The differential pressure switch is a small but critical component in the Vaillant ecoTEC boiler. It monitors whether the circulation pump is operating correctly by measuring the pressure difference between the pump inlet and outlet. When the pump runs, this pressure differential increases — the switch registers the change, closes its contacts, and sends a signal to the PCB confirming that water is circulating safely.
If the switch fails open (never confirming pump operation), the result is the F75 fault code. If it fails closed (always confirming), a dangerous condition exists where the PCB believes water is circulating even when it isn’t.
Why the Pressure Switch Fails
Vaillant pressure switches contain a small silicone rubber diaphragm that flexes in response to the pressure differential between the two monitoring ports. Over time, this diaphragm can develop micro-tears, harden, or the port connections can become blocked with scale.
Systems without adequate inhibitor treatment tend to experience pressure switch failures more frequently — scale and sludge from an unprotected system can block the monitoring ports.
F75 Pressure Switch Diagnosis
Before replacing the pressure switch, confirm:
The pump is actually running. If the pump motor has failed, the pressure switch is operating correctly — it’s simply reporting accurately that no pressure differential exists. Replacing the switch will not resolve the fault.
The monitoring tubes are clear. Two small-diameter flexible tubes connect the pressure switch to monitoring ports on the heat exchanger. Check these are not kinked, cracked, or blocked.
Test the switch directly. With the pump running, use a multimeter on continuity mode across the switch terminals. If the pump is running and the switch remains open (no continuity), the switch has failed.
Replacing the Pressure Switch
The pressure switch on the Vaillant ecoTEC is accessible via the boiler front casing. Replacement is a Gas Safe engineer task.
The replacement procedure:
- Isolate mains power to the boiler
- Remove the boiler front casing
- Disconnect the two monitoring tubes from the switch
- Disconnect the electrical connector
- Remove the switch mounting screw(s)
- Fit the replacement switch, reconnect tubes and connector
- Replace casing, restore power
- Test for F75 resolution
Important: After replacing the pressure switch, run the boiler through 2–3 complete heating cycles to confirm F75 does not return. If it does, the pump may be partially blocked and requiring a system flush.
Need a Recommended Installer?
Our Recommended Installers have resolved F75 faults across the full Vaillant ecoTEC range:
- HGB Heating Kent — Gas Safe engineers, Vaillant ecoTEC specialists, Kent and Medway
- Boiler Sense — Certified boiler specialists, Kent and South East London