The Glow-worm replacement thermistors decrease circuit resistance in line with temperature increases. A boiler's printed control board sends a current to the thermistor, deciding whether a decrease or increase in heat is needed by reading the relative resistance. It is designed for use in Eden, HXI, CXI, British Gas, C-CI and SXI boilers.

• Compatible with Glow-worm HXI, SXI, CXi and CI