Warm water from cold taps unvented cylinder

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Hi, please please can any one help. I have read lots but may be more dangerous as I have no idea and now have bits of info.

Basically when we turn on any cold tap we get cold water for about 5secs then 20-30secs of really quite warm water before it goes really cold and 'normal' and drinkable.

If we then turn the cold tap on after a short time then we only get cold but if left for,say, an hour, then it happens all over again. We even noticed warm water in toilet the other day.

The plumber has checked lots of things and is brand new but he says it's just the cold water pipes having heat transferred from the hot pipes but based on the amount of time we sometimes have to run the tap for, and the warmth of the water, surely it must be something else. He has also said he had spaced the pipes adequately throughout.

I have noticed the cold inlet pipe to the unvented cylinder gets very warm (26 degrees), especially if not used any taps, toilet etc and that heat rises up the copper pipe but the heat of the pipe feels more like hot water inside the pipe and rising. The plumber says the cold inlet balancing valve has a non return valve in and he even changed it anyway but it still happens.

Could it be this or something else or something to do with the electrics.

Please, please does any one have any ideas and could it be that the warm water is travelling up the cold inlet pipe and in to the cold supply due to too much pressure of something.

I just have no idea what I can do and the plumber has basically said that's how it is. I am also worried about legionnaires disease, especially as have 2 young children.

Thank you for your time. It is honestly really appreciated.
 
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Typically this type of issue is where water pipes are running side by side, or in the same restricted space with central heating or hot water pipework and there is thermal transfer, especially if it then runs cold after 30 secs.

The cold water supply to the unvented will have some expansion back up through the cold pipe though that isn't usually hot as the thermocline usually avoids that, it can get a little warm though if the cylinder is fully hot. It certainly shouldn't feed back into the cold mains as the combination valve has a built in NRV to stop that happening.

If you can't find the offending pipework then, as your plumber suggests, it's down to running the cold water until it goes cold. If you use your hot and cold water regularly every day then I wouldn't worry about legionella.
 
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Hi Phil,

I've just seen your message to me. I can only suggest you let your plumber see my post number 63 here https://www.diynot.com/diy/threads/warm-water-from-cold-taps.508083/page-5.

Do you have a Megaflo unvented cylinder and if so have you re-generated the air gap or had your water system drained down recently?

Can you physically see if there is a prv/non-return valve in the cold water inlet pipe to the cylinder? This is the pipe you say gets warm. The warmth should only go back as far as the NRV and the pipe should then be cold after that. My system did not have this NRV fitted so I got the same problem as you until I had one fitted.

I found a flow diagram in my Megaflo manual and worked out that your current problem, like I had, was normal for the system but I researched on-line and found modern Megaflo systems had the NRV in the cold inlet to the cylinder which removed the problem.

Hope this helps but keep asking and I'll refresh my memory of it later and see what I come up with.
 
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Find the NRV your plumber changed and see if there is a T piece between it and the cold inlet to the tank. If there is then you need an additional PRV/NRV fitted between the T piece and the cold inlet to the tank. This will stop the expanded hot water in the tank coming out into the bathroom cold taps and toilet cistern. I have the part number somewhere but make sure whoever fits it is unvented qualified.
 
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I've looked at my old notes from when I had the problem. Please remember that this is my experience with a Megaflo and I am not a plumber or heating engineer so always consult a G3 Unvented Engineer.

I looked at Megaflo FAQs on-line and found the question, ''Hot Water from a Cold Tap''. The answer was, ''In a Megaflo or unvented system, this may be an installation/design fault or missing check valve. If the cold water is warm for a short period of time, before returning to a normal cold supply; this is more of an inconvenience than a major problem''. (the punctuation may not be correct but it is exactly how it was written)

This was reassuring, but in my case the water was really hot and more than just an inconvenience.

My system was plumbed correctly as per the Megaflo manual but when I found a more modern diagram on line I noticed a fundamental difference...

In my then current system, the Mains Cold Water Supply entered the airing cupboard and into the combination valve (which includes a non-return valve). Coming out of the combination valve the pipe went to a tee where one branch went to the Cylinder Inlet and the other to the balanced cold supply to the bathroom taps.

This meant that when the water in the tank expanded after being heated and then a bathroom cold tap was opened, the hot water would escape (under pressure) back through the inlet and out through the cold tap (or even the toilet cistern if the toilet had been flushed) until the cylinder pressure dropped to normal. The hot water would never, however, go to the drinking water taps downstairs due to the NRV in the combination valve.

