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THERMOSTATS & CONTROLS

Heating Won’t Turn On

Heating may not start because there is no programmed demand, the thermostat is satisfied, the boiler is faulted, a motorised valve is stuck or the control signal is missing.

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Isolate electrical power before opening controls, receivers or wiring centres

Heating controls may contain mains voltage. Basic settings and battery checks are safe, but internal wiring and relay testing should be handled by a competent person.

MOST LIKELY CAUSES

Start with these

No programmed demand The schedule or override may be off.
Thermostat not calling The set temperature may be too low.
Boiler fault or lockout The boiler may not accept the demand.
Zone valve or relay fault The heating circuit may not open.
◫ Symptom variations Match the exact timing, display or response pattern. ⌄
  • Boiler stays idle
  • Hot water works but heating does not
  • Thermostat clicks but nothing happens
  • One zone remains cold
  • Programmer display is normal
✓ Quick checks Simple checks before removing covers. ⌄
  1. Check batteries and power.
  2. Confirm time, day and schedule.
  3. Cancel boost, hold, away and holiday modes.
  4. Create a clear heating demand.
  5. Check receiver or relay indicators.
1 Step 1 — Create a clear heating demand Isolate electrical power before removing covers or touching wiring. ⌄

Use a manual override and raise the thermostat setting.

2 Step 2 — Check boiler status Isolate electrical power before removing covers or touching wiring. ⌄

Look for error codes, pressure issues or lockout.

3 Step 3 — Check zone controls Isolate electrical power before removing covers or touching wiring. ⌄

Note whether valves, receiver or relays appear to respond.

⌁ Common causes explained Why heating controls fail or behave unexpectedly. ⌄

Heating may not start because there is no programmed demand, the thermostat is satisfied, the boiler is faulted, a motorised valve is stuck or the control signal is missing.

⌘ Advanced diagnostics Heating-engineer checks and repair work. ⌄
  • Test thermostat and programmer outputs.
  • Check motorised valve end switches.
  • Verify boiler switched-live demand.
  • Inspect wiring centre.
  • Test receiver relay.
▣ Parts that may need replacing Common components linked to this fault. ⌄
  • Programmer
  • Thermostat
  • Motorised valve
  • Receiver
  • Relay
! When to call a heating engineer Conditions that should end basic troubleshooting. ⌄
  • The control contains mains wiring.
  • The receiver or relay stays stuck on or off.
  • The heating ignores all controls.
  • A motorised valve or wiring centre may be faulty.
  • The boiler receives the wrong demand signal.
? Frequently asked questions Common thermostat and programmer questions. ⌄

Why does heating start before the set time?

Smart optimisation may start early so the room reaches temperature by the programmed time.

Why is the thermostat reading different from another thermometer?

Location, airflow and calibration can all affect the reading.