Warm Water From Cold Taps

[John D v2.0]

We had a brand new mixer tap fitted in the kitchen a few days ago by the plumbing people

I can't get it out of my head that the hot and cold water worked fine before the tank was drained down

The expansion is caused by the volume of water being heated by a given number of degrees in an enclosed space. Either you've made more water heat each time (unlikely) increased the temperature difference significantly (even less likely) or you've made it more enclosed than before. What was wrong with the old tap again?

 

[endaxi]

The expansion is caused by the volume of water being heated by a given number of degrees in an enclosed space. Either you've made more water heat each time (unlikely) increased the temperature difference significantly (even less likely) or you've made it more enclosed than before. What was wrong with the old tap again?

Hi John, thanks for your input.

The kitchen tap is a bit of a red herring. The old tap dripped but the new one is fine. I get hot water from the hot side and cold from the cold side with no backflow and no drips. The draining of the tank is what brought the problem to light, I think.

I can only reiterate that the problem remains that just after the hot water demand from the cylinder is met and the boiler switches off, if a bathroom cold tap is opened a backflow of very hot water from the Megaflo runs through the cold tap for up to a minute. At other times of the day once this initial burst of hot water is gone and the boiler hasn't been heating the tank again then the backflow through the cold taps is just warm for about 30 seconds so more of a nuisance than a danger.

Clearly the cylinder pressure when the water is hot is higher than it used to be. The water is set to 60 degrees as it should be and that is what I get at the hot taps. The problem is that I also get it at the cold taps in some instances and this is of course dangerous.

As I say, the up to date installation design for a Megaflo is that there is an 8 bar PRV and a check valve (NRV) between the cold water inlet to the cylinder and the tee piece to the bathroom cold taps (the balanced cold I believe) so backflow of hot water from the cylinder to the cold taps is prevented. My system does not have this as the the only check valve is in the combination valve before the tee to the bathroom cold taps. This stops backflow to the drinking water in the kitchen but not to the bathroom taps.

I can see a cure to the problem by fitting a new NRV at the tank inlet and moving the PRV but what caused it in the first place is a mystery. That's why I'm wondering if the floating baffle in the Megaflo cylinder has got stuck on the dip tube when the water was drained and this is causing the higher than normal pressure when the water is hot. If so, then would an external expansion vessel cure the problem?

I'm happy to have the 8 bar PRV moved and a new check valve fitted but as I said previously this might present a problem for an engineer as the tundish is more than a metre away from the from where the PRV would be fitted and also from the existing PRVs that are fed into it. I gather that this contravenes the G3 regulations. Therefore, if the problem can be solved with an expansion vessel then perhaps this is the way to go. Maybe even a combination of both fixes would be the preferred solution. Belt and braces, so to speak.

I will put these questions to Megaflo tech support. If anyone else would like to answer them then I'd be happy to hear any views.

Thanks as always.

 

[Hot&Cold]

I managed to find some Megaflo FAQs on line that says that ''if the cold water is warm for a short period of time before returning to cold then it is more of an inconvenience than a major problem''.

I'll get the Megaflo serviced for the very first time and see what is recommended then.

I'll have to get a Megaflo G3 person in.

Think you have answered your own question on this 15+ year vintage uv cylinder,ask heatrae sadia to service the mugaflow but please tell them you have a possible fault when booking the technicians visit.

 

[dilalio]

Can you upload some photo and/or a diagram of the cylinder set up. When the kitchen tap was fitted and the cold mains supply turned off, opening taps should not have drained the cylinder. It should have just depressurised and settled somewhere near the top when the hot taps were opened. Did the balanced cold taps continue to run when this was done and did the water from them become hot? In effect, did back syphoning occur thus pulling water out of the tank, via the cold inlet?
For hot water (very hot water even) to come out of the cold taps, means that the pressure from the cylinder is overcoming the incoming mains pressure and that sounds like a fault with the inlet control group/balancing valve.

 

[endaxi]

Think you have answered your own question on this 15+ year vintage uv cylinder,ask heatrae sadia to service the mugaflow but please tell them you have a possible fault when booking the technicians visit.

Thanks for the input, H&C. I think I have answered my own questions as you say but those answers have changed since I wrote the bits you quoted. I have since re-charged the air gap and learned a lot more about my hot water system and what may have gone wrong.

What I'm trying to do now is see if my current answers at #32 above are legal, logical and viable before I get a professional in.

 

[endaxi]

Can you upload some photo and/or a diagram of the cylinder set up. When the kitchen tap was fitted and the cold mains supply turned off, opening taps should not have drained the cylinder. It should have just depressurised and settled somewhere near the top when the hot taps were opened. Did the balanced cold taps continue to run when this was done and did the water from them become hot? In effect, did back syphoning occur thus pulling water out of the tank, via the cold inlet?
For hot water (very hot water even) to come out of the cold taps, means that the pressure from the cylinder is overcoming the incoming mains pressure and that sounds like a fault with the inlet control group/balancing valve.

Hi dilalio, thanks for the ongoing input.

