- Boiler control thermostat
Variable resistor varies with temperature commonly negative temperature coefficient (ntc) pcb knows temperature of water by the resistance of this device. Check resistance but can seem to be right even when faulty. May just need cleaning. Replace if in doubt cheap part, ask for a white one off the modern boilers backwards compatible and better made.
Resistance check
- safety thermostat
Interrpts a live feed is normally closed, opens when overheated or faulty preventing live feed going on to start boiler, sometimes combined in series with low system water pressure sensor and on condensing boilers flue temperature sensor (to prevent plastic flue parts melting).
Continuity test.
- relay
measure voltage at output both before and after demand applied. Usually there are 5 connections, a pair of low voltage dc coil contacts look for a low dc voltage 12 or 24v. Other two three are common NC and NO. After dc voltage applied to coil expect 230v at NO one probe of ac voltmeter on NO other on incoming neutral or earth. NC should not show a voltage at this moment.