Hot Water Tank Vent Pipe is Discharging in to Expansion Tank? All aspects of plumbing questions and answers, help, tips and information

[malcia]

Why is the hot water tank vent pipe discharging hot water into the expansion tank?
It seems to have a steady flow more than a trickle but less than a gush - I think its when the hot water is being heated or has been heated.
The thermostat is set at 60 degrees and the boiler is set on eco.

Thanks