The problem of your heating thermostat valve no longer opening after the summer is relatively common. Here are the most common reasons for this:
Valve pin is stuck:
There is a small pin in the valve that slides out when heating and back in when not heating. Over the summer, this pin can rust or become stuck due to deposits so that it no longer moves.
Dust and dirt:
Over time, dust, dirt or limescale particles can build up in the valve, blocking the pin.
Thermostatic head defective:
Sometimes the problem is with the thermostatic head itself, for example due to a defective mechanism or because the connection between the thermostat and the valve pin is not working properly.
Air in the radiator:
Air in the radiator can also cause the radiator not to heat up. However, this is more of a different problem and is usually indicated by cold spots on the radiator.
What can you do?
Move the valve pin manually:
Switch off the heating. Remove the thermostatic head (usually by simply pulling off or loosening a screw). Then carefully try to gently move the valve pin back and forth with pliers or a screwdriver to loosen it. It should be easy to move.
Check the thermostatic head:
Check that the thermostatic head is properly seated and noticeably moves the pin when adjusted.
Bleed the radiator:
Bleed the radiator if not already done.
Replacement or repair:
If nothing helps, it may be advisable to replace the thermostat or valve or have it checked by a specialist.