We would advise against using boiling water to clean out a radiator, given that this process could damage the finish of your radiator whilst also posing a scalding risk. Lukewarm soapy water is a recommended alternative – you should also place a bucket and towels beneath the radiator and cover any nearby electrical sockets.
You can take a look at our radiator cleaning guide for advice on the best way to clean out your rads, and take a look at our 10 home heating hacks to avoid article too.