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Discuss Pushfit connections under low pressuee in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Messages
7
Non plumber here, just a householder so please go easy on me!

Our house was renovated several years ago and all plumbing replaced. It is a mixture of copper and hep20 - listed building so hep20 was better in some places to use existing holes in joists. The system was fully pressure tested after install.

We had a major flood caused by a pushfit connection which had come loose. An engineer investigated and came to the conclusion that it had not been properly pushed originally.

Several months on now I wonder whether there was a bit more to the issue.

The connection was on the secondary hot water return (we have a circulating system). Reading the hep20 manual this is generally not allowed but there is a carve out for domestic installs with intermittent timing on the pump. So ok so far because this is what we have.

At the time of the leak, our expansion vessel on the secondary hot water circuit had failed and was awaiting replacement. We have a booster pump which pumps our water and so at the time of the leak, when turning on a hot tap there was initially a pressure drop to almost nothing and then when the pump kicked in, normal service resumed a few seconds later.

Prior to the leak, I had noticed an occasional splutter of air at the highest tap in the house, only when opening the tap.

After the leak had been repaired and when the expansion vessel replaced this air issue went away.

I would be interested on the views of the experts on my alternative hypothesis about what has happened

1. Expansion vessel failure meant when a tap was turned on, there was an initial drop in pressure.
2. If a dishwasher or some other long running use of water was taking place and an outlet was turned on higher up the house, the demand for water would exceed what was available for a few seconds.
3. In this circumstance it would pull water from elsewhere in the pipe (validate please - this may be nonsense!!)
4. In this circumstance there was potential for a section of pushfit pipe to have no pressure in it at all (validate again please!)
5. Should this be possible, the pushfit pipe may lose its airtight integrity because pressure must be maintained in order to activate the grip (please validate!)
6. At this point, air could be introduced due to a vacuum in that section of pipe (please validate)
7. When the pump kicks in, demand is fulfilled and pressure is restored, including the section of pipe which experienced negative pressure.
8. This motion could lead to the pushfit connection gradually working itself loose over an extended period of time. (Please validate!)
9. Eventually the connection worked loose enough that even with pressure the pushfit could not maintain grip and the system leaked (please validate).

Looking to really try and understand the root cause here to prevent something like this happening again. I am not that convinced about the official diagnosis as had the pushfit connection not been fully pushed, in a pressurised water system it would have failed a pressure test or failed within the first few weeks of installation.

More generally, are pushfit connections airtight during complete loss of pressure? If not, does this not mean that when any pushfit based system is drained for maintenance there is potential for the pipe to shift slightly each time the system is repressurised? After many occurrences of this happening, there could be potential for a leak?

Remember, just a householder here so go easy on this unprofessional diagnosis - just think there is a bit more to it than as was diagnosed.

Thanks!
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Messages
27,139
A sign of more to come hot water return starts to break the fittings down after around 7 years
 

scott_d

Plumber
Gas Engineer
Messages
5,380
Plastic shouldnt be used with secondary return pipework. Maybe that and the broken expansion vessel caused a pressure spike and it popped off.
 

Ric2013

Plumber
Messages
2,757
I can't see that 3 will happen. The water is pressurised in the pipes and opening a dishwasher or tap does not create a vacuum. You could get a slight vacuum if the downstairs taps are opened and the weight of the water in the pipes is pulling as it flows down - but this is minimal it is not really worth considering. Unless, by 'it', you mean the secondary return pump, which I suppose could pull a vacuum?

6. If there were a vacuum, I can't see any reason why the o ring seal (which bears against the side of the pipe) should be any worse at holding a vacuum than holding pressure (though you have got me wondering!).

8. The grab rings on pushfit tend to work like a ratchet - the pipe will move one way and not the other. So if there were a vacuum it would tend to pull the pipe further into the joint, at worst. If the pipe is already fully inserted into the fitting (as it should be), then no movement is possible. Fittings will not creep off.

9. The grab ring is a steel thing and a separate issue from the o ring water seal. The seal is made before the grab ring, so joint disinsertion failure would usually come before a slight leak.

Shaun - if hot water on a pumped return breaks down the fittings, how is this any different from the hot water in a hot distributing or supply pipe or in a heating system? Do these also break down the plastic, or is there something different about secondary returns? I suspect there is and thought plastic in these circumstances was not allowed (though the OP says the manufacturer says you can), but not sure what it is.
 
Messages
7
Really helpful many thanks. And just to quote what I had read in the hep20 manual:


Hep2O® should not be installed in Ringmain Installations.
A ringmain is a water-replenished circulating system maintained at a
constant high temperature to provide a permanent source of hot water to its distributing pipes.
Typical locations where ringmain systems are used are hospitals or hotels which distribute constant hot water to wards or rooms at a distance from heat source.
This type of installation is very different from conventional domestic hot water and central heating services.
This restriction does not apply to domestic systems operating intermittently at temperatures less than 65°C where a design life of
50 years as detailed in BS 7291 can be expected.

The carve out at the end applies in this case and as ric2013 says, how is this any different to a central heating circuit given intermittent heating to around 65c?
 

Ric2013

Plumber
Messages
2,757
The carve out at the end applies in this case and as ric2013 says, how is this any different to a central heating circuit given intermittent heating to around 65c?
Thinking out loud, and possibly making myself foolish in the process, a heating system might run at 80°C at worst. It would, however, be water with minimal oxygen content, whereas dissolved oxygen and chlorine etc. may be in DHW. Hot water that does not recirculate may be assumed to be normally below 65°C on most systems, and, particularly in a high occupancy house, is likely to be at that temperature for most of the day, and, like recirculated water, would be likely to contain dissolved gasses.

Hence my failure to understand the difference.
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Messages
27,139
It’s constantly oxygenated so this is bad for plastic also they only allow pir sensors if it’s timed it’s a no also I would email hep and you want a rep out as a fitting has failed
 

scott_d

Plumber
Gas Engineer
Messages
5,380
Any pictures of the failed fitting?
Was there a Hep insert fitted?
Was it fully inserted?
Any damage to the pipe?
 
Messages
7
Any pictures of the failed fitting?
Was there a Hep insert fitted?
Was it fully inserted?
Any damage to the pipe?
Fitting was totally fine - we even tested repushing it and fitting a new grabber and turning on the water again and it held until we could get a proper replacement part. It was all assembled correctly. No damage to pipe at all. The pipe had just worked loose
 

Ric2013

Plumber
Messages
2,757
Fitting was totally fine - we even tested repushing it and fitting a new grabber and turning on the water again and it held until we could get a proper replacement part. It was all assembled correctly. No damage to pipe at all. The pipe had just worked loose
So there were scratches on the pipe that evidenced the working loose as the grab ring had progressively moved up the pipe?
 

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