Reason for usual HW shutoff valve location

[phloaw]

In a conventional system, the standard arrangement is to have a gate valve on the (typically 28mm) cold feed pipe from cistern to cylinder.
Out of curiosity, I was wondering why you won't find it on the hw pipe (typically 22mm) going out of the cylinder instead.
Of course, positioned after the vent tee.

Since the valve would have a smaller diameter, it would be cheaper.

So why? Maybe because it would need to withstand possibly higher temperatures?

It's just an academic question, but I'm curious

 

[exedon2]

Simple really if you need to work on cylinder maybe change a immersion heater valve on cold feed is way to go.
Also if cylinder had 28mm cold feed your would normally see 28mm hot out , we have for some time used lever valves on cold feeds gate valves a pain!

 

[phloaw]

Simple really if you need to work on cylinder maybe change a immersion heater valve on cold feed is way to go.
Also if cylinder had 28mm cold feed your would normally see 28mm hot out , we have for some time used lever valves on cold feeds gate valves a pain!

I see, thanks. I am not a pro, just seen a couple of systems with 28mm in and 22mm out and assumed that was the norm.
Totally relate to the gate valve cry: sadly they are still there in older systems, waiting to be replaced by ball valves when they get stuck.
Actually, one reason for asking is that on my own system I do have a cold feed gate valve stuck, and it would be easier to add a 22mm valve on the out, rather than to bother touching the stuck one.

Would you think that sounds reasonable, @exedon2 ?

 

[SJB060685]

You don't put any valve in a position that could block the vent! Safety first

 

[phloaw]

You don't put any valve in a position that could block the vent! Safety first

Yes, that's why I specified "after the vent tee".

 

[SJB060685]

Ah yes, I missed that.

The valve, beit gate or lever is installed on the feed to the cylinder for safety reasons as we mentioned above but also so you only have to drain the cylinder and a small volume of water between the feed and cylinder when you have to drain the cylinder.
You could put a valve where you mention to isolate hot but if you needed to drain the cylinder you would also have to drain the entire header tank. In most common domestic properties thats only a small volume but in older and larger dwellings you could have much larger header tanks or even a few linked together.