Radiator valves are made by many different manufacturers all working to their own designs and tolerances, so there's always a chance that this might happen.
There are a several things that can help to solve this particular problem.
- Follow the correct installation procedure from the manufacturer - typically this will involve a calibration of the range that the valve moves through.
- Use the correct adapter - both the Danfoss and StellaZ come with different adapters to suit a variety of radiator valves.
- Ensure that you are fitting the Thermostat to the correct radiator valve - this needs to be the one with the "push-pin" that moves up and down, not the lock-shield valve!
- Make sure that the push-pin moves up and down freely - a squirt of WD40 can help and a gentle tap with a hammer can free it off if it is stuck.
- Fit a small "shim" on top of the push-pin, for example a small piece of plastic stuck on with some "no-more-nails" type adhesive.
The above tips should help in making the Danfoss and StellaZ Radiator Thermostats work correctly with your radiator valves!
The calibration process can be seen in the relevant manual: