Does an unvented cylinder really have to be connected to the mains?

[Benjy]

This is my first post on the forum... but I've been around a while, lurking. Hello!

I'm hoping someone can advise me with a question about unvented cylinders. I'm told they must be connected to the mains and can't be gravity fed, but I'm having trouble understanding why... if the pressure and flow rate are good enough, why does it matter?

Backstory / context

I currently have an open vented cylinder and cold tank in my airing cupboard. My hot water pressure is poor due to the lack of head height. So I'm looking to have an unvented cylinder installed to a) free up space in the airing cupboard, b) improve hot water pressure, and c) remove a very noisy (positive head) shower pump which only kicks in if the shower head is brought right down into the bath. The flow rate from the bath cold tap is 32 l/m, which indicates the flow would be good enough for an unvented

The issue is I have four supply pipes running through my airing cupboard, which unhelpfully are not labelled. One of them supplies the existing cold water tank in the airing cupboard and the bathroom. The other three pipes aren't connected to anything. We know there are communal tanks on the roof of the block, so we are not sure if this pipe is a rising main or fed by the communal tanks. But unfortunately we can't gain access to the roof to trace pipes. So there doesn't seem to be a way of determining whether the pipe is mains or gravity fed.

I spoke to Joule who told me unvented cylinders can't be gravity fed. When I asked why, the response I got was - "An unvented cylinder needs to have a constant pressure and supply and you would not have this on an unvented cylinder". I think they mean 'I would not have this on a gravity fed system'. I want to understand this better - what's the difference, if the pressure and flow is good enough from the communal tanks (given there's a 50/50 chance that's what the supply is).

 

[snowhead]

I spoke to Joule who told me unvented cylinders can't be gravity fed. When I asked why, the response I got was - "An unvented cylinder needs to have a constant pressure and supply and YOU would not have this on an unvented cylinder". ( If you used a tank supply.)

Joule mean what they say.

What was the pressure at the bath tap when the 32lpm flow was measured?
How tall is the building above your flat, bear in mind to achieve each 1 Bar pressure it needs a head (above your flat) of 10mtrs.
Is the tank feeding the Hot water cylinder fed from the mains or from the communal tank supply?
Have you had any plumbers in to look at what's there and what you want to do?

 

[gpbeck]

It is illegal to have one of these fitted unless following manufacturers instructions. Unless you find a manufacturer to sanction your plans, you just can’t do it.

 

[AWheating]

Well if its piped up like a rank feed cylinder and you run the discharge from the tprv to regs then I can't see the issue. It's just a cylinder designed to take the extra forces a mains supply puts onto it.

 

[Benjy]

Joule mean what they say.

What was the pressure at the bath tap when the 32lpm flow was measured?
How tall is the building above your flat, bear in mind to achieve each 1 Bar pressure it needs a head (above your flat) of 10mtrs.
Is the tank feeding the Hot water cylinder fed from the mains or from the communal tank supply?
Have you had any plumbers in to look at what's there and what you want to do?

• @snowhead I haven't measured but am planning to pick up a pressure gauge over the weekend to get an accurate reading. The filling loop for our sealed CH is off exactly the same supply and I can get that to at least 1.5 bar.
• There are two stories above us, the communal tanks couldn't be much more than 10 metres above us, if that. Perhaps that answers the question then? As we'd expect to have no more than 1 bar from that
• The cold tank is fed by the mystery pipe. Apart from feeding our cold tank... it also supplies the bathroom and the CH filling loop too. Our downstairs cold is serviced from different pipes.
• I've had two plumbers round (and will likely be going with the second for reasons completely unrelated to this matter). The first said it didn't make a difference either way (whether it was from mains or from communal tanks) and the second said it HAD to be mains!

@AWheating yes, exactly my thinking (though in my case, backed up with zero expertise!). Although it just occurred to me that it could void our warrantee, and might even be illegal as @gpbeck has indicated. But I still don't understand why.

Luckily the tprv can safely discharge into an iron soil pipe.

