Boiler F75 Fault Code: Pressure Sensor and Pump Diagnosis for London Homes
The F75 fault code is one of the most common boiler faults seen in London homes and affects Vaillant ecotec boilers as well as several other brands. It indicates that the boiler started the pump but detected no pressure change, pointing to a pump or pressure sensor problem. This guide explains the causes, the diagnostic process, and the typical repair options.
What the F75 Fault Code Means on a Boiler
The F75 fault code appears on Vaillant ecotec boilers, and an equivalent fault under different code designations appears on several other boiler brands including Glow-worm, which shares the same Vaillant platform, and some Baxi and Ideal models that use analogous pressure differential monitoring logic. On a Vaillant ecotec boiler displaying F75, the control board attempted to start the circulation pump and then checked whether the heating circuit pressure changed as expected during pump operation. When the pump runs normally, the pressure differential sensor detects a measurable change in pressure as water begins to circulate. If no pressure change is detected within the timeout period, the F75 lockout is triggered.
There are two distinct components whose failure produces the F75 code: the circulation pump and the pressure differential sensor. Distinguishing between them requires a qualified engineer to observe the pump behaviour during a restart attempt, check the wiring to the pressure sensor, and in some cases measure the sensor output directly. However, there are some indicators that can help narrow down the likely cause before an engineer attends.
Why F75 Is Particularly Common in London
London has one of the hardest water supplies in England, with typical calcium hardness levels of three hundred to four hundred milligrams per litre expressed as calcium carbonate. This hardness level means that limescale deposits form inside the boiler heat exchanger and, critically for the F75 fault, inside the circulation pump casing and on the pump rotor. Over time, limescale accumulation on the pump rotor increases the starting torque required to turn the impeller and eventually reaches a point where the pump motor can no longer overcome the static friction and the rotor fails to turn. The pump motor may hum but the rotor does not move, resulting in zero flow and triggering the F75 lockout.
In London properties that have never had a water softener installed and where the boiler inhibitor has not been maintained at the correct concentration, the combination of hard water and low inhibitor coverage accelerates limescale formation throughout the heating circuit. A pump that has seized due to limescale can sometimes be freed by an engineer using a descaling agent introduced through the pump service port, but in many cases the pump rotor is damaged beyond effective cleaning and replacement is required.
Pressure Sensor Failure as an Alternative Cause of F75
The pressure differential sensor on Vaillant ecotec boilers is a small component mounted on the hydraulic block that detects the pressure change produced by pump operation. Over time, the diaphragm inside the sensor can harden or fail, causing it to report no pressure differential even when the pump is running normally. This produces exactly the same F75 lockout as a seized pump, which is why diagnostic observation of the pump behaviour is important. If an engineer can hear the pump running and can feel vibration from the pump casing but the F75 code persists, a faulty pressure sensor is the more likely diagnosis. Sensor replacement is generally a faster and less disruptive repair than pump replacement and is a common fix for F75 faults on Vaillant ecotec boilers in London. Prestige Engineers carry both Vaillant circulation pumps and pressure differential sensors on their service vehicles when attending F75 call-outs in London, enabling same-visit resolution in most cases.