Jon Connell wrote:

All the radiators in my house have fairly suddenly developed a problem
with not getting hot (it's happened over the course of a week or so at
most). It's a sealed system and during some boiler trouble 2 years
ago, I had the system flushed at the recommendation of the boiler
installer (actually it turned out that the system didn't need
flushing ... grr). The water going to taps is very hot, but the water
going into the heating system is barely above body temperature feeling
the pipes coming away from the boiler. Does anyone have any
suggestions as to what might have happened?

Jon

Top of my checklist would be a failed zone valve motor. If it's a
system with 2-port valves you might find a little lever on the valve
body which manually opens the valve. If you do it should move very
very easily when the system is calling for heat (ie ON) and quite
slowly as you spin all the gears when the system is off. If it's
stiff in both cases the motor has failed which is not unusual (I keep
a spare in my DIY kit) and easy to fix. If that has happened you can
get heat in the short term by latching it open.

There are plenty of other possibilities but IME that's the Occam's
Razor solution.