Home › Heating › Boiler Runs but Radiators Stay Cold

PUMPS & MOTORISED VALVES

Boiler Runs but Radiators Stay Cold

If the boiler fires but radiators remain cold, hot water may not be circulating because of pump failure, closed valves, trapped air, a stuck motorised valve or blockage.

↻
!

Isolate electrical power before opening pumps, actuators or wiring centres

Heating pumps and motorised valves may contain mains voltage and hot water. Stop if a pump is seized, very hot, leaking or if a valve actuator is buzzing continuously.

MOST LIKELY CAUSES

Start with these

Pump not circulating The boiler heats but flow does not move.
Motorised valve closed The heating circuit may not open.
Airlock Trapped air can stop circulation.
System blockage Sludge or closed valves can restrict flow.
◫ Symptom variations Match the exact circulation or zone pattern. ⌄
  • Boiler burner runs
  • Flow pipe hot near boiler
  • Radiators remain cold
  • Pump silent or very hot
  • Boiler cycles quickly
✓ Quick checks Simple checks before removing covers. ⌄
  1. Confirm a heating demand exists.
  2. Check whether one zone or the whole system is affected.
  3. Listen for pump or actuator operation.
  4. Check system pressure and trapped air.
  5. Do not force seized valves or hot pumps.
1 Step 1 — Check heating valves and controls Isolate power before removing covers or touching wiring. ⌄

Confirm the heating zone is actually open.

2 Step 2 — Listen for pump operation Isolate power before removing covers or touching wiring. ⌄

Note whether the pump hums, vibrates or remains silent.

3 Step 3 — Check system pressure and air Isolate power before removing covers or touching wiring. ⌄

Verify normal pressure and bleed obvious trapped air.

⌁ Common causes explained Why pumps and zone valves fail. ⌄

If the boiler fires but radiators remain cold, hot water may not be circulating because of pump failure, closed valves, trapped air, a stuck motorised valve or blockage.

⌘ Advanced diagnostics Heating-engineer checks and repair work. ⌄
  • Test pump operation.
  • Inspect motorised valves.
  • Check bypass valve.
  • Assess flow and return temperatures.
  • Inspect for sludge or blockage.
▣ Parts that may need replacing Common components linked to this fault. ⌄
  • Circulation pump
  • Motorised valve
  • Automatic bypass valve
  • Air vent
  • Magnetic filter
! When to call a heating engineer Conditions that should end basic troubleshooting. ⌄
  • The pump or actuator contains mains wiring.
  • The pump is seized, leaking or overheating.
  • A valve body is stuck.
  • One or more zones remain uncontrollable.
  • Internal testing is required.
? Frequently asked questions Common pump and valve questions. ⌄

Can I force a motorised valve open?

Use only the normal manual lever as described by the manufacturer. Do not force a seized valve body.

Why is a heating pump hot?

Some warmth is normal, but excessive heat with no circulation may indicate a seized rotor or failed motor.