Warm Water From Cold Taps

...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...
So, the re-generation of the air gap could explain why the hot tap in the bathroom that has been dripping of late has suddenly stopped. I haven't seen it drip for days.

I'm now thinking that if I had re-generated the air gap periodically as I should have done, I may not have had dripping taps in the first place.

It's all going around in circles.
 
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So, the re-generation of the air gap could explain why the hot tap in the bathroom that has been dripping of late has suddenly stopped. I haven't seen it drip for days.

I'm now thinking that if I had re-generated the air gap periodically as I should have done, I may not have had dripping taps in the first place.
Exactly! But the safety devices need checking as you plan, as they should have protected the system.
 
Right... For anyone with a Megaflo cylinder, particularly one that was installed many years ago, that has similar problems of warm water from cold taps and/or frequent problems with dripping taps and by-passing toilet cisterns, and is reading this thread for the first time, I will edit the opening post to direct you to this one where I will attempt to summarise the symptoms, cause and cure of the problems...

My Megaflo cylinder was installed 15 years ago and was designed at the time so as not to have an 8 bar pressure relief valve and check valve between the cold water inlet to the cylinder and the tee to the balanced cold water supply. The tee to the balanced cold water supply and tank inlet is just after the cold water combination valve that reduces the pressure to 3 bar maximum, provides a check valve and currently houses the 8 bar pressure relief valve.

The air gap in the cylinder has never been knowingly re-generated as should have been done annually. The depletion of the air gap over time has meant there is less expansion room so there has been a resultant increase in pressure in the tank when the when the water gets hot (although not enough to cause any visible discharge through the tundish).

The resultant high pressure has been felt through the whole of the hot water and balanced cold water system which includes all water outlets (including toilet cisterns) except the kitchen and utility cold taps (which are fed direct from the incoming mains). This high pressure has probably contributed to the many dripping taps and toilet by-passes that have occurred over the years. As one was fixed another would start to drip.

Recently, during the replacement of the kitchen tap that was dripping, the hot water was drained sufficiently from the Megaflo tank to inadvertently re-generate the air gap. This has caused the pressure in the tank to be less than it was previously when the water is hot but has allowed the expansion of the hot water into the air gap as is supposed to happen. Suddenly, a previously dripping hot tap in the bathroom has stopped dripping.

The downside however, is that once the water has heated there is an amount of expanded water in the tank that would release with a short burst when a hot tap is first opened. This in itself is not a major problem. Without the pressure relief valve and check valve in the correct place however, if a cold tap is opened first (before any hot tap is opened or a toilet is flushed) then that expanded water will fight against the 3 bar balanced cold and force its way out of the cold tap until the hot and cold water pressures equalise. The cold water will then run from the tap as it should. In my case, the water from the cold tap could be as high as 60c for around 30 seconds before gradually getting cold, so quite dangerous if you don't expect it.

Pressures now equalised, everything will work fine until the hot water expands again, particularly if it has been re-heated, and the process repeats itself.

So, re-plumbing the system to put the pressure relief valve and check valve (NRV) between the cold inlet to the tank and the tee to the balanced cold, and re-generating the air gap periodically, will fix the hot water from the cold tap problem, keep the pressure at the balanced cold taps at 3 bar max (as per the 3 bar pressure reducing valve in the combination valve set) and will help keep the water pressure at the hot taps from being un-necessarily high, therefore hopefully reducing the number of issues with dripping taps.

I have learned that it's important to get the pressure relief valves checked annually for correct operation and that an air-gap re-generation once a year on top of any necessary re-generations (water coming through the tundish for instance) is also no bad thing.

A fascinating journey to this logical conclusion.

If I have made any errors in this appraisal then anyone can please feel free to correct me and I will amend it accordingly.

Thanks to all you generous people who have given their time and effort to help me reach a conclusion to the problem(s).
 
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Excellent summary, my only comment is the prv abbreviation could be confusing as it matches pressure reducing valve and pressure relief valve.
I have been avoiding confusion by using the non standard term blow off for the latter.
 
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Excellent summary, my only comment is the prv abbreviation could be confusing as it matches pressure reducing valve and pressure relief valve.
I have been avoiding confusion by using the non standard term blow off for the latter.
Thanks John. I have amended to long-hand accordingly.
 
A bit after the fact but to conclude, I can confirm that the fitting of an 8 bar PRV and check valve between the cold water inlet to the Megaflo and tee to the balanced cold water fixed the problem for around £130 parts and labour. Job done. Thanks everyone.
 
Good evening guys, apologies for digging up an old post but I feel I have feel I have similar issues to that stated. My hot water tap has warm water and a surge of high pressure when turn turning the cold water tap on. The hot water follows around 5 seconds after turning the cold tap on. I have isolated the hot water supply to the house from the top of the cylinder. So only the cold water runs (which isn’t so cold at times). Can anyone recommend any other checks I need to preform please to iron these issues out? I’ve attached a photo of the set up, I think it’s an invented cylinder with a system boiler? The cold water feed is very cold running from the Mains up through the back wall so don’t think it’s getting warm from the other pipes? Any help would be great. Cheers Ash.
 

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The cold water in our downstairs wc initially runs cold for a few seconds, then warm for a few seconds, then cold again. I just put it down to the pipes being routed through our airing cupboard warming that section of cold pipe.
 
Good morning, the pressure seems to be rather high in the line when opening, I know the heat expands the water in the line but the cold water supply is freezing in the boiling room, id very much doubt the cold water line is conducting that much heat from the run through the floor boards section?
 

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