Unvented hot water pressure build-up problem

[Winelight]

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

[Doctor Drivel]

Are the other tap mixers in the house OK? You can take off the shower mixer and connect up a hose and temporary tap and see if hot comes out of the hot pipe.

Unvented cylinders are famous for back flow from cold supplies Also you could fit a non-return valve in the hot supply of all the mixer taps in the house. Then any water passing crossing over through the mixer will be checked A compression non-return valve will fit easily onto the pipes. Or if the shower mixer pipe goes back to the cylinder (which it should), then maybe one non-return on the hot draw-off pipe to all the outlets except the shower would do it.

First see if hot water is actually at the shower mixer valve. You may have a blockage. Then install the non-return valves at the mixers as a matter of course.

sell them cheap enough.

[Aidan]

Backflow?

There's a connection on the multi-function/combination valve (check valve + pressure reducing valve + pressure relief valve) for the cold supply. This is so the cold is at the same pressure as the hot. It's often not used where, for example the system is converted from a storage tank supply or is connected to the mains at ground floor level & it's too inconvenient for the plumber to do the job properly. Read the manual.

If the cold is connected direct to the mains and doesn't pass through the combination valve, there can be problems. Connect it to the fitting provided or, failing that, install a pressure reducing valve on it.

[Winelight]

Yes, that's how it is. The original installer failed to take a separate cold feed from the pressure-reduced output but instead connected everything to the mains.

Thanks for the suggestions. The latter option is also what the plumber reckoned was the best bet.

However before embarking on that I wanted to be 100% certain that the shower is the cause of the problem. It just seemed odd that one shower worked for 4 years, and then three more exhibited a fault that the first one didn't, if you see what I mean.

Andrew

[Doctor Drivel]

Getting two pressure reducing valves to operate at the same pressure constantly is difficult. If you do fit one on the cold mains supply, then find out what the pressure is at the DHW draw-off at the top of the cylinder and set it to that. There is a pressure loss across the cylinder. Even taking the cold off the combination valve still leads to an imbalance between hot and cold, athough not great, as there is a pressure loss across the cylinder.

This is another point where heat banks (using plate heat exchanger) score over unvented cylinders. If the plate is say, rated at 10 bar and so is the shower mixer, then there is no need for a pressure reducing valve if the pressure is even 6 bar. A shower mixer with a pressure equalisation valve can trim off any slight imbalance. Also the lack of a cylinder means no crossover problems.

If it is difficult to get a cold supply pipe back to the combination valve, I would be tempted to use non-return valves. They are cheaper and less likely to go wrong.

Fist find out if there is hot water at the hot pipe at the shower.

[Winelight]

Well, yes, it's tempting to replace the whole thing with a thermal store. Expensive, yes, but at the right I'm running up plumber's bills to keep changing the shower...!

OK, thanks. I'll read what you said again more carefully to make sure I understand where they have to go.

Sorry, I should have been more precise. The current shower, fitted yesterday, does actually work, it's just that we are clearly getting mains cold water back into the hot water system through the outlet pipe. (The first shower, when it failed, did also behave like this for a while before the thermostatic cartridge failed altogether resulting in cold showers only.) So yes, there must be hot water getting to the shower.

Andrew

[Aidan]

Less likely to work, too

They'll stop the cold water flowing backwards up the hot pipes. However, the hot water at, let's say, 3 bar won't be able to flow into the shower valve if the shower valve is full of cold water at 6 bar.

Best to ignore Drivel, except for the purposes of entertainment. He's well known for his rantings.

You can buy shower valves that work with supplies at unbalanced pressures, usually mains cold and hot from a tank via a cylinder.

[Doctor Drivel]

It is easy to convert as a DIY task to a heat bank.

[Ed Sirett]

I agree. I'd be tempted to put the bathroom cold onto the balanced pressure outlet (located at the inlet to the cylinder) and fit NRV to the shower.

[Doctor Drivel]

Yep. You got it.

There could be a 3 bar differnce but highly unlikley.

You really understand these do you? The shower needs as a matter of course a mixer valve with an integral pressure equalisation valve or an external valve in the hot and cold to the feeds to the shower. All mains pressure system need one.

Nothing worse than a know it all amateur. Full of misinformation.

You are on about venturi shower mixers which are totally inapplicable to mains pressure hot and cold.

[Doctor Drivel]

He is on about an inapplicable venturi shower mixer. Just total babble.

The OP already has a new mixer. He is has problems getting pipes to various places. Ideally he should have the cold off the combination valve. For some reason this a big upheaval. His shower did work but now doesn't. There is a crossover somewhere. It may be via one of the basin mixers - this must be found out.

This best approach is put the non-return valves on the hot pipes on the points where a crossovers can occur. I assume his shower feeds do not go back to the cylinder. He is then better off fitting a pressure equalisation valve in the lines to the shower mixer - if the mixer doesn't already have an integral valve of course. The pressure equalisation valve can be anywhere along the two lines, not just on the mixer. You have to pragmatic about it.

[Doctor Drivel]

Another point. Has the water pressure gone up from the street after some works on the mains? If say they have upped it to 6 bar and shower mixer valve is rated for 4 bar maximum then it may be early failure for the valve.

[Winelight]

Not as far as I know (or have noticed). The shower mixer valve is rated at 3 bar. Whether or not the mains is greater than that I do not know. Even so it seems unlikely that two or three showers in a row would fail instantanously in this way, I'd have thought?

Andrew

[Ed Sirett]

My guess, based on the replies Aidan makes on this subject, is that Aidan probably earns some of his living dealing with unvented HW cylinders. He probably, like myself, has a "G3" card.

[Winelight]

That's a joke, surely? If not, please supply step-by-step instructions and diagram!

Andrew

[Doctor Drivel]

Which doesn't say much for a one day course then does it.

[Doctor Drivel]

It is not a joke at all. First, how many tappings and what size of the cylinder? What type of boiler and is it a sealed system?

[Doctor Drivel]

Screwfix sell a pressure gauge that you can connect to the washing machine raps or outside hose connection. I would buy one and see what the pressure is?

[Winelight]

Good idea. I need to put in a Screwfix order anyway.

Andrew

Doctor Drivel wrote:

[Winelight]

No boiler, but two immersion heaters, one normal and one boost. Only two tappings I am aware of, one for cold in and one for hot out.

I don't know if there is a sacrificial aluminium rod. We have very aggressive water.

I can see that it would be possible to connect the two tappings to a heat exchanger, vent something somewhere, and turn up the thermostats, and then run the mains through the other half of the heat exchanger to the hot supply, but how does the water from the cylinder get through the heat exchanger? Does it have to be pumped through or what? I know nothing of thermal stores.

Andrew

Doctor Drivel wrote: