Warm Water From Cold Taps

[endaxi]

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.

Thanks MM, this is great stuff.

I didn't know that the baffle could be 'got around' by air and water. What on earth is it there for? It just seems to confuse the issue as without it, re-generating the air gap would always be effective and surely the system would always work as it should.

I'm with you on the PRV/NRV bit. I'm just trying to understand what has changed. Yes, I'm putting it under extra scrutiny and opening the cold tap immediately after the hot water demand is satisfied and under normal use it could have been a hot tap opened first or, as John D has said, even a toilet being flushed. I've never checked the temp in a cistern. That said, both my wife and I noticed the problem separately as soon as the kitchen tap was changed.

I will open the stopcock under the sink up to near fully open as you suggest but I don't know what other stopcock you're referring-to.

Thanks for this help.

 

[MeldrewsMate]

I don't know what other stopcock you're referring-to

Didn't you suggest that Mr BG partially closed the mains in the street?

Didn't you also suggest, very early on, that the tap fitter had tried to turn off the water to the kitchen hot tap by 'doing something in the airing cupboard', not that I really want to re-read all of this thread to quote you accurately! Is it possible he/she adjusted the wrong thing, and thus precipitated this epic?

BTW is the pressure setting to your hot cylinder adjustable? (just a thought)

 

[endaxi]

Didn't you suggest that Mr BG partially closed the mains in the street?

Didn't you also suggest, very early on, that the tap fitter had tried to turn off the water to the kitchen hot tap by 'doing something in the airing cupboard', not that I really want to re-read all of this thread to quote you accurately! Is it possible he/she adjusted the wrong thing, and thus precipitated this epic?

BTW is the pressure setting to your hot cylinder adjustable? (just a thought)

No, the tap fitter didn't move from under the sink and only used the stopcock there. I opened the hot taps at his request. The BG guy said the tap guy should have used the stopcock in the street and not the one under the sink. He then closed the SC under the sink to two turns out as he thought this was causing the problem due to the flow rate.

I put a lot of stuff on this thread due to my ignorance but have tried to clear things in today's posts.

I don't see a way of adjusting the cylinder pressure and there's certainly no pressure gauge. All I've got is the reduction valve and the PRV so I know the pressure is (or should be if everything works) between 3 and 8 bar.

 

[endaxi]

OK, I got up early this morning and opened a cold tap first thing and got the usual hot water for a while. I ran a hot bath and let the tank heat up again until the demand was satisfied. This time I flushed a toilet first and witnessed it fill up with hot water. That's something I would never have noticed before as I don't check.

I rang Heatrae Sadia technical support and told my tale again. I was told that re-generating the air gap will cause the tank pressure to increase when it's heated due to the air being compressed (as we know and discussed here). He said I need the 8 bar PRV and a check valve between the cold tank and the tee to the balanced cold for the system to work properly (as we have already concluded). He seemed surprised that I didn't have that already in place and thought it hadn't been installed properly. I told him it was an old system and was installed as per my installation manual. Once modified, the initial burst of hot water would then be from the hot tap only and not through the cold of course.

I tried to get to the bottom of why we haven't noticed this problem before as I thought that if there wasn't a sufficient air gap then the hot water pressure would still be higher than the cold and so would still have backflowed. He said hot water doesn't compress. I know that but I still really couldn't get to the reason why the pressure in the tank is now higher than it was before the air gap was re-generated. He implied that any excess pressure would have been dealt with through the system. I told him there has not been any discharge through the tundish. He said the PRVs should be tested.

In conclusion, he advised a G3 engineer should fit the new PRV and NRV and service the system to ensure the PRVs are capable of doing their job and that will solve my problem.

The issue has clearly always been there but for some reason not as bad prior to the air gap being re-charged. We did get a bit of warm coming through the cold side sometimes but under normal usage we don't always open a cold tap immediately after the tank has heated and there's a load of hot water at the bottom near the primary coil and the cold inlet. We may flush a toilet first, open a hot tap or finish cleaning our teeth before the hot even gets to the cold tap.

