Percolation?

[Softus]

I agree. You've got to be absolutely sure that the obvious things aren't the cause of the problem before chasing after obscure phenomena.

I agree too. I'm trying to keep open-minded, but I reckon I've been through all the obvious things, in my mind, approximately four million times

Another outlet, slightly higher, is to the cold side of a Stuart Turner twin impeller pump.

The cold outlet should really be the lower of the two, so are you sure you've got them the right way round?

Thanks - you're absolutely right. I believe the pipes are right but I just wrote it down wrong, but I'll check again when I'm next there.

If the kitchen tap is a mixer type it could be allowing cold mains water to flow back up the hot supply pipe.

If course it should have had a NRV in the hot but nobody fits them as they should! Even if there is then it could be leaking.

Test would be to isolate the cold supply to kitchen.

Thanks Agile. One of my ideas was exactly that - to fit an isolating valve to the kitchen cold side so that the owner can turn it off overnight to see if the problem goes away.

Just to recap one point in case I didn't make it clear - the problem occurs when no water is being used at all, anywhere in the house. For example overnight. This is what made me concur with the porous casting idea, and what made me think that the NRV would stop it.

 

[chrishutt]

the places that main cold and pumped hot meet is at the three mixer valves in the house - kitchen, bath and shower

But you said before that the shower cold supply was pumped from the cold storage cistern. Anyway one of those must be the source of mains passing up the hot pipe.

 

[Softus]

the places that main cold and pumped hot meet is at the three mixer valves in the house - kitchen, bath and shower

But you said before that the shower cold supply was pumped from the cold storage cistern.

Yes - I've managed to make this confusing. There are two showers. One is generally unused, and takes mains cold and pumped hot. This is the one I included in the above list of three mixer valves. The other shower is the only outlet from the cold side of the pump.

Anyway one of those must be the source of mains passing up the hot pipe.

I tend to agree. When I go back I'm going to add a valve to kitchen cold side.

However, I can't get at the services to the other two mixer valves without breaking into walls and floors, and there's no clue whatsoever as to the pipe runs, and the house has been heavily modified (two knocked into one). Something of a f***ing nightmare really.

 

[chrishutt]

There are two showers. One is generally unused, and takes mains cold and pumped hot.

Hmm. I wish you'd mentioned that in the first place. This must be suspect no. 1 since shower valves often mix the supplies upstream of the main flow control, so even when turned off the only thing stopping the cold mains passing up the hot pipe is the built-in check valve, and these are not 100% secure as we know.

The intermittent nature of the problem could be down to higher night time mains pressure forcing it's way past the check valve. Mains pressures are often higher at night because of the reduced demand and flow.

 

[Agile]

Any mixer with mains water is a suspect.

I once went to an intermittent problem like this.

It turned out to be the cheap shower mixer but the leakage depended on where the last user had left the temperature knob!

Tony

 

[Softus]

You're confusing me a bit there Chris, because of the following:

I recently came across a similar situation...The hot and cold supplies mixed upstream of the shower flow control (which I think is quite common) so that the hot water from the cylinder was slowly pushed back into the cylinder and up the feed pipe to emerge in the feed tank and cause it to overflow.

...I wish you'd mentioned that in the first place. This must be suspect no. 1 since shower valves often mix the supplies upstream of the main flow control, so even when turned off the only thing stopping the cold mains passing up the hot pipe is the built-in check valve, and these are not 100% secure as we know.

Your rather significant December comment is the main thing that persuaded me that I should fit a NRV on the pump hot outlet, so I'm confused that with the 'new' information your analysis is pretty much the same.

Don't get me wrong - I'm immensely appreciative of you bringing your greater experience and wisdom to bear upon this problem, but this one is niggling at me, yet I can't just nip round to the problem every day because the customer lives miles away from me.

The intermittent nature of the problem could be down to higher night time mains pressure forcing it's way past the check valve. Mains pressures are often higher at night because of the reduced demand and flow.

Good point. Very good point, in fact.

Any mixer with mains water is a suspect.

I once went to an intermittent problem like this.

It turned out to be the cheap shower mixer but the leakage depended on where the last user had left the temperature knob!

Aha! A very interesting observation. I shall inspect the position of the customer's knob toute de suite

 

[chrishutt]

You're confusing me a bit there Chris,

Sorry, I'd completely forgotten that I'd already made that observation in the same thread - it was more than two months ago!

I think the mains pressure may be getting past the check valve that you've installed. Otherwise there's no explanation.

 

[Softus]

Yeees, this is a bit of a saga.

