To fix this, set your TRV to the highest setting then undo the thermostat removal nut by hand. You will uncover a piston that will usually spring up and compress down smoothly. Take some WD40 and apply a small amount until it moves smoothly once more.
A non-responsive piston means you should replace your TRV. Stay tuned until the end of this article when we present our selection of market-topping TRVs…