Replace rad valves - best method

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I'm going round and round in circles looking for the simplest method to replace the TRV and lockshield valve on a downstairs radiator. I have a combi boiler.. The rad is currently off the wall as we are decorating.

Is my only option to fully drain the system? Can I partially drain and then replace the valves?

To partially drain do I:
- Turn off boiler (do I have to close anything here?!)
- Close lockshield and TRVs on all radiators
- Crack nut on the valve I want to change to release pressure
- Change valves
- Refill

TIA
 
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With most sealed systems, all you need to do is as you suggest. Shut off the boiler, shut all the rads (count the lockshield turns to reinstate) and then drop the pressure out of the system using the valves you are going to change, both sides. Fit the new rad and use that one to add inhibitor, then refill and bleed and run the system up till all hot.
 
With most sealed systems, all you need to do is as you suggest. Shut off the boiler, shut all the rads (count the lockshield turns to reinstate) and then drop the pressure out of the system using the valves you are going to change, both sides. Fit the new rad and use that one to add inhibitor, then refill and bleed and run the system up till all hot.

If the rad is on the top floor, can you ignore the rad valves on the lower floors?
 
If the rad is on the top floor, can you ignore the rad valves on the lower floors?
You probably could but they hold pressure at whatever the system is set at so there would be more system water lost than if they were shut down, if you see what I mean.

How much, in the grand scheme of things depends on the number/size of rads and if it is being re-inhibited anyway, it may not really be much of an issue.
 
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I've had success in the past with using a pipe freezing kit to form a plug in the pipe feeding the valve, avoiding the need for any sort of drain down. The ice plug typically lasts around 20 to 30 mins giving plenty of time to replace the valve, as long as you've prepared all you need in advance. Assuming the valves are fitted with compression fittings, you'll have to remove (cut off or split) the old olive and put a new one on the joint between the pipe and the new valve.
 
I wouldn't worry about closing valves on other radiators. Just make sure they are fully vented before you start. The main pressure reservoir on a combi system is the boiler expansion vessel. There is also an auto air vent inside the boiler. Both of these can be taken out of the equation by closing the flow and return isolators under the boiler. This is safe to do on most boilers but should be avoided on some older models as they are likely to leak when opened again. If you post the boiler make and model, advice can be given on whether its likely to be ok. The system should now de-pressure and go to vacuum with only a small amount of water loss.
 
Thanks so much for all for the comments. I did consider a pipe freezing kit
 
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I wouldn't worry about closing valves on other radiators. Just make sure they are fully vented before you start. The main pressure reservoir on a combi system is the boiler expansion vessel. There is also an auto air vent inside the boiler. Both of these can be taken out of the equation by closing the flow and return isolators under the boiler. This is safe to do on most boilers but should be avoided on some older models as they are likely to leak when opened again. If you post the boiler make and model, advice can be given on whether its likely to be ok. The system should now de-pressure and go to vacuum with only a small amount of water loss.
Thanks so much for all for the comments. I did consider a pipe freezing kit

Elkato531 - this sounds like the kind of solution I am after. The boiler is a Glowworm Energy 7 30C. Would be grateful if you could please confirm if it would be safe to do?

So I would close the flow and return under the boiler, then go straight into changing the valves?
 
Should be ok with those flow and return iso valves. They look like they have a screwdriver slot but use a 4mm allen key.

If the rad is already removed...

Turn off boiler
Bleed all other rads to release trapped air
Close flow and return iso valves
If there are any other auto air vents on the system, close those as well
Open rad valve you're changing and allow to drain into tray or bucket
When water stops, change valve
 
Should be ok with those flow and return iso valves. They look like they have a screwdriver slot but use a 4mm allen key.

If the rad is already removed...

Turn off boiler
Bleed all other rads to release trapped air
Close flow and return iso valves
If there are any other auto air vents on the system, close those as well
Open rad valve you're changing and allow to drain into tray or bucket
When water stops, change valve
Small hiccup... no slot for a 4mm allen key and I cannot turn it with a screwdriver... any ideas?!
 
If they are the iso valves as shown in the manual, they should have a hole in the middle for an allen key. Post a pic of your valves.
 
If they are the iso valves as shown in the manual, they should have a hole in the middle for an allen key. Post a pic of your valves.
 

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Floor type matters, mothers with carpet on tiles had carpet washer on stand-by and lost around half a cup of water, which the carpet washer sucked up.

Here with compressed board floors I would not take a chance.
I have changed heads, but valve I got heating guy in to do it.
 
My apologies. Turn off with an adjustable spanner. Quarter turn.
 

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