Warm water from cold taps unvented cylinder

I'm not sure if you could tell from the photo but the cold supply pipe goes in to the combination valve and then the long cold inlet pipe comes from that all the way down and goes in to the cylinder.

The only pipe to come off the long inlet pipe is situated just under / after the combination valve and that connects to a flexi hose which connects to a pressure gauge which then connects two ways, one goes to the tundish, the other to the red expansion tank.
 
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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)

You need to confirm if there is expansion for the hot water within the cylinder (as in a bubble/baffle, like Megaflo's have).

The expansion vessel in the photos is for the CH.
 
Also, surely any back pressure would hit the prv before the nrv in the combination valve
Apologies, of course ...the water hits the NRV before it gets far enough back to contaminate the mains cold, was worded badly. It's part of the design of the combination valve.
Where is the expansion vessel plumbed in for the cylinder
It's an internal bubble.
 
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This is getting complicated for me.

If that expansion tank is in the cold inlet to the tank and the tank has a bubble then any expanded water could occupy the bubble and the expansion tank.

There is also a pipe coming from the bottom of the pressure gauge and I’m guessing that is your balanced cold. If so, any expanded hot water will go that way if a cold tap is opened before a hot tap.

Presuming your cold mains comes in at the top of the combination valve, if your balanced cold was teed off immediately after the combination valve (inlet group) and below that T you had an additional NRV/PRV, and then below that you had the T to the expansion tank and then below that the pipe went to the cylinder inlet I think that would work.

Expanded water could go to the expansion tank but not get to the balanced cold due to the new NRV. The expanded water would only be released with an initial gush when a hot tap was opened and the balanced cold taps would be unaffected.

This is what happens in my system, the only difference being I don’t have the expansion tank.
 
In the 3rd picture -

The cold mains comes down off the combination valve and heads down into the bottom of the cylinder, the balanced cold comes out the back of the combination valve, it has insulation round it, and heads backwards to the left of the cylinder.
The Tee further down heads right, through the ISO valve and braided hose (filling loop) and over to the multi coupler to supply the central heating(CH) circuit.
The pipe heading off and round to the expansion vessel is to protect the central heating expansion not the cylinder
The pipe from the bottom of the pressure gauge is the feed into the central heating to allow pressure topup and access for the CH to the PRV and expansion vessel.
 
One final thought...

when your cylinder water has just finished heating up and therefore the water has expanded, you could open a hot tap for a while first to get rid of the expanded water. Then open a cold tap and see if it runs cold. There may be a little warm water at first but if it is markedly improved then it would prove that it’s expanded hot water that’s causing the problem.
 
Following from what Madrab has just said, if the balanced cold is coming off the back of the combination valve then it will be after the nrv and so expanded water would be able to flow through a balanced cold tap but would be unable to get to your kitchen tap.

This means the design of your system is causing the issue.
 
UK Industry standards dictate that CAT2 water (warmed water) cannot be allowed back into the cold mains that could be used for drinking etc. Hence the design of the combination valve with integrated check valve that stops the warmed water getting back into the cold.

If it didn't do it by design it would be WRAS approved and therefore would be allowed to be fitted to the mains supply. Checking the install docs it mentions that the supplied combination valve has a built in check valve to protect the mains and balanced supply.

The check valve sits in between the Pressure Reducing Valve and the Pressure Release Valve as part of the valve
 
UK Industry standards dictate that CAT2 water (warmed water) cannot be allowed back into the cold mains that could be used for drinking etc. Hence the design of the combination valve with integrated check valve that stops the warmed water getting back into the cold.

If it didn't do it by design it would be WRAS approved and therefore would be allowed to be fitted to the mains supply. Checking the install docs it mentions that the supplied combination valve has a built in check valve to protect the mains and balanced supply.

The check valve sits in between the Pressure Reducing Valve and the Pressure Release Valve as part of the valve
Right, so that would mean the balanced cold would be taken off between the pressure reducing valve and the check valve so that expanded hot water could not backflow to the balanced cold taps as well as the mains water?
 
The balanced cold comes from the bottom of the pressure reducing valve, so it's upstream of the check valve, the check valve stops the expansion water getting back to both the mains and balanced supply

Img_0070.jpg
 
The balanced cold comes from the bottom of the pressure reducing valve, so it's upstream of the check valve, the check valve stops the expansion water getting back to both the mains and balanced supply

View attachment 226702
Thanks for explaining. Every day’s a school day.

I’m out of ideas now.
 
Guys, thanks very much for exchanging comments as it is way beyond me but i think you seem to have come to the same conclusion but as Rob has said, the cold comes in to the valve at the top and the cold balanced out immediately of that valve to the left / behind the valve.

However, what i can highlight having kept trialling things:
1. The same issue of warm out of the cold tap happens in every cold tap, bathroom, ensuite, kitchen etc
2. If I run a cold tap and go through the 30sec process of getting rid of the warm to eventually ge tthe cold constant supply, if i then open another tap in any part of the house the warm that then comes through is only for a few seconds and far less than normal.
3. Whilst the warm is coming out of the cold tap, if i then turn off the cold, and turn it back on, and then do it again, it seems as though this stpos the warm coming through far quicker than waiting the 30secs. The only problem is, i don't know if it was going to change to cold anyway but it defiently seems to speed the process up.
4. As Endaxi asked, if i run say a shower, the cold taps then tend to run cold immediatley and no warm water comes out at all
5. But if i run a hot tap for a short while, therefor enot usng as much hot water as a shower, this tends to speed up the process of the warm water not coming through for as long when the cold tap is turned on, although it still happens.
6. By the time i check the cold inlet pipe in a morning it is very very warm / hot. This was 26 degrees when tested the heat of the pipe the other day. I assume becuase UFH, CH and DHW are all on so making it work hard. However, this morning I got up early so all of these had been on but only for about an hour. The cold inlet pipe had not got warm. I ran the cold water tap and this still did the same thing, 20/30 secs of warm water. The pipes above the combination valve weren't freezing cold and even though the cold tap was on, they did not turn freezing cold until about 20secs later, at which time there was still warm coming out of the tap but then soon stopped. However, the cold mains supply pipe coming out of the floor wit the sto pcock on, wnet freezing cold once the tap had been turned on. Could this mean that the water between the stopcock in the cylinder room up to the combination valve is warming in the pipes as i think that pipe runs up the wall cavity, across the ceiling and then out throgh the pipes you can see coming out of the wall.

I don't know if that helps with anything but thought i would explain in case it made a difference to any of your thinking.

Thanks again as always
 
Sorry, forgot to also say, the outside taps also have the same issue but i assume that is just because they are teed off the tap inside that also has the problem so not really surprising news.
 
You need to establish if the balanced cold is being warmed by poor piping (thermal convection), failure of the inlet control group or crossover from mixer valves.

When you can recreate the fault, get someone to open a cold tap whilst you feel the balanced cold upstream of the inlet control group... Can you feel a slug of hot water passing until it then runs cold?
 

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