Vented cylinder connected to mains.

tails_naf

I just discovered after 10 years that my 300L DHW cylinder is marked as vented. I've been using it all this time connected directly to the mains. Is this an issue? What is the difference between a cylinder that is marked as vented vs one marked as unvented? Is it pressure tolerance?

John.G

A unvented cylinder is designed to have a safe working pressure of 7bar which is what the cylinder PRV (Safety Valve) is set to but would normally operate at 2.5/3 bar depending on the mains pressure or, if a pumped system, the pump design head. A unvented cylinder should/must be fed from a CWST (cold water storage tank) in the attic with a vent taken off the cylinder top so that the pressure can never exceed say 1 bar.

You are very lucky that the cylinder hasn't ruptured, is there a white/red expansion vessel near this cylinder? and what is the mains pressure?, if whoever installed it he/she may have foolishly/dangerously installed a pressure reducing valve on reduce the mains pressure to give 1 bar but also have installed a PRV (safety valve) on the cylinder or on the cold water at the cylinder set to say 1.5bar.

There will be a label on the cylinder stating the working pressure and maybe its test pressure.

Wearb

"A unvented cylinder should/must be fed from a CWST (cold water storage tank) in the attic with a vent taken off the cylinder top so that the pressure can never exceed say 1 bar"

I expect John meant "vented" in above paragraph.

John.G

These are the different grades of copper used in vented HW cylinders.

Grade 3 test pressure is 1.45 bar and is suitable for 10m working head

Grade 2 test pressure is 2.2 bar and is suitable for 15m working head

Grade 1 test pressure is 3.65 bar and is suitable for 25m working head

These are the details from my vented cylinder. (Grade 3)

 

 

 

 

 

antoinolachtnai

Heating water under pressure in unsuitable vessel without the proper safety mechanisms and regular servicing is a very dangerous thing to do

All these things can work without incident for a few years but it gets riskier and riskier every year. Every month that passes after the tenth year makes it much more likely that a fault will occur with a thermostat for instance or that the tank will just rupture (which is probably the best thing that can happen while mall the risks are considered).

tails_naf

Thanks for the comments. It's not a coopper cylinder, it's an insulated steel cylinder, but it's marked as vented. There is a pressure reducing valve on the mains, and a pressure/temperature release valve on the tank. I don't see markings as to what the rated pressure is.

John.G

Strange all right that a steel cylinder is, or is marked vented, is this on a label or just written on it, any other info, manufacturer etc? or a photo.

tails_naf

IMG_20220324_132004.jpg

Heres a photo. The brand is Telford, but little else marked on it, can't see model or anything. Its a pretty cramped location, so can't see the back, etc.

John.G

Can you post a photo of the temperature and pressure (T&P) relief valve on the cylinder? and the expansion vessel, if it hasn';t got a E.vessel then its a vented cylinder.

tails_naf

IMG_20220324_211632.jpg

Here is the photo of the T&P valve. There is no expansion vessel on this cylinder, it's a DHW and the tank output at the top goes straight to all hot taps. There are expansion vessels on the closed loop items like solar, and under floor eating / oil boiler as all those are closed loops.

John.G

There must be a E.vessel on a unvented HW cylinder to allow for expansion while heating the cylinder, 300 litres of water will expand by almost 5 litres when heating from 20C to 60C , if no E.vessel then this 5 litres will be ejected to drain when the T&P valve lifts at 7bar, even if only reheating say 50 litre, the T&P valve will lift each and every time and eject a litre or two of water. Also the T&P valve should be installed right at/in the cylinder to give a accurate measurement of the hot water temperature, in the above installation it will not give a true reading as the temperature rod won't reach down far enough. You might have a look on the top of the (red) T&P valve and note its setting in case its just a PRV which may have a far lower setting than 7bar.

That whole set up needs looking at ASAP as it doesn't conform to any standard and IMO is dangerous in its present state.

tails_naf

https://www.boards.ie/discussion/comment/118831886#Comment_118831886

Thanks. Plumbing really is a cowboy trade in this country, or at least was 10 years ago when my house was built.

I remember when installed by oil boiler and asking whether the return should come from the top or bottom of the cylinder - as condensing oil boilers only work in condensing mode when the flow and return temperature has a high differential, so in that case taking the return form the top (hot) part of the tank is incorrect. Absolutely crickets when I asked a few plumbers and boiler technicians about this.

In terms of the vented vs unvented - I have never heard the T&P valve go off in the normal course - but know it did one time when the heat went above 90C - due to a particularly hot day and good solar heating and the tank was (almost literally) boiling!

Furze99

I'd have thought you'd must have an expansion vessel on that setup. The hot water is expanding in volume and you are reliant on what is an 'emergency' pressure valve to stop it blowing? Mad!

John.G

https://www.boards.ie/discussion/comment/118833681#Comment_118833681

Thats fair enough as regards the solar but a complete mystery as to where the expansion is going, some unvented cylinders used the top of the cylinder as a air "bubble" for expansion but yours can't be as the HW is drawn off the top.

Have you any tanks in your attic??

John.G

I have seen installations on the continent where they use a "expansion valve" and don't have a EV which can never be used as a safety device anyway as it can rupture/fail at any time, this expansion valve is set to 6bar so lifts without disaturbing the T&P valve, it should be fitted as well even with a EV.

https://www.boards.ie/discussion/comment/118833781#Comment_118833781

John.G

The only other way I can see the expansion being accomodated is by back flowing through the PRV, these are usually a combination valve with the reducing valve and a NRV (non return valve) combined to prevent backflow., so a photo of the (mains) PRV will help as well.

Furze99

https://www.boards.ie/discussion/comment/118833857#Comment_118833857

Maybe it's being contained in the cylinder? But then surely you'd know as when you'd turn on a hot tap, I'd imagine the flow would be a lot stronger at times. Was on holidays in Italy a couple of times and any rental places we stayed seemed to have a pressurised system, thought they all had expansion vessels built in, but memory is hazy.

John.G

It could have a air pocket if a dip tube was inserted (inadvertently or other wise in the top of the cylinder), The ones that have the air pocket designed in (with no dip tube) are recharged with air by shutting the cold feed and draining off water by opening the T&P valve until no more water comes out, then reclosing it and reopening the cold feed. I came across this "dip tube" in a vented HW cylinder as someone kept burning out the top mounted dual immersion because a portion of it was immersed in air, I just removed the dip tube and all was well.

tails_naf

No tanks in the attic. 100% mains connected.

John.G

The most likely reason that the 7bar T&P isn't lifting then IMO is that the expanded water is back flowing through the mains PRV, the most unlikely scenario of a air bubble in the top of the cylinder would require a dip tube of ~ 9ins to provide a approx 50 litre air space to give around 5 bar when/if cylinder heated to 60C.

Can you get sight of this mains PRV with any details and a photo would be useful.