Not sure how to post diagrams but if you refer back to page 11 of the link I put in at post #18 on page 2 of this thread you will see how it should be if it was installed today.

My installation is exactly the same except for the fact that my 8 bar PRV is part of the incoming cold water control group to the right of it (before the tee to the balanced cold) and not at the cylinder inlet with a second check valve next to it (after the tee to the balanced cold). My only check valve is in the inlet control group, so where this stops expanded water getting to the drinking water downstairs, it doesn't stop it getting to the balanced cold.

This was acceptable at the time of installation and up until now has only resulted in a few seconds of warm water at the bathroom taps before they run cold. Even then it was only on occasions and a barely noticeable minor nuisance. Megaflo put it down to an installation design fault and more of an inconvenience than a major problem.

Now, clearly something has changed and I do have a major problem as I get water at nearly 60 degrees coming from the cold taps. My reasoning is that when the hot water was partially drained from the tank (cold mains off... hot taps on...cold taps off) it essentially re-charged the air gap. In any case I have re-charged the air gap properly on Friday.

So what has gone wrong? I can only think that the floating baffle has got stuck and doesn't rise and fall fully any more and that as a result the pressure builds up in the tank as the water tries to expand. It could potentially reach just under 8 bar and so not show a discharge through the tundish but obviously would be a good few bar higher than the 3 bar maximum cold inlet. This would then cause the backflow through to the balanced cold.

I have felt the pipes in the airing cupboard so I'm as confident as I can be as an amateur that things are happening as I've explained. I just have no way of seeing inside the tank so cannot prove my theory is 100% correct.

As to a solution, well I guess a new Megaflo is out of the question hence I want to know if an external expansion vessel could be installed in the hot water outlet of the tank. Even if the floating baffle is working, or partially working, I can't see that an external expansion vessel could do any harm. Surely the at worst it could only make no difference.

If it did make no difference then that's the point at which I feel I should move the 8 bar PRV to where it is in the diagram I've referred you to and put a check valve between it and the balanced cold water tee piece.

An external expansion vessel and moving the PRV/fitting another check valve would definitely fix the problem and bring the system up to date as far as I can see but I need some opinions as to whether I'm right before I head down that road. Then there's the issue of the length of the discharge pipes from the PRVs to the tundish that I mentioned earlier. That could throw a spanner in the works.

I've explained as best I can I'm afraid. What are your thoughts?

 

[John D v2.0]

The old tap dripped

Exactly my point. So the old one released the pressure gradually as the water warmed, but the new one doesn't so the pressure builds up in the tank. That all perfectly explains your symptoms.

If you're still not convinced, try cracking the new tap on a tiny drip the same as the old one did and leave the tank to warm from cold, and the problem will magically go away!

 

[endaxi]

Exactly my point. So the old one released the pressure gradually as the water warmed, but the new one doesn't so the pressure builds up in the tank. That all perfectly explains your symptoms.

If you're still not convinced, try cracking the new tap on a tiny drip the same as the old one did and leave the tank to warm from cold, and the problem will magically go away!

I can see where you're coming from John but if the drip was from the cold side of the mixer tap then it was direct from the cold water main and before the NRV in the airing cupboard that feeds the Megaflo and the bathroom taps so would have no effect on the backflow from the tank at all. If it was from the hot side of the tap then I guess the pressure in the tank would not get so high as it would with no dripping so that could in theory mask a problem with the tank over-pressurising... a sort of inadvertent PRV I suppose. An interesting thought and your theory would probably work if I left the hot side of the tap dripping but doing so is not a permanent solution.

Even so, the drip from the tap was a relatively new thing and before it started to drip the was no problem with the hot and cold water. The new tap hasn't caused the problem as far as I can see but may have contributed to showing it up.

Thanks for the thoughts.

 

[Hot&Cold]

What I'm trying to do now is see if my current answers at #32 above are legal, logical and viable before I get a professional in.

Doubt that can be fully answered without actually doing onsite test's and carrying out observations of your plumbing system,consider getting HS to service & repair if possible.
Dont leave it to long

Let us know how you get on Good luck.

 

[endaxi]

Doubt that can be fully answered without actually doing onsite test's and carrying out observations of your plumbing system,consider getting HS to service & repair if possible.
Dont leave it to long

Let us know how you get on Good luck.

Thanks H&C, will do. Will phone HS tomorrow for advice then get BG back (I have a maintenance contract). If there's a fault then they should fix it under contract. If it's purely a design/installation problem then it's down to me I guess.

 

[John D v2.0]

An interesting thought and your theory would probably work if I left the hot side of the tap dripping but doing so is not a permanent solution

Did not realise it was fine before the dripping tap, so that does rule out my theory. Unless there was a drip elsewhere after the pressure reducing valve which was also sorted as part of the change eg the old tap was also passing between the hot and cold sides or something.
I am certainly not suggesting leaving a drip as a permanent solution, merely trying to address your point about it must have been something to do with draining down the system, when of course that is not the only relevant change made.
I certainly agree as per my first post about the nrv situated after the balanced cold takeoff would solve the problem at the expense of the cold not always being balanced, but the change would need to be made by a qualified engineer to ensure no safety system would be negated.

