feed & expansion tank boiling over

[Mcfly1]

Hi, Ive searched this topic but no other problem was identical.

Water gets very hot in our CH header tank and every so often overflows out of it. When this happened in the past we turned the pump down and this solved it for several months. Though it has now returned and the pump is on the lowest setting.

The system is gravity fed, thanks.

 

[bab]

sounds like u might have a blockage on the cold feed out of the tank to the system. often on the tee to the cylinder return if gravity hot water,does water trickle out of the vent over into the tank. vent may not be high enough either.