Unvented cylinder install query??

[macka09]

Hi guys. Is there any reason you can’t can’t downsize the hot water draw off from 22mm to 15mm straight away? Other than the flow rate being affected would there be any other concerns?
Cheers

 

[oz-plumber]

You would end up getting hot water quicker with less water drawn off?

 

[macka09]

You would end up getting hot water quicker with less water drawn off?

So if there’s no bath there’s no issue with it? Is that right? Cheers for your reply

 

[Ben-gee]

Flow rate and potentially a bit of noise are the only issues I can think of.
But considering the unvented was probably out in to ensure a good flow rate I’d check what the customers future plans are / eg if multiple showers then you should stick to 22.

 

[oz-plumber]

Also depends on incoming mains pressure.
No point in smaller diameter pipe with low mains pressure.

We have high pressure over here - reduced at water meter to 500kPa.
Generally most houses have a 1/2 inch hot water pipe.

 

[macka09]

Thanks for the replies folks. Where do you stand with isolation valves on the hot draw off?

 

[Ben-gee]

Not needed. There should always be a shut off as part of the inlet control set.
Also by shutting off the cold this gives you a good idea of the state of the expansion vessel or the air bubble- by way of how long the hot runs before it stops. This function is lost if you shut off the hot on its way out.

 

[RoryD]

You would end up getting hot water quicker with less water drawn off?

Actually it would fill at the same rate but because of the smaller diameter the velocity would increase.

 

[macka09]

These cylinders will only feed one shower, one basin and one sink. Very small apartments

 

[macka09]

Just for clarification. These cylinders have been fitted to spec on a job with lever valves on the hot draw off. I questioned it because it seemed daft when you can just isolate the cold. Also there’s a risk of implosion when draining down if somebody doesn’t know what they’re doing. I’ve been advised to label the valves to ensure they are left open when draining.

 

[Ben-gee]

So why fit them?
It’s best not to let a customer tell you how to do your job - even if they think they know better.
State what you intend to do and if necessary explain why, if you they still insist you do something you’re not happy with - that is the time to explain they need to find someone else to do it.

 

[fowlerboi]

I fit lever valves to the hot on all my unvented installs. If you wanted to isolate the hot and not the balanced cold you would be stuffed

You will only get implosion if you try to drain the cylinder with the hot lever valve off, but this is no different than having the hot taps shut anyway. Realistically someone who's G3 should be working on the cylinders and they should know the procedure for draining an unvented cylinder. I'm not a big fan of making things easier for DIY when we pay lots of money to have our qualifications.

As far as I see it there is no problem with immediately reducing the hot to 15mm.

 

[Timmy D]

Actually it would fill at the same rate but because of the smaller diameter the velocity would increase.

You misunderstand. Smaller pipe means less cold water sitting in the pipe, which has to be drawn off before hot arrives.

Smaller pipe also increases the frictional losses, so it’s not exactly the same rate.

There are also velocity regs to adhere too.

 

[RoryD]

You misunderstand. Smaller pipe means less cold water sitting in the pipe, which has to be drawn off before hot arrives.

Smaller pipe also increases the frictional losses, so it’s not exactly the same rate.

There are also velocity regs to adhere too.

No I haven't mate, It would take the same time to fill a bath. A larger diameter would flow at a lower rate whilst the smaller diameter with a smaller area flows faster.

 

[macka09]

So why fit them?
It’s best not to let a customer tell you how to do your job - even if they think they know better.
State what you intend to do and if necessary explain why, if you they still insist you do something you’re not happy with - that is the time to explain they need to find someone else to do it.

Alright mate. I haven’t fit these cylinders and it’s not the customer who’s saying anything. It’s an apartment block with a load of cylinders going in. Builder would like to be able to isolate the hot if need be. I personally would just rely on the cold mains iso above the combi valve.
There’s no issue having a valve on the hot as long as whoever works on the systems knows to leave the valve open when draining down.