In the good old days of rented accommodation, a simple call to the landlord would get a broken radiator working (possibly with another call to make sure he really does get a plumber out). However, now we own a house, we have to pay for this sort of thing.
The radiator in our spare bedroom hasn't worked for ages and we hadn't done anything about it (apart from trying to bleed it and all we get is cold water and no air), but we've got family staying over at Christmas so we need the room warm. Katherine rang a couple of plumbers last week and it was sounding expensive.
I decided it couldn't be too hard to do it myself. I figured the thermostatic valve must be jammed, so headed off to B&Q to get a new one. £15 spent.
Now I had to figure out how to replace it without flooding the place. A quick internet search stated that for a sealed combi boiler system you should get professional help. Undeterred, I ploughed on and found lots of stuff explaining how the system will be hooked up and realised it's fairly simple to drain a single radiator without causing Noah's flood (a page explaining how to replace a bathroom radiator with a heated towel rail was the breakthrough!).
Turned off the system so the pump wouldn't send any new water around. Close the valve at the other end of the radiator, bleed the thing until the pressure stops pushing the water out. Then start undoing the connections between the dodgy valve and the radiator. A couple of litres of water came out (what was in the radiator itself), but was easily caught in a jug. Then disconnected the valve from the in pipe. This was the bit I was unsure about, because it there was any pressure left it would spray the whole bedroom in slightly rusty water. Thankfully, just a mere dribble came out.
My theory was correct, the valve was jammed up tighter than a man on a red meat only diet! Replaced it with the new one. Tighten everything up, re-pressurise the system and voila, a whole house full of working radiators.
I reckon a plumber would have charged a fortune for that - a dodgy plumber could have bullshitted to charge for replacing the entire radiator. All it cost me was £15 for the new valve, plus an hour or so research and thinking time, followed by another hour actually doing it.