Why is water flowing through my expansion pipe

[Tellboy]

Water is flowing through my central heating expansion pipe even when cold with the pump running. Hot water flows through it when the system is heated. Pump has been replaced but still happens

 

[exedon2]

Some clear pictures of how your expansion pipe and cold feed connects to system would be useful.
Plus a sketch of pipework?

 

[Tellboy]

Some clear pictures of how your expansion pipe and cold feed connects to system would be useful.
Plus a sketch of pipework?

I have an open vented solid fuel system. It all started when hot water and steam was blowing out through the expansion pipe in the attic (bungalow). I ran the hot water off and thinking it was the pump, had it replaced. But when I put on a fire yesterday the hot water was flowing through the expansion pipe but the f and e tank was not overflowing. The water in my cylinder did not get hot. Just warm. Some of the radiators got slightly warm. It's a dual system but I haven't used the oil heating in years. I've attached a few photos of the cylinder and pump. The house is about 30 years old. The solid fuel was connected about 15 years ago during an extension build. When I run the pump even cold there is a water flowing through the expansion pipe. The tank is not overflowing.