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Radiators - bleed valve replacement

Does what it says on the tin
bungeejumper
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Radiators - bleed valve replacement

#208188

Postby bungeejumper » March 17th, 2019, 12:59 pm

Following on from the query about drain cocks, here's a quick job that I've been threatening to do for several years but haven't got round to it. Will it be as straightforward as I think?

We have a few pressed steel double radiators dating from the mid-1960s - inefficient and heavy by today's standards, but still performing well enough for our purposes. Anyway, the rad in question has a badly seized bleed valve pin - but it's deeply recessed and very worn, so it's very hard to get a grip on the pin. So I can't bleed the rad any more. :(

My local plumbers' merchants laughed when I asked them about getting a new pin for this ancient rad, and instead they sold me a compete new bleed valve that will need to be screwed into the radiator once I've spannered the old valve out.

That won't be a problem (I hope!), but if I can make the swap without draining down the radiator it would be a bonus. If I shut down the lockshield valve and screw down the TRV back to zero (both about 25 years old), would that be enough to stop the flow while I do the swap? Or would it be safer to wait until the next time I'm doing a total drain-down?

What would you do, chums?

BJ

Itsallaguess
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Re: Radiators - bleed valve replacement

#208189

Postby Itsallaguess » March 17th, 2019, 1:08 pm

bungeejumper wrote:
If I shut down the lockshield valve and screw down the TRV back to zero (both about 25 years old), would that be enough to stop the flow while I do the swap?

Or would it be safer to wait until the next time I'm doing a total drain-down?


Closing both valves at either end of the radiator will isolate the body of water in the radiator from the rest of the system. It'll then be a case of releasing the small amount of water-pressure in the radiator, and then draining the top level of water in the radiator down to a level slightly lower than the bottom of the valve you're replacing.

Where you drain that top level of water from will depend on what type of capture mechanism you can furnish - you could crack the radiator-side of either the lock-shield valve or the TRV if that would suit your circumstances, or you could simply start to remove the valve you're replacing, and just let it bleed out until it stops. Even if you crack one of the lower sections, it's likely that you'll still need to crack the top bleed valve to get any water to flow from the radiator.

This job shouldn't need a total drain-down so long as you get the lock-shield and the TRV in the fully closed position. Make a note of how many turns you need to close the lock-shield, and then you know how many to re-open it to get the same system-balance back...

Cheers,

Itsallaguess

stewamax
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Re: Radiators - bleed valve replacement

#208209

Postby stewamax » March 17th, 2019, 3:05 pm

Itsallaguess wrote:Closing both valves at either end of the radiator will isolate the body of water in the radiator from the rest of the system.

Fine in the present weather, but turning down a TRV to 0 ("closed") may not shut off the flow completely in cold weather. Manufacturers such as Drayton include a small plastic cap that replaces the TRV and positively locks down the valve. You can also use a 5p coin between the TRV itself and the valve stem.

Many years ago I found out the hard way when redecorating: I had removed a radiator (with both valves shut off) and left things overnight in below 0degC weather. Next day the TRV's outlet was spouting!

jfgw
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Re: Radiators - bleed valve replacement

#208238

Postby jfgw » March 17th, 2019, 6:02 pm

bungeejumper wrote:That won't be a problem (I hope!), but if I can make the swap without draining down the radiator it would be a bonus. If I shut down the lockshield valve and screw down the TRV back to zero (both about 25 years old), would that be enough to stop the flow while I do the swap? Or would it be safer to wait until the next time I'm doing a total drain-down?


Those old valves may shut off fully if you are lucky. When you reopen the lockshield valve, it may leak around the gland and drip onto your carpet. It is usually possible to tighten the gland nut to stop the leak. A new lockshield valve is a better fix.

Bleed valves come in at least five sizes: 1/2"BSP (approx. 21mm thread), 1/4"BSP (approx. 13mm thread), 1/8"BSP (approx. 10mm thread), a smaller one used by Myson (7mm (approx?)), 6mm used by Barlo.

Bleed valves and screws may be had from here, https://www.bleedscrews.co.uk/radiator-bleed-screws/

Julian F. G. W.

bungeejumper
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Re: Radiators - bleed valve replacement

#208256

Postby bungeejumper » March 17th, 2019, 7:04 pm

jfgw wrote:Those old valves may shut off fully if you are lucky. When you reopen the lockshield valve, it may leak around the gland and drip onto your carpet. It is usually possible to tighten the gland nut to stop the leak. A new lockshield valve is a better fix.

Ah, at least the lockshield and the TRV aren't completely ancient. I replaced them when we bought the house, but OK, that was 25 years ago. As I recall, I found at the time that the old valves were on the wrong ends of the radiator. :D It worked a lot better once I'd got the water flowing in the right direction.
Bleed valves come in at least five sizes: 1/2"BSP (approx. 21mm thread), 1/4"BSP (approx. 13mm thread), 1/8"BSP (approx. 10mm thread), a smaller one used by Myson (7mm (approx?)), 6mm used by Barlo.

Myson? LOL, thanks for the memory. Our main kitchen radiator was a shiny new king-size duplex thing from Myson which I bought new in the 1990s - only to find that both the brass bleed valves sheared off the second time I tried to use them. (It appeared that the brass had stretched so that the pins were too tight in the threads and fatigued straight off.)

None of the local plumbers' merchants could offer any advice, and they were all agreed that they'd never seen anything quite like the sheared-off heads that I showed them. There was nothing for it, then, but to drill out the failed brass pins; tap some new threads into the radiator in a larger size (which was itself a bit dicey on a modern radiator made of wafer-thin tinplate!); screw in a pair of short M5 bolts (I think?) where the old bleed pins would have been; and bung in a bit of silicon sealant around the edges just in case!

Twenty years on, my bleed valve bodge is still holding up nicely, and I've bled the radiator at least half a dozen times. Won't be buying any more Myson radiators, though.

BJ


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