I'm happy to read your comment/conclusion about the check valve - this was the only conclusion I had come to. In your opinion, does this mean that the valve is faulty, or are they not designed to cope with mains pressure when the flow is nil?

 

[Slugbabydotcom]

I did have one like this many many moons ago and the problem was indeed a mixer tap. and another one not quite so many moons but quite a lot of moons nevertheless ago where someone had mis-undertood the principles of a 'y' piece adapter for a washing machine.

 

[Softus]

...where someone had mis-undertood the principles of a 'y' piece adapter for a washing machine.

The washing machine had occurred to me, but I dismissed it and didn't look because there's no mixing valve. However, the "reversed Y connector scenario" hadn't occurred to me. I shall look....

 

[Softus]

Guess what ? I returned to the job, and the overflow was running

Fitted service valve to kitchen cold - waited - overflow still running.
Looked behind washing machine for Y connector - none found.

Finally, looked at the unused shower - temperature control was set to fully hot.

I set it to fully cold, and waited, and the overflow petered out. Set to fully hot again - waited - overflow started again.

Showed customer - general dancing and merriment.

They still insisted that the shower hadn't been touched for months at the time that the symptoms started, so my guess is that an 'O' ring had finally given way under the continual onslaught of main pressure against alternating 0.5 to 2.0 bar hot pressure. (Either that or the porous casting as mooted by some earlier in this topic.)

I'm grateful to all who contributed towards solving the problem - the solution is definately one I'll keep in the back pocket in case I see it again.

Thanks guys.

 

[Agile]

Agile said on 27 Feb:-

"""Any mixer with mains water is a suspect.

I once went to an intermittent problem like this.

It turned out to be the cheap shower mixer but the leakage depended on where the last user had left the temperature knob!

Tony"""

QED !!!

 

[Softus]

QED !!!

Agile, that's tacky. FYI, I was giving equal credit to everyone who contributed, but you're attempting to take it all. Here's how your advice actually appeared in the sequence of postings:

I think I can envisage an explanation-mains pressure on a bathroom mixer crossing via a porous casting into the hot side of the mixer- backflowing to the top of the cylinder...

and

I recently came across a similar situation...The hot and cold supplies mixed upstream of the shower flow control (which I think is quite common) so that the hot water from the cylinder was slowly pushed back into the cylinder and up the feed pipe to emerge in the feed tank and cause it to overflow.

and

If the kitchen tap is a mixer type it could be allowing cold mains water to flow back up the hot supply pipe....

Test would be to isolate the cold supply to kitchen.

To which I replied:

Thanks Agile. One of my ideas was exactly that - to fit an isolating valve to the kitchen cold side so that the owner can turn it off overnight to see if the problem goes away.

... which I then carried out.

Also:

My reasoning has been that the only way hot water could be forced up to the CS cistern would be under a pressure that greater than the head of the water in that cistern. The only thing that fits the bill is mains pressure, and the places that main cold and pumped hot meet is at the three mixer valves in the house - kitchen, bath and shower.

So:

I once went to an intermittent problem like this.

It turned out to be the cheap shower mixer but the leakage depended on where the last user had left the temperature knob!

To which I replied:

Aha! A very interesting observation. I shall inspect the position of the customer's knob toute de suite

Which I did. However, it was vital that I could see the symptom at work so that I could test the outcome of the actions I was taking. Anything else would be a process of guesswork, not engineering.

 

[ChrisR]

It would be interesting to know if the offending shower mixer has any check valves or not.

Though they can be of meagre dimensions the integral ones usually have (I see from the ones I've removed in disgust) an O ring or flat rubber ring as a valve seat. One 15/22mm double check valve I used didn't, relying on some sort of white plastic. I tested it with a pipe tester, to find it passed noticeably at about 10 bar. It wasn't a concern in that application (undersink unvented), bit it was a new dcv, and I wonder how it would perform when older.
I didn't measure the leak rate, which was a slow dribble. At that it would take a long time on normal mains pressure to overflow a cistern.

 

[Softus]

It would be interesting to know if the offending shower mixer has any check valves or not.

One day I'm gonna find out, because the customer now wants it removed

...leak rate, which was a slow dribble. At that it would take a long time on normal mains pressure to overflow a cistern.

Hm, interestingly (to a nerd like me), the leak rate on "my" problem was also a slow dribble - this much was clear by observing the flow rate of the water escaping from the cistern - scarcely more than fast drips, but at a 30 deg. angle (of the warning pipe) it gave the impression of a deluge.