I can't thank you all enough for taking the time to put forward your thoughts and educating me. It's been really interesting.

Every day's a school day.

 

[John D v2.0]

I write a reply on the bus this morning but it was lost somehow but here is the essence

r normal use it could have been a hot tap opened first or, as John D has said, even a toilet being flushed

That's not actually what i meant, i actually meant that if the pressure increases more than usual due to no space for expansion but doesn't open the blow off, the toilet might start dripping in much the same way as a leaky tap. This would take up the extra 3.5l which would disappear down the pan unnoticed.
The regeneration of the correct expansion capacity may keep the pressure low enough to not lose the extra down the loo, so it's available to come through the cold tap normally.

 

[endaxi]

I write a reply on the bus this morning but it was lost somehow but here is the essence


That's not actually what i meant, i actually meant that if the pressure increases more than usual due to no space for expansion but doesn't open the blow off, the toilet might start dripping in much the same way as a leaky tap. This would take up the extra 3.5l which would disappear down the pan unnoticed.
The regeneration of the correct expansion capacity may keep the pressure low enough to not lose the extra down the loo, so it's available to come through the cold tap normally.

Yes, I realised after the fact that you probably meant what you've just explained.

See my previous post for my hot water in loo experience.

Thanks.

 

[dilalio]

@endaxi when you have the alterations made to the install, get the g3 chap to add gauges at relevant positions so that you can monitor pressures during usage. This will help you in future for any further issues.

 

[endaxi]

@endaxi when you have the alterations made to the install, get the g3 chap to add gauges at relevant positions so that you can monitor pressures during usage. This will help you in future for any further issues.

That's a good idea dilalio. How about one before the new check valve and one after it (or in the hot outlet from the tank). That would tell me the pressure in the tank and the pressure in the balanced cold.

The only downside is the added cost. Maybe I'll get a quote with gauges and without and decide from there.

As an aside, I owe people a lot of thanks and see there's a 'thanks' button thing at the bottom of all replies. Can anyone tell me, do I hit that every time someone helps me? If so I'll go back through this and the other thread I started last year and use it. I see I've got a 'Thanks Received'. I've no idea when, who by or why. All I've ever done is ask questions. Maybe someone hit the button by mistake ha ha.

Anyway, thanks again.

 

[HERTS P&D]

I write a reply on the bus this morning but it was lost somehow but here is the essence

On the bus??? Are you not a tradesman?

Andy

 

[endaxi]

I write a reply on the bus this morning but it was lost somehow but here is the essence


That's not actually what i meant, i actually meant that if the pressure increases more than usual due to no space for expansion but doesn't open the blow off, the toilet might start dripping in much the same way as a leaky tap. This would take up the extra 3.5l which would disappear down the pan unnoticed.
The regeneration of the correct expansion capacity may keep the pressure low enough to not lose the extra down the loo, so it's available to come through the cold tap normally.

Sorry to quote you again John but the same question is stuck in my head...

Re-generating the air gap is always cited as a cure for excess pressure in the tank (ie. when there's water going through the tundish). In my case it seems that re-generating the air gap has increased the pressure in the tank. Why would that happen?

What you've said about dripping taps and loos is making more and more sense. The kitchen tap dripped for months, sometimes worse than other times. If I opened the hot side of the tap for several seconds it stopped dripping for a while. I thought this was due to the temperature rise in the tap but now see it was more likely due to relieving the hot water pressure... if only for a while.

Prior to the kitchen tap leaking we've had at least three cases of bathroom taps dripping for months on end, another case of a leaking kitchen tap and two cases of water overflowing from cisterns into the toilet bowls. Over many years, now I think about it, if it wasn't one dripping tap or another, it was a toilet. I currently have no dripping taps (ok, one intermittently actually but that one is ok when it's turned down fully and I haven't seen it dripping for days) or overflowing loos.

Couple that with the fact that there's often one loo or another getting flushed (all on the balanced cold) and hot taps being opened then I can now see that there was plenty of pressure relief going on throughout the day so maybe the potential backflow problem went un-noticed.

