pressure problem

[AND]

Is there anyone that can help with this problem?
I have fitted an unvented cylinder for my DH supply. I have mains cold water running into a 3 bar pressure regulating valve, this then runs to the non return valve situated just above the top of the tank. Off the valve is an 18 ltr expansion bottle and also a feed to the pressure relief valve.
Here is the problem. When I turn on the hot tap, the hot water flows just until the hot tap is nearly fully open. Just as the tap is almost fully open, it sounds like the NRV slams shut and all the water pressure drops. I am left with a feeble gravity trickle. If I then turn the tap off and on again ,the same thing happens. I have removed the regulating valve to see if it makes any difference, which it does not. I have decent mains pressure and cannot understand how if I have reasonable input pressure, the valve can slam shut against the flow. If the valve was to chatter, it would make more sense ( to me anyway ) but it just shuts off the mains pressure. I have inspected the NRV and all seems to be as you would expect. Any ideas?

Andy

 

[BLOD]

Can you post a picture of the pipework & valves ?

 

[AND]

Hi BLOD. Thanks for the quick reply. Does this help?

 

[AND]

 

[BLOD]

Difficult to see from the photos but I'd start by checking the reducing valve is the correct way round - same with the non return valve.
Is your expansion vessel a potable one - red ones are usually for central heating and aren't.

 

[AND]

Hi BLOD

Thanks again. I have checked the pressure reducing valve, it has an arrow on it so I could not have got that one wrong. I also tried taking it out of the system. As for the NRV surely it can only go in one way, cant it? If I got that the wrong way round I simply wouldnt let the water through. I did wonder if I had put it in the wrong way round, but after inspection, I cant see how that is possible.

Regards
Andy

 

[AND]

The expansion vessel came with the kit, and the system is potable.

 

[BLOD]

If you're sure the prv & the check valve are th right way round and are working ok, I'd look for a faulty stop valve further back up the supply.
A blockage in the cylinder can also cause this, but usually in an older cylinder - is this one new ?

Re the ex vessel - I've only ever seen white or blue potable ones, never red.

 

[AND]

Thanks again. It has me completely baffled. The cold water supply is reasonably good, with no loss of pressure when the cold is turned on. This suggests to me the stopcock is not faulty.. The tank is SS and fairly new, so shouldnt be a problem with that, but I will see if I can find anything. I did notice alot of rusty water come out of the expansion vessel when I took it off to re-pressurise it though.

Andy

 

[BLOD]

Probably because it's a red one and not potable.
With all due respect i think you'd be well advised to get someone qualified (more familiar with unvented cylinders)
to sort this out for you - especially if it's a second hand purchase.

 

[GQuigley67]

does seem strange that there is a red expansion vessel fitted, since this is associated with heating and probably not as good quality as potable one.

 

[AND]

I will replace it an see if it has any effect. Thanks BLOD.

 

[diamondgas]

Hi Andy,

I'd be checking the NRV is the right way around and piped correctly. Something is restricting the flow of water through the cylinder; that'll be either its cws or hws. you could try hitching a hosepipe up TEMP to the hws outlet on the cylinder and see what you get then? Are all hot water taps the same?

 

[fuzzy]

swapping the expansion vessel is required because its the wrong one, but it wont fix your problem.

check the filter on the composite valve. be aware you must be qualified to install these, they are potential bombs if installed incorectly, they have killed people in the past. pressurised hot water was banned in the Uk until the 80s such was its danger, so id get somebody in mate

 

[fuzzy]

YouTube - Water Heater Blast

YouTube - Water Heater explosion

 

[diamondgas]

Nice one fuzzy.

I've had experience of a composit valve installed incorrectly, water flowed great but the pressure kept building and the prv would open up. Turned out the exp vessel had been hooked up incorrectly!
I'd sugest you get someone UV qualified Andy, it appears that the cylinder stat has no wires entering into it! Just an observation. That could indicate no zone valve on the heating pipework?
Incidentaly I'd agree with the others regards exp vessel!

 

[AND]

Thanks diamondgas,

I have checked the NRV. Surely it can only be fitted one way can't it? I have a mixture of taps. Some are mixers and some conventional stand alone. The fault is on all hot taps, so I dont think it can be faulty mixer. I like the idea of the temp pipe. At least I can identify which side of the tank the problem is. Thanks.

 

[AND]

Thanks to all of you for you help. I have solved the problem. As I was taking diamondgas's suggestion of a temporary isolation of the dhw ( tap side) I noticed an unnecessary
in line NRV fitted in the dhw pipe, downstream of the tank. I removed this and problem solved.

 

[GQuigley67]

I would still advise you to replace expansion vessel

 

[gassafe]

Good advice about the expansion vessel, surely a red CH vessel would have a lower pre-charge pressure than a proper unvented cylinder model!? Just a guess tho.
What with that cylinder stat not being wired and also a dodgy old check valve left on hot water supply pipe, and the blow off lagged... Sounds like a right DIY job to me!!

 

[GQuigley67]

is it illegal to do a DIY unvented install ? I'm guessing it is since you need to be qualified

 

[diamondgas]

Glad I could help to you out Andy.

 

[fagsanbooz]

lets all be honest here, it looks a bit of a mess, diy, bodge job!! the vessel is wrong! and that flexi looks a bit kinked! and judging by the way it has been described, i wouldn't want to live next door to it!!! anyway, i am wondering why you have removed a check valve that was fitted already? why? and i'm also wondering if the original description you gave was indeed 'backflow' occuring.