John Stumbles wrote:

I'm wondering whether I've got my calculations right: anyone like to look
them over?

If the cold feed is at 10degC & 10 m , I make it that the height of the
water column in the open vent would be 10.167m at 60 degC. You couldn't
exceed 10.5m if the temperature was below boiling.

TMVs are a prime suspect for backfeeding because they're fitted
upstream of the outlet.

Aidan wrote:
The water levels in the tank and the overflow aren't the same because
of the air-lock.

That bit is rubbish. That could only happen if there was a non-return
valve or stop-cock on the cold feed.