This looks to be a potentially dangerous problem. As a DIYer, I don't know enough to determine what exactly might be wrong (or certainly not enough to advise someone what to do). I suggest that you shut down the system and get a plumber in!
Redkev, I'm struggling to see how a blocked cold feed would result in the Feed & Expansion tank being full of hot steamy water and overflowing. Do you mean the feed from the bottom of the F & E tank into the system or the mains water feed into the ball valve to fill the tank?
You seem to have 2 problems here. An FE tank that is overheating AND not topping itself up.
As explained the overheating could be dangerous as the water in it could reach boiling point or thereabouts and come through the ceiling. At the very least make sure no one sleeps below it! If you turn the thermostat down it COULD cool things down sufficiently to solve the problem depending on your system but I would advise a plumber to sort it out asap.
If i am right that the second problem is the FE tank not refilling this could be the result of damage because of the hot water in the tank. However if that were the case it is more likely that the FE tank wouldnt stop refilling and you would have more than a few drips coming out of the overflow. Luckily the problems shouldnt be too expensive to solve so get someone in.
Good luck with it all.
"I'm struggling to see how a blocked cold feed would result in the Feed & Expansion tank being full of hot steamy water and overflowing."
The water in the F&E is hot because the system is pumping over. This is because there is a magnetite blockage (at the usual place) where the feedpipe tees into the primary. As the hot water circulates (driven by the pump) it comes up against this blockage and has nowhere to go except up the vent pipe into the F&E tank - hence same gets hot.
"Do you mean the feed from the bottom of the F & E tank into the system"
Yes he does.
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