Error S.27

Safety first

- Do not remove boiler covers or work on live electrical or gas parts. If you are unsure at any point, turn the boiler off and call a Gas Safe registered engineer.

- If you must isolate the boiler, switch it off at the boiler’s on/off switch and the fused spur or the consumer unit. Only perform electrical isolation if you are competent and safe to do so.

Initial checks homeowners can do

1. Observe timing: If the pump has only just started overrun, wait 10–15 minutes. Normal pump overrun will stop on its own. If it stops, no further action is needed.

2. Check the programmer/thermostat: Make sure the heating timer, room thermostat or hot water selector is not set to call for heating. Temporarily set the programmer to OFF and the room thermostat down to see if the pump still runs.

3. Look for other codes or messages: Note any additional fault codes or unusual display messages; these help diagnostics.

4. Check boiler pressure: Verify system pressure on the gauge (usually around 1–1.5 bar cold). If pressure is very low (<0.7–1.0 bar) top up per your manual. Low pressure can be associated with other faults but will not normally cause pump overrun alone.

5. Listen and watch: Confirm whether the pump is running (vibration/noise) and whether it keeps running after the boiler has been turned off at the programmer and at the boiler switch.

Simple diagnostic steps you can try

1. Reset the boiler: Use the boiler reset procedure in the handbook. If S.27 reappears immediately or the pump continues to run, note times and conditions and proceed.

2. Power-cycle: Turn the boiler off at its fused spur or consumer unit for 1–2 minutes and turn back on. If pump still runs continuously after power-up with no demand from the thermostat/programmer, do not continue experimenting—call a professional.

3. Check controls: Temporarily isolate the room thermostat or set the programmer to OFF to verify whether an external control is wrongly calling for heat. If isolating external controls stops the pump, the problem may be with those controls.

4. Bleed radiators: If circulation is poor or there is a large flow-return temperature spread, air in the system can reduce circulation. Bleed radiators to remove trapped air and then re-check.

5. Check for obvious leaks or faults: Visually inspect pipework and the area around the pump for leaks or obvious damage. Low water level from leaks can cause other safety trips; if there is evidence of leaking, switch the boiler off and call an engineer.

When to call a professional

- If the pump runs continuously for hours or S.27 persists after the simple checks above.

- If you cannot identify any external control calling for heat but the pump still runs.

- If resetting or power-cycling does not clear the status, or if other fault codes are present.

- If you hear unusual noises from the pump, suspect electrical faults, or find water leaks.

What the engineer will check / information to provide

- Tell the engineer the exact code (S.27), model (Vaillant EcoTEC Plus Regular), how long the pump has been running, and whether other fault codes or symptoms are present. The engineer will check the pump and pump wiring/relay, the programmer/thermostat signals, the boiler PCB, the diverter/3-port valve or bypass valve, temperature sensors and wiring, and system pressure and flow. They will have the tools and training to safely repair or replace defective parts.

Final note

A short pump overrun is normal and needed to protect the boiler. Prolonged overrun or repeated S.27 faults usually requires a Gas Safe registered engineer or Vaillant-authorised service technician to diagnose and repair. Do not attempt internal electrical or gas repairs yourself.