Central heating venting through expansion pipe

The problem:
Water is erupting violently through the expansion pipe as the boiler shuts down (not every time). The water in the expansion tank is very hot. The problem occurs after the heating as been on for about 1½ - 2 hours, and sometimes a screeching noise is heard at the same time (pump?)

Setup:
Pumped C/H circuit with thermostat (hall and landing) and motorised isolation valve for upstairs and downstairs. All radiators have TRW’s except for one upstairs and one downstairs.

Pumped H/W circuit with two hot water tanks, each controlled by a motorised isolation valve contolled by tank thermostat.

Action taken so far:
Have checked all isolation valves and thermostats, and these are shutting down and opening up as expected. Boiler thermostat seems okay as it can be heard clicking in and out. Have run the system with sludge remover for two weeks.

Coincidences?:
Have replaced two of the isolation valves, but am sure they are wired up correctly, and are working as expected.

Removed the C/H pump some months ago to replace leaky valve, but it’s working fine. Pump speed has not been changed.

Notes:
All radiators heat up as normal, as do the hot water tanks.


Does anyone have any ideas?

I had a similar problem.Could be your boiler thermostat need replacing or cleaning. I took the boiler thermostat apart and blew out the dusts & hoover and turn the setting slightly down.It's now okay ! I think what happening, the boiler is overheating the water too quickly and therefore expanding to the expansion tank via the vent. I also turn up the pump setting to number 3 to speed up the water flow to reduce overheating. You could also try and turn the thermostat down and see what happen as this is the cheapest way of finding out ! Good luck

I had a similar problem and it was eventually discovered that the coil inside the hot water cylinder was defective/fractured and the hot water and central heating systems were "crossing over" causing hot water going back to the expansion tank. Also it meant that the expansion tank was almost constantly overflowing. Hope this might be of help.After changing the hot water cylinder all is well.
Jim