Unvented Cylinder discharge pipe

[madge1]

Hi

Just come back from a couple of days away, and our hot water cylinder is overflowing vast quantities of hot water via the discharge pipe. Doesn't always happen; when the boiler is heating the CH obviously the heating come second - when the house is hot, and the boiler can concentrate on the water it does.

Obviously turning off the hotwater at the timer, and running a hot bath solves the problem, and equally obviously I need a plumber to come around. However, time is not the best for this! And I don't really know what to say when he does.

Could someone explain what the problem actually is - a thermostat has failed maybe? - so I can explain it to a friendly plumber. And no real way I can alleviate until then except cutting back the heating time for the water? And what sort of price would I be looking at for this? Only we're moving house in two months, and don't really want to be spending vast amounts!

ta

madge

 

[dcawkwell]

The most likely cause is the expansion vessel (Big ball shaped tank somewhere near the unvented cylinder) of the unvented cylinder has lost the air pressure like a car tyre.

To fix

Turn water off at stop cock or if you have a valve for the unvented cylinder
to cut the hot water off use that.
Open hot tap and let any water vent. Keep the hot tap open.

On the top of the expansion vessel is a car tyre valve. Use a foot pump and repressurise to 2.5bar (25psi). Close tap and restore the supply to the cylinder. Job done.

If you have an unvented cylinder that uses an air bubble (megaflow) so there is no external expansion vessel is it just a matter of cutting the supply to the tank opening a hot tap until the water stops and then closing tap and restoring supply to recreate the air bubble.

There is also the possibility that your tank is becoming too hot
but I would have thought this unlikely since you don't mention the water being especially hot.

 

[madge1]

cheers - I don't think the water is too hot - the showers have been the same as normal, and so has the washing up!

I had a look in there - we've two expansion vessels. A red one for the CH (which seems fine - hasn't moved at all recently), and a white one for the HW, which doesn't seem to have a car valve, and I can't even see a pressure guage - I'm assuming it's not under the black plastic cover in the picture below?

Untitled

• madge1
• 30 Dec 2011
• 1

ta

madge

 

[madge1]

funnily enough there is a car valve under the black plastic cap - so theoretically i get my high tech bike pump, and just bung it up to 25psi after turning the water off?

 

[Onetap]

funnily enough there is a car valve under the black plastic cap - so theoretically i get my high tech bike pump, and just bung it up to 25psi after turning the water off?

No, wait. You may be barking up the wrong tree and, if this is the right tree, you need to read the manual about how you go about this.

Someone may advise, I'm otherwise occupied at the mo.

 

[madge1]

well I did check, the valve was up at 30psi anyway, so I'm assuming it's not that.

thanks for all your help though - just wondering now if the water is too hot. Hard to tell cos the boy is always fiddling with the temp gauge in there, so never actually have a consistency of temperature!

I've put the hot water back on, with gauge in the right place, so hopefully it shouldn't be running as there's nothing to heat up

 

[dcawkwell]

well I did check, the valve was up at 30psi anyway, so I'm assuming it's not that.

thanks for all your help though - just wondering now if the water is too hot. Hard to tell cos the boy is always fiddling with the temp gauge in there, so never actually have a consistency of temperature!

I've put the hot water back on, with gauge in the right place, so hopefully it shouldn't be running as there's nothing to heat up

The other possibilities are the temperature and pressure relief valves.
Usually located on the tank. Little twist top things.
See if you can tell if water is coming from one.
If may be just a case of twisting one to reseat it otherwise one may need replacing.

 

[madge1]

one of the T and P valves is on the side - leads to the discharge pipe. Twist it a bit and more does come out of that valve, into the discharge pipe and tundish.

 

[dcawkwell]

one of the T and P valves is on the side - leads to the discharge pipe. Twist it a bit and more does come out of that valve, into the discharge pipe and tundish.

Yes both the pressure and temperature relief valves should be piped via the tundish. See if one is leaking when the water is again reheating.

 

[Onetap]

The other possibilities are the temperature and pressure relief valves.
Usually located on the tank. Little twist top things.

I did advise you to wait.

The discharge of water from either indicates a fault; if the valves are working as intended, then either the pressure or/and the temperature is too high. The advice is then to turn off the power and get a G3 qualified technician to examine and repair the cylinder. You must fit spares supplied by the manufacturer, you should be G3 qualified to work on these things. Do NOT fit anything other than the appropriate relief/safety valve on the discharge pipe.

The pressure relief valve (PRV) is on the cold water inlet. The temperature & pressure relief valve (T&PRV) is in the top of the cylinder.

If it is discharging hot water, it is most likely coming from the T&PRV. It is not likely to be opening due to excessive pressure since the PRV usually has a lower pressure setting than the T&PRV; either, or both, valves could be defective, so you can't take that as being infallible.

If the water is too hot, the valve is doing its job.

If the water is not too hot (>90 degC) it would suggest the T&PRV is defective. The temperature and pressure settings will be in the installation manual for your cylinder, available on the internet if you don't have a copy. The T&PRVs sometimes fail and discharge hot water at a lower temperature than they should.

The air charge setting in the expansion tank should be set after relieving the water pressure in the cylinder. If most of the air has leaked out (as it does) and the water is at 30 psi, a pressure gauge would read 30 psi; it tells you nothing.

 

[madge1]

Cheers. Just had the hot water on for 30 minutes, and no overflow yet...the t&p valve just open and let hot water out if tried.

I shall monitor, but thanks for all your help.

 

[BARNY78BG]

Hi mate just thought i'd warn you that the unvented cylinder you have is nothing like the old open vented cylinders and that someone with the qualifications and know how should be called as a previous thread has advised!! cos the mess these unvented cylinders can make if they explode due to tampering with safety devises such as you are, is a lot worse than a gas explosion!!!! just advising you mate. be carefull!!