Flow switch to suit Bypass style heaters up to 24V. Check with a licensed gas-fitter or boiler repair person for suitability.
Comes with 3/8" (12-13mm) hose barbs and flow switch. Must be wired into existing flow switch. (Polarity is reversible)
Dega / Bosch / Everdure / Chaffoteux / Pyrox and other bypass heaters.