorrectly placed / too small a pump overrun creae a similar problem? valves are 2 way and I dont think pipework for overrun will hold much water before reaching valves (which would be closed)
The problem only starts (and hot water enters ex tank and then vents) once I have turned heating off, which would indicate that valves being closed and water having nowhere to go is the problem?

 

[footprints]

With zone (two port valves) there should normally be a bypass with a valve to adjust it fitted.
It might be if it only happens when the whole system is switched off, but not when the boiler stat operates, that the over run (if fitted) loses power at once, as a rule the over run has it's own permanent supply so it can run the pump till cool even when the controls are switched off.

 

[FiremanT]

It sounds an add scenario, but sometimes these have an obvious answer once you get "eyes on"
However, I am a bit intrigued as to why you have 2 plumbers coming out. Are you asking for quotes/estimates to repair, or re you getting them to sort it.?

 

[footprints]

It sounds an add scenario, but sometimes these have an obvious answer once you get "eyes on"
However, I am a bit intrigued as to why you have 2 plumbers coming out. Are you asking for quotes/estimates to repair, or re you getting them to sort it.?

Might be the Mario Brothers!

 

[FiremanT]

Might be the Mario Brothers!

Doubt it.I read the other day one is retired.

http://twinfinite.net/2017/09/mario-retired-plumbing/

 

[John D v2.0]

We had that exact problem, apparently was caused by a non return valve in the heating return added when it was converted to fully pumped by a heating engineer. We also had air and black water spraying on the wall from an automatic air vent by the boiler.
Eventually the landlord got British gas along who removed the non return and converted to a sealed system, after that no trouble at all. They might have also changed the zone valve to a diverter valve but they said the nrv was the issue.