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I have a vented indirect hot water cylinder with a system boiler. It has lost pressure slowly from 1 bar down to about 0.7 bar over a few weeks for a few years now. Then over the summer it refused to hold more than 0.5 bar read on the boiler which is in the garage or 0.2 bar read upstairs in the airing cupboard at the filling loop. If I fill it upto 1 bar it drops in less than 1 minute. I swapped out the 3 port valve a few weeks ago and was supprised that the radiator water was very very clean when I drained it. This started me thinkning that the coil in the hot water tank might be broken so the boiler, radiator, ho****er coil water was mixing with the hot water in the cylinder. The pressure at the filling loop and the boiler are just becuase of the height of water above them.
Today I have done the following:
1. Started by isoltaing the feed and return under the boiler to remove any chance of an internal leak in the boiler or issue with the expanision vessel. Pressure still drops at the same rate.
2. I have shut off the incoming water to the hot water cylinder from cold water storage tank in the loft, when I presurise the boiler filling loop upto 1 bar and let it drop 3 or 4 times water comes out of the expansion overflow from the hot water cylinder into the cold water storage tank. I think this is conclusive that the coil is broken and the water from the central heating loop is mixing with the water in the hot water cylinder. Do you guys agree.
Many thanks for anybody that can check my logic.
Yeah you’ve got it.
The pressure readings (0.5,0.2) also tally with the boiler being pressurised only by the vented header. The upstairs gauge must be ~2m below the header, the garage one must be ~5m below the header tank.
Thanks goldfish thats what I thought now is just the decision weather to replace with another vented cylinder which I can do myself or upgrade to an unvented cylinder which I would need a plumber to do as I am not certified for G3 stuff.
As a double check. If you have no cold water feed into the cylinder as it's off at the isolation valve and the water return to the expansion tank is warm.( If your hot water tank is warm as well) then then I think the only thing can be a split coil .
The pressure in the coil pushes out into the cylinder, this will mix with the hot water and the only place it can go is up into the expansion tank. If the pipe that goes back from your vented cylinder to the expansion tank is warm or hot then that's it.
Unvented cylinder any day of the week...best shower ever.