The Vaillant F75 fault code is the most searched boiler fault in the UK, generating over 1,300 searches per month. It is triggered when the differential pressure switch — which monitors the pressure change across the pump during operation — does not register the expected response when the pump is commanded to start.
Understanding the Vaillant F75 Fault
F75 occurs when the pump confirmation does not arrive. There are three typical causes:
1. The pump is blocked (impeller blocked by sludge/magnetite) The pump motor runs, but the impeller cannot turn freely due to accumulated black iron oxide (magnetite) sludge. This is the most common cause of F75 on systems without magnetic filtration.
2. The pump motor has failed The motor itself has burned out or seized. No impeller movement, no pressure.
3. The differential pressure switch has failed The pump is running correctly and generating normal pressure, but the pressure switch is not registering the change. This presents identically to a failed pump from a fault code perspective.
Diagnosing Pump vs Pressure Switch
To distinguish between a failed pump and a failed pressure switch:
- Listen and feel: Place your hand on the pump body. Can you feel the pump vibrating/running? If yes, the pump motor is running — proceed to pressure switch.
- Check pump head temperature: A running pump will be warm at its head after a few minutes of operation.
- Pressure switch test: With the boiler running, use a multimeter to test continuity across the differential pressure switch terminals. If the pump is running and the switch remains open, the switch has failed.
- Clear the pump impeller: On some ecoTEC models, there is a pump access bleed screw. With power off, open the bleed screw and check for dark sludge. If sludge is present, the system needs flushing.
Pump Assembly Details
The Vaillant ecoTEC Pro and Plus use a Grundfos-based pump assembly. The assembly includes the pump motor, impeller, pump housing, and connection manifold.
Part numbers vary between ecoTEC Pro and ecoTEC Plus generations. Confirm your specific model number before ordering. We stock pumps for:
- ecoTEC Pro 24, 28, 35
- ecoTEC Plus 24, 28, 35, 48
- Glow-worm Energy range (same pump assembly in many models)
Need a Recommended Installer?
Pump replacement on a Vaillant ecoTEC requires a Gas Safe engineer. Diagnosis to distinguish pump vs pressure switch failure benefits from hands-on testing.
Our Recommended Installers have experience diagnosing and resolving Vaillant F75:
- HGB Heating Kent — Gas Safe engineers, Vaillant specialists, Kent and Medway
- Boiler Sense — Certified boiler specialists, Kent and South East London