 

[Ben-gee]

It doesn’t have to be fed from the mains.
I have customers where the unvented cylinder is fed from a cws tank via a single impeller pump. The system works very well, I didn’t install it by the way but I have fitted two bathrooms there.

 

[Benjy]

A little update from me. I'd really appreciate hearing your thoughts...

So the first thing is that unfortunately the management agents won’t help us gain access to the communal tanks until after lockdown

I've measured the flow rate & pressure from a 15mm run coming off that mystery pipe servicing everything in the airing cupboard / bathroom. The flow was 28 l/m and pressure was just a fraction under 1 bar. Looks like most unvented tank manufacturers need a minimum of 1.5 bar. So it's not looking good so far. And still no clearer if pipe is mains or from communal tanks. I’m suspicious that it is indeed from the communal tanks, as there was sediment in the bucket after measuring the flow rate. On the other hand, 1 bar seems a bit high... as there can’t be much more than say 7 metres from our airing cupboard to where the tanks would be.

The pressure in our kitchen was a fraction under 2 bar with a substantially lower flow rate of 12 l/p.

 

[exedon2]

Attaching a specification for 3 cylinders we are changing later in year.
They will currently be tank fed but when water supply is upgraded in approximately 18 months will be converted to fully unvented.

 

[Knappers]

The cylinders will be tested and certified to EN standard for the purpose its designed, the manufacturer is unlikely to 'allow' deviation from this as it would need total redesign of the package as a whole.
The multifunction control may not pass enough water without mains pressure and if you get rid of it you may aswell throw the whole manual away and report yourself to the HSE.
I don't see a problem with using a boosted supply tho and a standard unvented setup.
I like the 'DAB E.Sybox' if you can satisfy its requirements (and have a spare £500)

 

[Knappers]

Attaching a specification for 3 cylinders we are changing later in year.
They will currently be tank fed but when water supply is upgraded in approximately 18 months will be converted to fully unvented.

Will they have vent pipes and anti gravity loops?
Does the cold feed directly into the base of the cylinder?
If they are unvented cylinders what labels will they have to avoid confusion, how will an engineer know if its in gravity or unvented mode?

 

[Brian O]

A little update from me. I'd really appreciate hearing your thoughts...

So the first thing is that unfortunately the management agents won’t help us gain access to the communal tanks until after lockdown

I've measured the flow rate & pressure from a 15mm run coming off that mystery pipe servicing everything in the airing cupboard / bathroom. The flow was 28 l/m and pressure was just a fraction under 1 bar. Looks like most unvented tank manufacturers need a minimum of 1.5 bar. So it's not looking good so far. And still no clearer if pipe is mains or from communal tanks. I’m suspicious that it is indeed from the communal tanks, as there was sediment in the bucket after measuring the flow rate. On the other hand, 1 bar seems a bit high... as there can’t be much more than say 7 metres from our airing cupboard to where the tanks would be.

The pressure in our kitchen was a fraction under 2 bar with a substantially lower flow rate of 12 l/p.

Have you looked at a negative head shower pump but use for entire property hot water. More expensive pumps but should alleviate having to drop the shower head all the way down to kick in.

 

[Benjy]

Have you looked at a negative head shower pump but use for entire property hot water. More expensive pumps but should alleviate having to drop the shower head all the way down to kick in.

Hi Brian O, thanks for your thoughts. This kind of pump was actually my first port of call, but the pump manufacturers were recommending 50 gallons of water storage as a minimum to supply them. Unfortunately, the airing cupboard where the tank is doesn't have much space at all and to make matters worse has ducting running through. Current tank is only 15 gallons. I started looking into custom made cold water storage tanks... but it started seeming like too much of a faff, especially as I wasn't even confident a heaver tank could be supported by the beams already in place.

Long story short, we are now looking to have a combi installed and reclaiming 100% of the space in the airing cupboard. Yes, it's an expensive option, but hoping to save money on bills over time and there has got to be value in reclaiming all that space back too. What I've got to do first is get new hot & cold pipework over to where the combi will be. But hopefully, it will all be worth it