 

[endaxi]

Did not realise it was fine before the dripping tap, so that does rule out my theory. Unless there was a drip elsewhere after the pressure reducing valve which was also sorted as part of the change eg the old tap was also passing between the hot and cold sides or something.
I am certainly not suggesting leaving a drip as a permanent solution, merely trying to address your point about it must have been something to do with draining down the system, when of course that is not the only relevant change made.
I certainly agree as per my first post about the nrv situated after the balanced cold takeoff would solve the problem at the expense of the cold not always being balanced, but the change would need to be made by a qualified engineer to ensure no safety system would be negated.

Your theory was sound John and was something I hadn't thought of. It's difficult to convey everything pertinent in one place when I'm finding out more all the time but I think I've about covered everything in post #36 above.

Any thoughts about my theory that the floating baffle is stuck or damaged?

Thanks again.

 

[John D v2.0]

Any thoughts about my theory that the floating baffle is stuck or damaged?

As I'm not a heating engineer i am coming from a place of physics rather than baffles. But i don't see there could be any problem with the expansion capability of your system.
what i do know is that if you have 200l water at 15c and heat it to 60c you will have an extra 3.5l of volume. So regardless of baffles or whatnot you'd have to run 3.5l (of hot water) through your cold tap before getting any cold at all.
If you put external expansion vessels or whatever else it won't change that fundamental fact. The suggested nrv would prevent that 3.5l to flow backwards out the cold side.
If you had problems with the expansion vessel ie insufficient expansion capacity you should have water coming out of the pressure relief. I can speculate all day where else it could have dissipated possibly elsewhere if something else were to let by before the set pressure were reached. For all i know that could have been the case before (UN noticed into a wc cistern), that could even have caused the original tap fault.

 

[MeldrewsMate]

I can only think that the floating baffle has got stuck and doesn't rise and fall fully any more and that as a result the pressure builds up in the tank as the water tries to expand.

No, no, no. You assume the floating baffle completely separates the water from the air gap above it, sealing the air in. It does not. It is a loose fit around the central hot water discharge pipe. If it has somehow stuck then water and air will pass it, but it will not reduce the expansion volume one iota. Consider, if you must, the condition where a notional item had reduced the internal expansion volume to a size that would allow the expanded hot water to approach the 8 bar relief valve setting. In that instance it is clear to see that running off hot or cold water would rapidly reduce the pressure in the tank to the level of that supplied by the inlet pressure control set. So it would be clear that the kind of malfunction you described would NOT cause the hot water to run through the cold pipework for as long as it would with a fully operational baffle. Indeed it would shorten the hot-in-cold symptom to a minimum.

Please consider both this and the kitchen tap change to be the red herring(s).
I also suggest that the mains inlets (both stopcocks) should be within 1/4 turn of being fully open. They will have no influence on your problem.

My suggestion is to modify the inlet pipework to include the extra relief valve and the non-return valve.
There is no regulation that prevents you having a second or even third tundish, if needed, so long as the vertical run below each is the minimum 300mm before any bend.

 

[endaxi]

As I'm not a heating engineer i am coming from a place of physics rather than baffles. But i don't see there could be any problem with the expansion capability of your system.
what i do know is that if you have 200l water at 15c and heat it to 60c you will have an extra 3.5l of volume. So regardless of baffles or whatnot you'd have to run 3.5l (of hot water) through your cold tap before getting any cold at all.
If you put external expansion vessels or whatever else it won't change that fundamental fact. The suggested nrv would prevent that 3.5l to flow backwards out the cold side.
If you had problems with the expansion vessel ie insufficient expansion capacity you should have water coming out of the pressure relief. I can speculate all day where else it could have dissipated possibly elsewhere if something else were to let by before the set pressure were reached. For all i know that could have been the case before (UN noticed into a wc cistern), that could even have caused the original tap fault.

Hi John,

I'm loving this.

I get about 5-10 seconds of cold (existing water in the pipes?) then about the same of warm (some conduction from the tank to the cold water inlet?) then about 30 seconds of hot (about 3.5ltrs at the rate I'm running the tap would not be an unreasonable estimate), then about another 10 seconds of warm (cold water taking heat from the hot pipework?) then back to cold again. That sits nicely with what you've just said.

Now, if there's a problem with expansion space in my tank, say through a floating baffle that doesn't float any more (stuck at the top or bottom or anywhere), the water would not expand but merely increase in pressure. Would this increase in pressure be more likely to cause my problem than if the water had been allowed to expand into an air gap. There would be a bigger volume of water for sure but at a lower pressure than if it didn't expand. Am I right in saying that?

This hot water is battling a 3 bar cold inlet so if there was no expansion the pressure would be much greater I presume and more likely to cause my problem. If the air gap is not good enough then a symptom is the 8 bar PRV letting-by. This doesn't happen but the hot water could still be as much as 7 bar or more if there's no room for expansion.

What do you think?

Thanks for helping.