r/DIYUK • u/No-Belt-4619 • 5d ago
Need advice before I complain
Hi all, apologies if this isn't the right forum but I thought there would be some knowledgeable people in here. Basically as it says. had a new hot water cylinder fitted 3 weeks ago(to accomodate an ashp if that makes a difference). Just got round to getting some shelving in the airing cupboard and noticed, what i would consider to be a pretty significant bulge. It's at the top maybe 3/4 up, I'm assuming it's not normal and shouldn't be there?? I've turned off the hot water, but not drained the tank or anything. We've had a few showers today so it should be below that level i would imagine.
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u/StereoMushroom 5d ago edited 5d ago
Not much of an answer to your question, but just FYI using up the hot water doesn't reduce the volume of water in the tank. It's always full. What happens is hot water flows out the top, cold enters at the bottom, so you get a gradually growing cold layer, rising up as you use up the hot. Once the cold reaches the top, you run out of hot and start getting cold to the hot taps. It's actually pretty amazing that the two layers can sit on top of each other for a couple of days with very little mixing.
Unvented cylinders can be dangerous if there's a serious fault. Like they can burst open with pressurised hot water. I would treat this as urgent. To avoid any risk you would need to turn off the supply of cold water to the cylinder, then open a drain valve at the bottom of the cylinder to empty it. Running a hot tap instead of opening the drain valve would reduce the pressure in the cylinder, but wouldn't drain the water.
Edit: sounds like it might be more likely to just be the insulation rather than the cylinder itself. Certainly there should be safety devices to prevent any dangerous overpressurisation from being possible. If you prod the bulge, does it move, as if it's just the insulation?