Thermostat
Switches a heating or cooling load on and off to hold a temperature at a setpoint.
What it is
A thermostat senses temperature and switches a heating or cooling load to keep it near a chosen setpoint. A high-limit (cut-out) thermostat is a safety device that disconnects if temperature gets dangerously high.
How it works
A sensing element (bimetal, gas-filled bulb, or electronic sensor) operates a contact at the setpoint. Simple thermostats switch on/off with a small dead-band; better controllers modulate to reduce overshoot.
Where the sensor is placed matters: a poorly-located or slow sensor makes the control overshoot and hunt around the setpoint.
Where it's used
Hot water systems, ovens, space and slab heating, and HVAC. A separate high-limit/cut-out protects against runaway heating if the main control fails.
Safety first
A tripped high-limit means a previous over-heat — find the cause before resetting. Don't simply turn a thermostat up (scald/over-temp risk).
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Heater overshooting or oscillating around setpoint
The process temperature overshoots the setpoint then cycles above and below it, instead of settling — poor control that can spoil product or trip high-limits.
No hot water (electric storage system)
An electric storage hot water system has gone cold — no hot water at the taps. Usually the element, thermostat, supply, or (for off-peak) the tariff/timing.
Thermocouple open or reversed (wrong temperature reading)
A thermocouple gives a wrong temperature — reads way off, shows an open/sensor-break, or moves the wrong way — so the heat control can't work properly.
Related definitions
Heating element
A resistive conductor that turns electrical energy into heat — the business end of most electric heating.
SSR (Solid-State Relay)
Switches a load electronically with no moving parts — fast, silent, ideal for frequent switching like heaters.
Thermocouple
A temperature sensor that produces a tiny voltage proportional to the temperature at its junction.
How HVAC control works (electrical view)
A thermostat/controller calls for heating or cooling; the electrical side energises the unit — refrigerant is separate.