Header tank overflow

All help appreciated. Today I noticed water coming out of the expansion tank overflow. I found that the tank was full, well above the ballcock and the water was steaming hot and a thick reddy brown. Hot water was flowing in from the vent pipe of the HW cylinder.
I turned off the mains supply to the expansion tank. Scooped out most of the water. turned the mains back on and checked the ballcock was working OK - it was. Hot water still coming from the vent pipe. Then turned the CH pump off and flow of water stopped.
The flow of water from the vent pipe into the expansion tank is a lot greater than the flow out of the overflow pipe (dripping) - so mostly its recirculating but with a little new water entering the system somehow.
The overall set up is a standard gravity fed system with traditional CH boiler etc. Any ideas?

Yet another badly designed pipe layout/ sludge blockage . The short term "fix" may be to turn down the pump speed, and raise the height of the vent loop as much as possible

Hot water was flowing in from the vent pipe of the HW cylinder.

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Hope you mean the vent from the boiler primary pipes, NOT the one from the top of the HWC?

We've recently had a couple of very similar questions - do a search or scroll down topic list - you'll find that most points will have been covered.