Error F.75

Safety first

- If you smell gas, evacuate the property and call the gas emergency number immediately. Do not touch the boiler or electrical switches.

- Turn the boiler off at the power switch before opening any panels or attempting any internal checks. Do not open sealed boiler covers or attempt gas or electrical repairs unless you are Gas Safe qualified.

Initial quick checks a homeowner can do

1. Confirm the exact code shown (F.75 or F75) and note any other messages or flashing indicators.

2. Check the boiler pressure reading on the display or the analogue gauge. Cold system pressure should normally be around 1.0–1.5 bar (consult your boiler handbook for your model’s recommended cold pressure).

3. Look for visible leaks, damp patches, drips from the pressure relief valve, or signs of corrosion around pipework and radiators.

4. If the boiler is in a cold location or outdoors/garage, check the condensate pipe for freezing or blockage. If external and frozen, thaw it using warm (not boiling) water or a warm cloth; do not use open flames.

Steps to attempt to clear the fault (only if you are comfortable following the boiler manual)

1. Repressurise the system if pressure is low:

- Locate the filling loop (usually a braided or metal flexible hose) and the two isolation valves beneath the boiler or on the pipework. Follow your boiler manual for exact locations and procedure.

- Open the valves slowly until the pressure gauge rises to about 1.0–1.5 bar (or the manufacturer’s recommended cold pressure). Close both valves fully and ensure they are secure. Do not leave the filling loop permanently connected unless designed to be.

- If you are unsure where the filling loop is or how to operate it, stop and call a qualified engineer.

2. Reset the boiler:

- Use the boiler reset button (often marked with a flame icon) and hold for a few seconds to reboot. Wait a few minutes for the boiler to attempt a restart.

3. Trigger the boiler and listen for the pump:

- Run a hot tap or call for heat from the thermostat to make the boiler fire and the pump run.

- Put your hand gently on the pump casing (if accessible) or listen closely. A working pump typically gives a quiet hum and slight vibration. If you hear/feel nothing at all when the boiler is trying to run, the pump may be seized or defective.

4. Bleed radiators to remove airlocks:

- Air in the system can prevent pressure change. Starting with the lowest radiator, use a bleed key to release trapped air until water flows, then check system pressure and repressurise if needed. Each time you bleed, system pressure will drop, so recheck and top up as necessary.

5. If the pump runs but the F.75 returns or you have adequate pressure and no water delivery:

- This points toward a pressure sensor fault, a blocked sensor port (magnetite/debris), or internal valve issues. Sensor ports can become blocked by debris; cleaning is sometimes possible but replacement of the sensor is commonly recommended and safer.

When to call a professional

- Call a Gas Safe-registered engineer if the pump is silent or seized, if the fault returns after repressurising and resetting, if you find leaks, or if you are uncomfortable performing any of the steps above.

- Do not attempt pump or pressure-sensor replacement, internal inspections of the gas pathway, or electrical repairs yourself. These require a qualified engineer and specialist tools.

- Ask for an engineer familiar with Vaillant EcoTEC models and advise them the fault is F.75 so they can bring appropriate parts (pump, pressure sensor, or diagnostic tools).

Preventive notes

- Regular annual servicing reduces the risk of this fault by catching debris, magnetite build-up, pump wear and minor leaks early.

- If magnetite or debris is suspected repeatedly, discuss system cleaning or a powerflush and corrosion inhibitor with your engineer.

If the steps above do not clear the F.75 fault, stop further attempts and arrange for a Gas Safe-registered engineer to diagnose and repair the boiler.