I have tied up the ballcock in the tank, attached a hose to a downstairs radiator's drainage pipe, opened it up and water has come out. I have then opened the upstairs rads' bleed screws and more water has come out. Downstairs, however, whilst some of the rads are ok to open the bleed screws, some still have water escaping from them and I cannot leave them open. I have tested whether it is only a small amount, but it just keeps coming and if I take them almost out it's a gusher!
Water can only flow downhill, so if your downstairs radiators are all plumbed from above then you cannot drain them all from one drain point. The drainpoint you have used is likley to have drained the upstairs radiators but any pipework not uphill from this drainpoint will still contain water. Hence the other downstairs radiators still contain water and when you open the bleed valves the water in the pipework is coming out. I would guess that this is not likley to be more than a few litres.
Thanks. The one I drained from is downstairs and I assumed it provided an outlet for all of them - it does not it seems. How should I drain the others? Will I need to undo the valves I am changing and catch the water in a tray do you think? There is also a drainage point on the boiler - is that relevant? Should I put a hose on that and undo the tap? Cheers, J
The way I have done this previously is the put towels down under the radiator, close the valves and then quickly as possibly undo, remove and flip the full radiator before taking it outside to drain. If the radiator is too big then you may have no choice other than to drain it in place.
Once the radiator is out the way you can slowly open the valve you plan to replace and allow the residule water to run off before fitting the new valve, alternatively if there is a lot of low pressure water left to come out you can once again use towels to catch the water and change the valve quickly. Please be aware if you are doing this that central heating water can stain so don't use the best towels and make sure you catch all the water. (You could also do this with the radiator left in place but it is fidlier.
When you are working on the upstairs radiators it is also worth remembering that a little residual water might remain in the radiators so be ready to catch it if required.