A timer key plugs into a boiler and is part of a control mechanical for a boiler system, allowing you to set the timer or schedule for when the heating and/or hot water switches on and off. It's a small key that you insert into the timer unit of the boiler, and by turning the key, you get heating control features, such as setting a specific time for turning the boiler on and off. These timer keys are part of a heating control system, where it connects to a boiler programmer, giving you boiler controls and helping make your system more energy-efficient.
