My issue on the face of it has been covered many times, but specifically in this instance it makes no sense and I have not yet found a reasonable answer, if you can help it would be very much appreciated.
I have an open vented CH/HW system. For some time there has been a reasonable trickle of hot water being emptied into the header tank via the vent. The pipework is installed correctly, the vent and the cold feed are less than 150mm apart, and in the right sequence from the boiler outlet.
What makes no sense to me as that this ONLY happens when the HW is demanded of the boiler and the Y valve is set to provide both CH and HW. if there is NO demand for HW then the vent remains dry (CH on full tilt)
Given that when there is a demand for both HW and CH the water has more pipework to flow around and therefore the pressure should be reduced, therefore making the overflow LESS likely, So why is this happening?
I have carried out many power flushes, treatments of all kinds - today I have bled the system right down after 2 days of sentinel 800 and still the problem persists.
It happens on pump speed of 2 or 3, and my vent pipe is the correct height above the header tank.
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