To me, this was a design fault. When I found the more modern system diagram there was an 8 Bar Pressure Relief Valve (combined Expansion Relief Valve/Check Valve) fitted between the tee to the balanced cold and the inlet to the cylinder. This prevented the pressurised hot water getting back through to the bathroom cold taps as well as the kitchen taps.

I phoned Megaflo help line and the chap I spoke to was surprised I didn't have this valve fitted and that it must have been an installation error but I explained that my installation matched the manual.

My fix was to purchase a Heatrae Sadia Megaflo 8 BarPressure Expansion Relief Valve Part No. 95605893 (which has a Non-Return Valve incorporated) for about £45 and then get a G3 Engineer to fit it immediately prior to the cylinder inlet but after the tee to the cold taps. It is also then connected to the inlet to the tundish. This work cost me £85. The combination valve should remain in place, although some of it will become redundant.

It fixed my problems there and then for a total spend of £130 which I was pleased with. It took a lot of research and help from the good members of this forum but was worth it. It took some explaining to the guy in the Heating Parts shop and to the G3 engineer as to why I thought this would fix my problem but they understood in the end and agreed with my thoughts.

I hope this helps you get yours fixed too.

Good luck with it.
 
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Hi, firstly thank you to the both of you for your time and replies, really appreciated.

Just to confirm, the plumber is G3 qualified. I have also attached a few pictures in case it helps.

THe plumber fit a pre-plumbed Tornado Indirect Unvented cylinder from Telford Cylinders, this is used to provide hot water for the central heating, hot water and underfloor heating system (which i wondered if that has any impact).

I appreciate the hot pipes may warm the cold but as another test, i have turned off the hot water, and then the heating for a few days each and we still get the same problem. Does this not mean that the pipes in the ceiling weren't getting hot and therefore the cold water pipes shouldn't be getting really warm like they do normally as there is less heat going on in the ceiling.

However, during this time we still had the UFH still on throughout the day, therefore the cylinder was still working I assume.

Today, i have tried turning off the UFH throughout the day (and yesterday afternoon) and the heating and hot water went off first thing. During today there seemed to be less warm water coming through the cold tap and didn't need to run it off for 30secs. SO i suppose i was just wondering if the piping to the UFH could be causing the issue of heat / pressure build up in the tank and somehow getting to the cold supply.

On that point, the thing the plumber changed, just in case it was faulty, was the cold inlet group. I understand this to be a Caleffi Inlet control Multibloc valve. From what i have researched, this contains a pressure reducing valve, pressure relief valve, strainer, balanced cold water connection AND a check valve.

This is fitted at the top of the cold inlet pipe and i assume this check valve would stop the warm water going in to the cold supply? The pipes before this valve are only really cold when we run the cold tap. If i check it in a morning before anything is used and the system has been on for a few hours then those pipes are not very cold at all and when i runa cold tap, it seems to take a while for them to turn really cold.

I understand the external expansion tank (which i wasn't sure if it was needed as it says in the manaual there is an internal one in the cylinder) was set at the correct level when it was set up less thana year ago but not had anyone check it recently. This is piped to a pressure control which is around 1.5bar but on the set up diagram it seems to indicate this should be piped to the Caleffi valve but mine isn't.

Based on one of your comments earlier Endaxi, i notice that just below the Caleffi valve there is a T off the cold inlet pipe and that runs to a flexi hose that goes to a pressure gauge (as previously mentioned set at 1.5bar) and then continues to the external expansion tank. I have no idea what that means or if correct but could cause the problem?

Thank you both, and anyone else, for your help. As i said, i don't have much of an idea but trying to do all i can to see it is as my plumber and Rob has said in that it is just the way the system works.

Many thanks
Phil
 

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Hi Phil,

It will take a G3 person to diagnose this of course but it just sounds so much like the problem I had.

If you had your heating and hot water pretty much off all day then the issue of hot water backflowing through the inlet to the tank and out to your cold taps would be hardly noticeable, if at all, as the water in the cylinder would only expand after it gained temperature.

Mine was more of a problem first thing in the morning after the water had heated or after a bath had been run. This would mean the water in the tank had gained many degrees and had expanded.

