A Mystery overflow.....

[Twinkle]

A question regarding a mystery overflow.

1st my system. I have a recently installed Worcester condensing system boiler. I have a normal vented HW cylinder and a CW storage tank. The FE tank has been removed when the system boiler was installed.
The boiler works fine - HW and CH are both fine.

My problem seems to be with the HW cylinder. When either the immersion heater or boiler are switched on the CW tank overflow starts emitting water to the outside of the house. I have checked that the ball valve in the CW storage tank is fine. The water is not coming into the CW storage tank via the HW cylinder vent pipe either. By elimination I assume the water is coming from the HW cylinder via the pipe that feeds it from the CW storage tank. I have read about this problem on the board before and it seems that this is due to the cylinder coil cracking- do people agree?- is there any way to verify this for sure? If the CH/DHW are mixing in the cylinder wouldnt the HW from the tap be discoloured?-it seems clean.
It seems it cant be the boiler or cylinder stats as the water isnt coming in via the vent pipe/and it seems to happen even with the immersion heater!

Is my only option to replace the HW cylinder?- If so are they all much the same or is one brand better!

Please help with my mystery!!!

 

[PTH]

I'm assuming that after a while the flow from the overflow stops or reduces to a dribble, if so it is expansion. As the water in the cylinder is heated it expands. There are only four places it can go:
1 out of a tap
2 up the open vent
3 up the cold feed
4 through the cylinder wall ie leak.

If we discount 1 and 4 the expanding water will push up the open vent and cold feed at eaxactly the same rate (from a hydraulic point of view they are parallel pipes both with open ends). The open vent goes higher than the cold feed which stops at the water level in the CW tank.

So, the water level rises and out the overflow.

To stop it you need more expansion space in thee CW tank, therefore you need to lower the water level when the system is cold. To acheive this, bend the float valve arm down a bit.

Simple. Job done