CH drain valve won't shut

[phloaw]

Today I drained my CH/HW. This is a standard vented, pumped S plan with two electric valves, and I'm in UK.
I was lucky enough to have a drain valve underneath ground floor, relatively accessible, see pics:

However, upon refilling the system, I found out that that valve will rotate endlessly, and won't guarantee proper isolation: it's stripped.

I was thinking to add an isolating (aka service) valve past the old one, i.e., along the pipe in the bottom part of the images.
Something like this:

Do you think that will work? Or is there a risk that the old valve will leak when the new one is shut? Do you foresee any other possible problem?

Any feedback welcome!

Thanks!

 

[breplum]

We don't have completely similar systems here in the U.S. but you need not worry about that existing valve spinnning. It will, in the 90 degree cross way position, shut off.
I don't even know what you are talking about: "CH/HW. This is a standard vented, pumped S plan " ?

 

[phloaw]

We don't have completely similar systems here in the U.S. but you need not worry about that existing valve spinnning. It will, in the 90 degree cross way position, shut off.
I don't even know what you are talking about: "CH/HW. This is a standard vented, pumped S plan " ?

Thanks!
I guess the terminology is British: CH is central heating, HW hot water. In the classical UK system, you have them fed together by piping fed and venting into a loft cistern.

 

[breplum]

Oh yes. I've only seen one such reservoir tank in an attic from a very old system.
Here we call it a hydronic heating system.
I don't know what that specific valve does, but piece of info: The picture you showed has a choked flow. Ideally, you might want a "full port" valve.

 

[phloaw]

Oh yes. I've only seen one such reservoir tank in an attic from a very old system.
Here we call it a hydronic heating system.
I don't know what that specific valve does, but piece of info: The picture you showed has a choked flow. Ideally, you might want a "full port" valve.

Ah, yes, I know. I thought that for a drain valve full flow is not so important. Am I wrong?

 

[breplum]

No, you aren't wrong.
As long as there is suffecient difference in elevation and the valve is on the lowest portion, it shouldn't be an issue.

 

[phloaw]

No, you aren't wrong.
As long as there is suffecient difference in elevation and the valve is on the lowest portion, it shouldn't be an issue.

I finished, hoping I did a decent job. Thanks for your help!