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!

 

[Chuck]

Can you provide a few photographs and some more details about the system, please? In partiular, how many radiators. What are the dimensions of the expansion tank in the roof and what is the water level in it when the system is off? The photos need to show where the motorised valves are located in the pipework with respect to the pump and the pipes going up to the smaller feed/expansion tank in the roof.

 

[Last Plumber]

Sounds dangerous.
How many pipes leave the Boiler?
As chuck asked, any photos ?

Boiler and pipes
Cylinder cupboard and pipes

 

[Rizme]

Call some one who knows open Vented system, it usually comes down to poor system design, when the water starts to overflow.
If you can combine the feed and vent in the loft by expansion tank to start with and see if itstill is over flowing.

 

[deg]

Are you saying your boiler is doing this after being turned off? Sounds like your motorised valves need wiring properly to interlock with the boiler at a guess.