so where is the immersion and cylinder on the diagram just curiousThanks guys.
Yes there is an immersion fitted, but it's never used.
JohnG and EvilDr, ok I could try that, you think the water is travelling too fast through the coil whereas I was thinking it was flowing too slow! I could try to use the red gate valve as a balancer and restrict the flow with that one, if that fails I will drain down and fit a different valve there.
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Just tried setting pump speed to 2, and adjusting the red gate valve further and further closed in increments, I'm afraid the water never stops coming out of the vent unless the gate valve is closed fully, which of course knackers the boiler.. back to pump speed 1 and all is ok again, so it doesn't seem that throttling the cylinder coil makes any difference.