Hi, I installed an Albion Mainsflow thermal store about 5 years ago. It's this type here to save explaining how it works:
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's the direct contractor version on page 3 where the store has a float valve at the top and acts as the feed and expansion tank for the boiler/radiators as well as providing hot water. The problem is its been overflowing (about half a bucket per day). It's not the float valve as I've cut and capped the pipe going to it, so that only leaves the DHW coil. I connected a pressure tester to the coil and didn't get any bubbles or noticeable drop in pressure, but there's no other way for water to get into the store. So I'm left with two options. Either replace the coil with an external plate heat exchanger, pump and flow switch. This'll cost about £250 in parts to do. The downside is if the copper coil is failing, will the cylinder itself go in time? If so, it might be better to replace the whole thing, maybe going for a more conventional unvented cylinder, which are pretty cheap (e.g. Vaillant Unistor 155 is less than £600 + fitting). I could also switch to a sealed system for the boiler/rads. But unvented cylinders aren't DIY (how much would a pro charge do you think to put one in?). Any advice appreciated. Cheers, Antony.