One of my checks was to run a bath and let the tank heat up again fully without running any water from any tap anywhere. When the tank has reached its temperature, feel the cold inlet to the tank which might be a bit warm by now due to conduction. Then feel along the pipe until it starts to feel cooler. Now get someone to run a kitchen cold tap and I would guess it would run cold immediately and stay that way due to the NRV in the inlet group (what I called the combination valve). I would bet that there would be no change to the temperature of the pipe you are feeling.

Next, turn off the kitchen tap and turn on a bathroom cold tap for a second or two at a time and see if the tank inlet pipe gets hot. Follow it along as far as you can, seeing if the hot water moves along the pipe and then out of the cold bathroom tap.

If you feel the tank inlet pipe getting hot and then going cold after a while and this corresponds with the water at the cold tap doing the same but a few seconds later then I would say you have the same problem as I had and you would need the extra PRV/NRV valve I mentioned earlier in your tank inlet.

If you don't feel the hot water flowing from the tank back along the inlet pipe then you have a different problem.

I hope you follow what I'm suggesting. I'm happy to keep at it if it helps but I'm running out of ideas.

Let me know how it goes.
 
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Thanks again. I will try this tomorrow.

When you say turn the bathroom tap on for a second or two at a time should I turn it on, then off, then back on etc, etc?

Just want to make sure I get it correct.

Also, what I did try is to turn the cold tap on in a morning when the pressure had been building and as you say, hold the cold inlet pipe which by this time is very warm at the bottom and a bit cooler further up. I was hoping to feel the inlet pipe get hotter if it pulled the hot water up and out in to the cold supply but that didn't happen and the temperature stayed the same, hot at the bottom and warmer further up but in that test the pipes above the combination valve didn't go very cold for a while which made me think, is the warm seeping out as it was for about 20-30secs, or is the cold supply actually warming up before it gets to those pipes above the valve but then I suppose that would mean the water is warm for the length of the supply pipe before it reaches the inside pipes i.e. under the drive, but based on the warmth it feels more like hot mixing with cold but who knows.

If you can just confirm about the bathroom tap process and I'll try tomorrow evening.

Thanks again
Phil
 
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The NRV is downstream of the balanced cold supply, to stop exactly that occurrence where the balanced cold could be backfed by the cylinder expansion. In the combination valve the NRV it is the first thing the returning water hits as it tries to enter the valve.

I'll bet that the balanced supply is sitting next to a CH pipe somewhere and it's warming the cold pipe up .... I wouldn't say it's "just how the system works" rather that it's how the system has been designed and run/laid out by the installer

The T off is just feeding the CH filling loop and looks shut. The red expansion vessel, after the fill valve double checkvalve and blow off, isn't for the HW cylinder, it's for the Central Heating. The cylinder will be using its internal bubble, it could need re-charged but I would hope that's already been checked by your plumber.

Really need to map where the pipes are running, even just to rule it out.
 
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I suggest turning the cold tap on and off just to slow things down a bit otherwise everything could happen so fast you’ll miss what’s happening.

It sounds like you’ve pretty much tried this.

You didn’t say if you were getting the hot water from just the balanced bathroom taps or if was happening in the kitchen too. I would be amazed if it was happening in the kitchen as these cold taps should be straight from the mains.
 
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Thanks both, I can't see the layout now as all carpeted but plumber said all spaced properly but then I suppose he would say that but obviously want to trust him.

The warm water comes out of all cold water taps as I think everything comes via the cylinder.
 
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Madrab, in the case of my system the combination valve NRV only stopped a backflow to the mains inlet and to the kitchen taps. Hence I had to fit another prv/nrv between the tank inlet and balanced cold.

Also, surely any back pressure would hit the prv before the nrv in the combination valve. Otherwise the prv would be of little use.
 
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Phil, I’m sure the kitchen cold should come straight from the mains supply to the house and not from the balanced cold but I stand to be corrected.
 
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Phil, I’m trying to find a way to get my main point across.

If you follow the cold inlet pipe to your cylinder does it go straight to your inlet group (combination valve) or does it reach a T first?

Mine went to a T first. The back flowing expanded hot water from the tank could not turn right at the T and pass through the inlet group (combination valve) due to the NRV but it could turn left and flow through any balanced cold tap that was opened.

I now have an additional PRV/NRV between the cold inlet to the cylinder and the T so that no water is capable of back flowing anywhere from the cylinder inlet.

l hope you get what I’m trying to say.
 
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