Sorry but this is a bit of a long story.
Earlier in the year we had a problem with intermittent discharge from an unvented hot water cylinder (the installation was around 4 years old).
The book says this could be caused by low pressure in the expansion vessel, so I checked it by shutting off the cold water feed to the cylinder, reducing the pressure in the cylinder by operating a hot tap, and putting a tyre valve on the expansion vessel. The pressure was extremely 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 would expect, it would actually run warm very briefly, then go icy cold, and then eventually hot again.
It seemed clear that cold water was being admitted into the hot water circuit somewhere.
Then the shower started to go wrong: it gradually got colder and we had to keep turning the thermostat up and eventually it would only run cold.
The shower is of an interesting design, which is that the hot and cold feeds go to the thermostatic cartridge first, which then feeds the flow valve, rather than having flow valves on the hot and cold feeds.
It seemed obvious that the thermostatic cartridge was failing and allowing cold water back into the hot water system. In any case, it seemed impossible for the problem to be caused by anything else (the only other places the hot water goes to are four mixer taps and the washing machine). The problem occurred without any of these taps being operated. Some of these other taps have isolating valves and when I shut them off the problem was still there. So, surely, it was the shower.
We had the shower replaced and the problem seemed to go away but quickly came back. Could the shower have failed again so quickly? Anyway, we got a replacement, but the problem was still there. So we got another replacement, fitted today, and the problem is *still* there.
Now, you might say, the thing is surely to operate the isolating valves to the shower and prove that beyond any doubt that it is the fourth faulty shower in a row. However, there are no isolating valves, and to fit any, and make them accessible, would mean ripping the bathroom apart and taking out the bath, and running long pipe loops to somewhere accessible and putting isolating valves in the loops - costing a small fortune and creating huge disruption. (Alternatively, I suppose, we could fairly easily gain access to the pipes from outside, by ripping off the external cladding - timber framed / walled house.)
In any case, I really don't see how it could be anything else apart from the shower. Mixer taps don't tend to admit cold water to the hot water circuit, especially when they're not being operated.
Any ideas anyone please?
Andrew