I had no idea that all the water outlets in my house (and there are many), with the exception of my kitchen and utility cold taps, were feeling the pressure of the hot, expanded water in my Megaflo cylinder and so relieving the pressure every time one dripped or was opened.

Also, I've been playing with the tank stat so for all I know I've increased the temp a degree or two which would also have an effect.

So, I'm wondering if re-generating the air gap has made any difference at all or is this hot water backflowing to the cold taps all down to nothing dripping for once and me now focusing on the output of a bathroom cold tap immediately after the tank has heated?

I'm still going to get the new check valve and PRV fitted of course.

Any further thoughts appreciated.

Thanks.

 

[John D v2.0]

Sounds like you're spot on with what happened.

In my case it seems that re-generating the air gap has increased the pressure in the tank. Why would that happen?

What makes you think that? Don't confuse pressure with volume/quantity.
Before the pressure was too high but the taps and loo released the pressure before the pressure relief (maybe the pressure relief was blocked?) and the volume of water was lost as it expanded.
Now that you have got the air bubble, the pressure is lower so the expansion quantity is retained in the system at higher than the pressure reducing set pressure and comes out when the tap opens.
I would consider your existing pressure relief valve suspect as it sounds like it's been not doing it's job and the taps have been doing the job which would be dangerous. If the tank were to get to 100c for any reason, opening a tap could cause the contents to turn to steam very quickly and expand outwards

 

[John D v2.0]

Duplicate

 

[endaxi]

Sounds like you're spot on with what happened.

What makes you think that? Don't confuse pressure with volume/quantity.
Before the pressure was too high but the taps and loo released the pressure before the pressure relief (maybe the pressure relief was blocked?) and the volume of water was lost as it expanded.
Now that you have got the air bubble, the pressure is lower so the expansion quantity is retained in the system at higher than the pressure reducing set pressure and comes out when the tap opens.
I would consider your existing pressure relief valve suspect as it sounds like it's been not doing it's job and the taps have been doing the job which would be dangerous. If the tank were to get to 100c for any reason, opening a tap could cause the contents to turn to steam very quickly and expand outwards

Thanks John, you've explained that well and I think I understand what's going on at last. I just thought that if I had excess pressure in the tank before the re-generation, due to lack of air gap, then I would have had the backflow I'm now getting.

When I get the work done on the system I intend to get a brand new 8 bar PRV and check valve fitted in the correct place and leave the existing 8 bar PRV redundant in the combination valve set (or have it removed and discarded). I will also get the T & P valve checked for correct operation at the same time.

Fascinating stuff this.

 

[dilalio]

That's a good idea dilalio. How about one before the new check valve and one after it (or in the hot outlet from the tank). That would tell me the pressure in the tank and the pressure in the balanced cold.

The only downside is the added cost. Maybe I'll get a quote with gauges and without and decide from there.

As an aside, I owe people a lot of thanks and see there's a 'thanks' button thing at the bottom of all replies. Can anyone tell me, do I hit that every time someone helps me? If so I'll go back through this and the other thread I started last year and use it. I see I've got a 'Thanks Received'. I've no idea when, who by or why. All I've ever done is ask questions. Maybe someone hit the button by mistake ha ha.

Anyway, thanks again.

Most, if not all, pressure reducing valves come with provision for connecting a gauge and for in line pipework it's a small cost to insert one on a Tee. I'd put one on the main so you know what your incoming supply is. One after the pressure reducing valve (inlet control group) and one on the cold inlet to the cylinder. That will tell you all.

 

[endaxi]

Most, if not all, pressure reducing valves come with provision for connecting a gauge and for in line pipework it's a small cost to insert one on a Tee. I'd put one on the main so you know what your incoming supply is. One after the pressure reducing valve (inlet control group) and one on the cold inlet to the cylinder. That will tell you all.

Ok, that makes sense. If it's only going to increase the cost by a fraction then it's worth doing.

Thanks.