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.
What it looks like when it fails
Contacts stick closed — temperature sails past the setpoint, the overshoot call — or fail open, which is a plain no-heat. Capillary and bulb types lose their gas charge and simply never call again. And everything drifts with age: the dial says 60, the water says 48.
A high-limit cut-out that keeps tripping is usually doing its job — the control stat has failed closed and the limit is the only thing stopping a runaway. Treat the limit as the witness, not the culprit.
How to test it
Meter across the contacts and swing the dial through the current temperature — you should hear and measure a clean changeover either side of it. No change in either direction condemns the stat (or its lost bulb charge).
Check calibration against a real thermometer at the sensing point, not the dial markings. On hot water systems, confirm which device you're actually on — the control stat and the high-limit sit together and look similar; the limit usually carries the manual reset button.
Gotchas
A kinked or cut capillary tube is a dead thermostat regardless of what the contacts read — route and clamp the capillary like it matters, because it does.
Don't fix overshoot by turning the dial down: sticking contacts only get worse, until the high-limit is doing all the switching — and that's a safety device, not a control. Replace the stat.
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.