Electric Combi Boiler - No heating!

Use this guide when the room thermostat is calling for heat but the boiler does not respond, or the call-for-heat signal is intermittent.

---

1. Confirm Boiler is in Winter Mode

Before any fault-finding, confirm the boiler is set to heating mode.

• Check the boiler display:

  • Snowflake symbol = Winter mode (heating + hot water enabled)

  • Sun symbol = Summer mode (hot water only, no heating)

If the boiler is in Summer mode:

• Press the Menu button until the snowflake symbol appears

• Re-check whether the boiler responds to heating demand

If already in Winter mode, continue.

---

2. Safety and Initial Setup

• Confirm the system is safe to work on

• Ensure electrical isolation is available before opening any controls

• Record starting conditions:

  • Time on site

  • Ambient temperature

  • System pressure

  • Thermostat setpoint and room temperature

  • Boiler flow temperature setpoint

---

3. Verify Heating System Flow (Rule Out Blockages)

Before investigating controls or wiring, confirm the heating system can circulate properly.

Check the following:

• All boiler flow and return isolation valves are open

• Any zone valves are open and operating

• All radiator TRVs are fully open (set to maximum)

• Lockshield valves are not closed

• If using underfloor heating:

  • All actuator heads are open

  • Manifold valves are open and not stuck closed

• Pump runs when heating demand is present

• Flow pipe warms up and return responds

If circulation is restricted, correct the flow issue first.

If flow is confirmed OK, continue.

---

4. Check Call-for-Heat Signal at Each Stage

Confirm the call-for-heat is present through the full control chain.

1. Room thermostat

• Raise setpoint above room temperature

• Confirm thermostat shows demand for heat

1. Receiver/control box

• Confirm thermostat and receiver are connected/paired

• Confirm receiver indicator shows call-for-heat

1. Boiler

• Confirm boiler display shows call-for-heat input (if applicable)

If thermostat and receiver show demand but boiler does not, the signal is being lost between receiver and boiler.

---

5. Check Wiring Inside the Receiver Box (Most Common Fix)

Most faults of this type are caused by poor terminations inside the receiver.

Steps:

• Isolate power

• Open the receiver/control box

• Inspect call-for-heat terminals and wiring

• Check for:

  • Loose screws

  • Poorly stripped conductors

  • Stray copper strands

  • Damaged or stretched cable

• Remake and tighten all call-for-heat signal connections

• Restore power and retest boiler response

If fault is resolved, close out.

If not resolved, continue.

---

6. Prove Boiler Operation with Temporary Bridge Test

If receiver wiring does not resolve the issue, prove whether the boiler can fire correctly.

Steps:

• Isolate power

• Locate boiler call-for-heat input terminals

• Temporarily bridge the call-for-heat input (diagnostic use only)

• Restore power and observe operation

Results:

• Boiler fires and cycles normally:

  • Boiler is functional

  • Fault remains in controls, wiring, or receiver output

• Boiler does not fire:

  • Boiler-side fault likely (PCB, sensors, internal wiring, lockout)

Remove bridge after test and reinstate correct wiring.

---

7. Continuity Test the Call-for-Heat Signal Wire

If the fault is intermittent, test the signal cable between receiver and boiler.

Steps:

• Identify call-for-heat conductors

• Test continuity end-to-end

• Leave tester connected during operation to detect dropouts

Interpretation:

• Continuity stable:

  • Wiring likely OK

  • Suspect receiver switching output or boiler input stage

• Continuity drops in and out:

  • Cable break, loose termination, or faulty receiver terminal

---

8. Fix Terminations and Connections Properly

If wiring faults are suspected:

• Strip conductors cleanly

• Ensure no insulation is trapped under terminals

• Ensure no stray strands

• Use secure connectors (e.g., Wago) if required

• Add strain relief so cables cannot pull loose

Retest for at least one full heating cycle.

---

9. Swap in a Known-Good Receiver/Control Box

If receiver output remains unreliable:

• Replace with a known working receiver/control box

• Pair with thermostat

• Confirm call-for-heat appears at:

  • Thermostat

  • Receiver

  • Boiler

If fault disappears, original receiver/control box is faulty.

---

10. Verify Heating Operation and Cycling

Once repaired, confirm normal heating performance:

• Boiler fires reliably

• Pump runs correctly

• Radiators warm up

• Boiler reaches flow setpoint and modulates

• Boiler shuts down at setpoint and refires after differential drop

---

11. Domestic Hot Water Changeover Test (Combi Systems)

If boiler provides DHW:

• Run hot water and confirm DHW mode activates

• Stop hot water and confirm boiler returns to heating demand

• Heating should refire once temperature drops below threshold

---

12. Final Stability Check and Close-Out

• Leave system running on normal controls

• Observe at least one complete heating cycle

• If possible, return later to confirm continued stable operation

• Document any outstanding issues for supplier escalation

---

Common Fault Causes Identified by This Process

• Boiler left in Summer mode (sun symbol)

• Closed valves or restricted circulation

• Loose termination inside receiver box (most common electrical cause)

• Intermittent call-for-heat signal cable fault

• Faulty receiver/control output

• Boiler PCB or input recognition fault