Could you describe a bit better what you are trying to do?
Is there a rad or towel rad already in the bathroom?
Anyway I'll go in at the deep end. Assuming you your bathroom is on the first floor you do not need to drain the whole system.
First locate the system drain cock (probably clos to the lowest rad in the system). Attach a hose and run it outside to take the water away.
Second turn off the mains at the stopcock - not really becessary but a uesful precaution.
Close down the valves on all upstairs rads to avoid draining them.
Turn the boiler off.
Open the draincock and allow the water to start to drain out - not full blast!
When the presure gauge on the boiler drops to zero then water will begin to descend from above.
Go upstairs, release the bleed valve on the existing radiator in the bathroom and you should hear air being sucked in as the water flows out.
Next may seem a littte bit hit and miss but you'll get the hang of it as what you are going to do is lower the level of water in the system so that it's just below the level at which you want to work.
When you think the existing rad is empty or getting near to it then close the drain cock and just crack one of the nuts on the rad tails a little. If you are still getting water coming out tighten it up and let a little more out at the drain cock.
Keep doing this until you are satisfied that the rad is empty and you've dropped the water level down to just below the level of the pipework.
You can then remove the exisiting rad. There will still be some water in it and you may get a little spillage but nothing you won't be able to cope with.
Making sure the drain cock is fully closed again remove the hose.
You'll probably have to adust the piping to fit the new rad. Murphy's Law clearly states that the new one will have different dimensions to the old!!
When you've finished connecting the new rad open up the mains stopcock again and open the filling loop valve on the boiler. Go up to the towel rail and open the bleed valve. When the water starts to come out of the bleed valve close it off and do a leak check. Then go to the boiler and continue filling until you reach the required operating pressure of around 1.5 Bar. Do another leak check.
Now you can open the valves again on all the other rads.
You'll probably need to bleed them all a little bit. If so when you've finished adjust boiler pressure again to 1.5 bar or whatever the operating pressure is of your boiler.
Turn on boiler and switch on CH. Clear any airlocks. When your new rad is hot do another leak check.
PS If you don't have an existing rad/towel rail in your bathroom then you'll have to drain one of the other rads to let the air in and you'll have to refill that one as well as the new towel rail afterwards.
What i need to do is modify the pipe work to allow the towel rad to fit [ taking an old rad out ] What i really need to know is what to do with the boiler do i need to turn it of completely or could i get away with just turning the stat right down so it doesnt kick in also do i close all rads then drain it. Forgot to mention i live in a dormer bungalow 2 rads upstairs 7 downstairs thanks again Gaz
Isolate boiler elctrical supply, find a suitable drain point - either on a radiator valve or on the piepwork near the boiler. Attach a hose and open the drain valve - open all upstairs bleed vents this will drain all upstairs radiators and pipework - if the bathroom is downstairs, drain from the bathroom radiator.
There's no need to mess about closing all rad valves or keep checking whether there's still water at the radiator valves - when the hose stops running it's empty (it's a pressurised system so wont take long to refill and wont be any major airlock problems)
Alter pipework to fit towel rail - flush if soldering.
Fit one of the valves (closed) to the towel rail,
Pour inhibitor into other end then fit other valve (closed)
Hang towel rail
Attach and open valves
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