Unvented hot water pressure build-up problem

Winelight · 2006-11-15T02:43:18+00:00

Sorry but this is a bit of a long story.Earlier in the year we had a problem with intermittent discharge froman unvented hot water cylinder (the installation was around 4 yearsold).The book says this could be caused by low pressure in the expansionvessel, so I checked it by shutting off the cold water feed to thecylinder, reducing the pressure in the cylinder by operating a hot tap,and putting a tyre valve on the expansion vessel. The pressure wasextremely low so I repressurised it using a car foot pump.This didn't solve the problem.The book says the other reason could be back pressure (crossflow).Another symptom we experienced was that when you operated a hot tap,instead of the water running warm or hot from previous use as you wouldexpect, it would actually run warm very briefly, then go icy cold, andthen eventually hot again.It seemed clear that cold water was being admitted into the hot watercircuit somewhere.Then the shower started to go wrong: it gradually got colder and we hadto keep turning the thermostat up and eventually it would only runcold.The shower is of an interesting design, which is that the hot and coldfeeds go to the thermostatic cartridge first, which then feeds the flowvalve, rather than having flow valves on the hot and cold feeds.It seemed obvious that the thermostatic cartridge was failing andallowing cold water back into the hot water system. In any case, itseemed impossible for the problem to be caused by anything else (theonly other places the hot water goes to are four mixer taps and thewashing machine). The problem occurred without any of these taps beingoperated. Some of these other taps have isolating valves and when Ishut them off the problem was still there. So, surely, it was theshower.We had the shower replaced and the problem seemed to go away butquickly came back. Could the shower have failed again so quickly?Anyway, we got a replacement, but the problem was still there. So wegot another replacement, fitted today, and the...

Nov 15, 2006 117 Replies

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