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It's interesting you have mentioned using mains pressure as I've read about someone doing this to remove a blockage caused by sludge.You could shoot the system up with mains pressure ...find a rad with a good type mt cock
connect hose with a jubilee clip and put the other end on to outside tap. close off boiler
flow and return then shoot it up with mains pressure water, start at the bottom and bleed every rad going upwards and along so the last rad is highest and furtherest away.
In case anybody wants to do this with open vented it works but be carefull not to overfill
the F & E tank. blip the pump on and off ...its a bit like heart surgery for heating systems.
then undo the lot and maybe dump all the water and do again..its not rocket science
centralheatking
In my attempt to remove the blockage I was going to do the following:
1. Empty some water from the CH system, enough to empty the loft radiators and loft pipe work, but ideally still keep the bedroom radiators full. This will be a guessing game!
2. Disconnect the loft radiator piping from the two 15mm pipes currently feeding the loft from the bathroom and then connect these two pipes together. At this point I have isolated the loft radiators from the rest of the system.
3. Re-pressurise the system so that it can continue to be used in the rest of the house.
I then have 2 pipes going to each radiator in the loft and I can connect, in turn, each radiator circuit to a hose pipe as you have suggested. As it is the loft, I would need a long piece of hose (just bought a 30m hose) connected to the outside tap, so I'm a little unsure what pressure I could achieve given it is going up to the 2nd floor. The other side of the circuit I would connect to a hose that terminated in the loft toilet. I can then turn on the outside tap and see if any water flows into a toilet. For the cold towel rail and cold radiator I suspect nothing will flow through. For the warm radiator I may get a trickle.
I'm not sure what the mains water pressures involved would be in doing this but suspect it will still be higher than that produced by the CH system.
If this fails I was planning to access the piping under each radiator. I haven't mentioned before, but the 15mm feed and return pipes to the loft are copper (as is the rest of the house), however, the 15mm pipes to the loft radiators are all plastic (and yes I've checked they are the correct CH pipes given this was installed 15 years ago). My understanding of plastic piping is that the joints, especially elbows, can reduce the diameter of the pipe, so I suspect if there is a blockage it is likely to be on the elbows underneath each radiator, where the plastic pipe is connected to the copper tails going to the radiator valves. Of course this is all guesswork