Stainless steel vented hot water cylinder to the mains

[Mick54]

Hi All
I know the answers will be "no" but I was theorising the following:
Install a direct, vented stainless steel grade 1 (2.5 bar max) hot water cylinder and connect it directly to the incoming main via a PRV.
Run the insulated vent pipe to a height of about 6m above the the cylinder, so it vents into the gutter of the two storey dwelling (I'm on the ground floor).
Adjust the PRV so that water just starts to emit from the vent pipe and turn back so that the head in the vent pipe is about 5m, equivalent to about 0.5bar.
In this way I dont need a header tank, any heat expansion is catered for via the vent pipe and I get a decent pressure on the hot water.
Any increase in incoming pressure is catered for by the PRV, or as a fail safe pushed out via the vent pipe.
I'm in the Thames water area and the maximum pressure is no more than 2 bar.
Looking forward to the replies.
Mick54

 

[muggles]

OK, I'll bite...

Absolutely no

On a proper unvented cylinder the pressure relief discharge (your "vent") is not allowed to rise, it must fall continuously.

You've got no temperature relief discharge to deal with an overheat situation

You've got nowhere for expansion to go - water expands as it heats, meaning that the pressure will rise. PRVs often won't re-seat after being operated more than a couple of times, so you can't just dump excess pressure out of your badly-installed discharge pipe

Proper unvented cylinders have a pressure safety factor of 5 - they run at 3 bar and are tested to 15 bar. This means that, if pressurised, your Grade 1 cylinder shouldn't ever run at more than 0.5 bar as that is one fifth of the test pressure of 2.5 bar, you're currently proposing a safety factor of 1.25

How do you know the pressure in your area won't get much higher at, say, 3 in the morning?

Generally a terrible idea that will inevitably lead to a large flood in your home. If you want higher pressure, have a normal cylinder and a pump

 

[Mick54]

Thanks for the reply Muggles.

Firstly this post is for a vented cylinder.

For the over heat issue. I plan to use an immersion heater which will have the auto cut out and stop heating. Expansion will be treated the same as if a header tank was used and expelled via the vent pipe.

The input to the PRV will be via the same feed for the cold water, thus already reduced in pressure.

Mick54

 

[picasso]

So you end up with a half a bar pressure at the taps (on the ground floor) you would have the same if you put a tank in the loft and it would be a lot safer than relying on a prv, the water regs dont allow wasteage of water so the vent pipe to accommodate the expansion is out.

 

[Mick54]

Thanks Picasso
Regs point taken. Good theory though 😊
Mick54