TRVs - I bought the wrong ones but why?

[chewymix]

I am installing a drain down valve, as i dont have one, in my open vented CH system and at the same time taking each radiator off and giving them a good flush. I recently took one off for decoration and the amount of thick black sludge that i flushed out the bottom was alarming to say the least. Once they are all manually flushed out i will stick some cleaner in the system, run it around for a week or so, and then empty and refill adding some inhibitor in.

While the system is drained down i will swap 5 old rad valves to TRV's - All relatively simple.

I bought the drain down valve which i have pictured which says its a 15mm / 1/2 inch and looking at one of my current rad valves it looks looks like a straight swap. I therefore ordered spome 15mm / 1/2 inch TRVs but when they came they seem to be 15mm / 15mm.

Can anyone let me know of my basic error and which TRV i actually need. The drain off valve has 15mm / 1/2 inch on the packet hence me 'assuming' a TRV of the same description would fit.

Link to TRV but pic also attached - https://www.screwfix.com/p/white-chrome-angled-trv-lockshield-15mm-x/31811

cheers!!

 

[denso13]

The TRV and new lockshield you have linked to are fine but you'll have to fit the new tails into the radiator after removing the old ones.

 

[chewymix]

thanks for that although i'd rather avoid swapping the tails if thats possible with a different TRV? I dont understand why a drain off valve of 15mm / 1/2 inch is different to the same TRV?

 

[denso13]

i'd rather avoid swapping the tails if thats possible with a different TRV?

You won't get a TRV to fit the existing tail but they are usually easy enough to swap out.

I dont understand why a drain off valve of 15mm / 1/2 inch is different to the same TRV?

The 15mm refers to the pipe size and the ½ inch refers to the thread size which are the same, it is just the connection from the valve to the tail that differs. You can find lockshield valves which might be a straight swap but, as above, the TRV will need a new tail in the radiator.

 

[chewymix]

You won't get a TRV to fit the existing tail but they are usually easy enough to swap out.

Is there a particular reason a TRV body is not made to fit onto the larger diameter tail? These radiators are old imperial units and i am not convinced i'd be able to get the tail out, they have been in for a while!!! Ideally i'd swap the rads too but they are in decent enough condition and the pipework comes up through an oak floor and so cant be modified economically.

The 15mm refers to the pipe size and the ½ inch refers to the thread size which are the same, it is just the connection from the valve to the tail that differs. You can find lockshield valves which might be a straight swap but, as above, the TRV will need a new tail in the radiator.

Thanks for the info on what the units of measurement actually mean!!

 

[dal5band]

The basic difference is that trvs have 15mm compression fitting onto the radiator tail which is a piece of 15mm pipe sticking out with 1/2 inch bsp male going into the radiator. Whereas the drain off valve has what looks like a union fitting not requiring an olive, the other end is ofcourse 1/2 inch bsp going into the radiator.
You can get manual valves with 15mm compression both sides but not sure if they have a drain off option.

 

[denso13]

and i am not convinced i'd be able to get the tail out, they have been in for a while!!

It really is quite straightforward, if you are not confident in doing it yourself then you should perhaps get someone in.

 

[chewymix]

It really is quite straightforward, if you are not confident in doing it yourself then you should perhaps get someone in.

Perfectly confident - Its just they look a bit seized up having been in there for many decades. I guess i just need to give it a go.

 

[dal5band]

The old radiator valve is also a union fitting and you will require a large hex key to fit inside the radiator tail toget it out. If you undo the union on your new valve with drain, you will see the internal hex "socket" .

 

[chewymix]

The old radiator valve is also a union fitting and you will require a large hex key to fit inside the radiator tail toget it out. If you undo the union on your new valve with drain, you will see the internal hex "socket" .

OK thanks for the tip - i see what you mean. I'll order one in.