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

 

[Jeff Handy]

Huh?

 

[vlepouce]

If I understand the question, when you need to do maintenance to or replace the hot water heater, you would have no way to shut off the water to the hot water heater.