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Key takeaways
- Each time the pump starts, the boiler checks that system pressure rises slightly. F75 means it saw no change, so it cannot be sure water is circulating.
- Vaillant lists four causes: a defective pressure sensor, a defective pump, air in the heating system, or insufficient water in the boiler.
- Check the gauge first. Below 1 bar, Vaillant allows a competent homeowner to top up to 1 to 1.5 bar. At normal pressure, call an engineer.
- F75 often appears after radiators are bled or the system is drained, when air is still trapped around the pump.
- Repeated resets will not fix a failed pump or sensor and can leave the boiler cycling in and out of lockout.
The Vaillant F75 error means the boiler started its circulating pump and did not detect the small rise in water pressure it expects to see. Vaillant’s installation manual describes it as “no pressure change detection when starting pump”. The usual causes are air in the system, too little water, a failing pump or a faulty water pressure sensor. You can safely check the pressure gauge and top up if it is below 1 bar, but anything beyond that is a repair inside the boiler for a Gas Safe registered engineer.
What does F75 mean on a Vaillant boiler?
Vaillant ecoTEC boilers use an electronic water pressure sensor rather than relying on a simple mechanical switch. That lets the boiler run a quick self-test. When the pump starts, the water it pushes round creates a small but measurable change in pressure at the sensor. If the change appears, the boiler knows the pump is turning and the circuit is full of water. If it does not, the boiler shows F.75 and locks out.
In Vaillant’s own fault table in the ecoTEC plus installation and maintenance instructions, F.75 is listed as “Fault: No pressure change detection when starting pump”. The possible causes given are:
- the water pressure sensor is defective
- the pump is defective
- there is air in the heating installation
- there is insufficient water in the product
For engineers, the same table also points to checking the adjustable bypass and the expansion vessel arrangement. Those are installation checks that need the casing off.
F75 vs F22 and other Vaillant pressure codes
Vaillant has several codes around water pressure, and it helps to know which one you are looking at.
| Code | Vaillant description | What it points to |
|---|---|---|
| F.22 | Safety switch-off: low water pressure | Too little water, or the sensor or its wiring |
| F.73 | Water pressure sensor signal in the wrong range (too low) | Sensor or wiring fault |
| F.74 | Water pressure sensor signal outside correct range (too high) | Sensor or wiring fault |
| F.75 | No pressure change detection when starting pump | Air, low water, pump or sensor |
F22 is about the pressure being too low full stop. F75 is about the pressure not changing when it should. They share causes, which is why some boilers show one and then the other. Our guide to the Vaillant F22 error code covers low pressure lockouts in more depth.
What you can safely check yourself
There are three things a homeowner can look at without opening the boiler.
1. The pressure gauge
Vaillant’s UK fault code advice gives a normal range of 1 to 1.5 bar. If the gauge shows below 1 bar, Vaillant says a competent homeowner can top up using the filling loop, slowly, to 1 to 1.5 bar, at their own risk. If it is above 1 bar and F75 is showing, that is a case for an engineer. Our guide on how to repressurise a combi boiler explains the filling loop.
2. Recent work on the system
Think back. Have radiators been bled, a radiator changed, or the system drained in the last few days? Air that has worked its way to the boiler can sit around the pump, which then spins without moving water properly. The pressure change never arrives and F75 appears.
3. A single reset
If pressure is back in range after a top-up, one reset is reasonable. Follow the steps in your user manual or our guide to resetting a boiler safely. If F75 returns, stop there. Resetting again and again will not fix a mechanical or electrical fault.
Do not take the boiler casing off. Inside are live electrics, the gas valve and sealed combustion parts. Vaillant’s own advice is not to open the casing.
Common causes of F75 in more detail
Air in the system
Air is the most fixable cause. It often turns up after bleeding, after a radiator has been replaced, or in systems with a small leak that lets air in. Engineers can release air at the pump and the boiler’s automatic air vent, and check the vent is working. If air keeps returning, they will look for the reason, such as a leak, a faulty auto air vent, or corrosion producing gas in the water.
Low water
If the pressure is low enough, the pump has too little water to move and the expected change does not register. A top-up may fix it, but you then need to know why the pressure dropped. Regular top-ups mean water is leaving the system. Our article on a boiler losing pressure with no visible leak covers where it usually goes.
A failing or seized pump
Boiler pumps can seize, particularly after a summer standing idle, or wear out after years of use. Sludge in the system makes this more likely. A pump that hums but does not turn, or runs weakly, will often trigger F75. The engineer checks the pump is receiving power, that it turns freely, and that it moves water.
A faulty water pressure sensor
The sensor converts pressure into an electrical signal. If it fails, or its connector is loose or corroded, the boiler cannot see the pressure change even when the pump is working perfectly. Testing it means checking the readings on the boiler’s diagnostic menu against a gauge, which is an engineer task.
Sludge and blockages
Heavy magnetite sludge can clog the pump, the heat exchanger and the sensor connection. If the engineer finds dirty black water, the repair may need to include cleaning the system and fitting a magnetic filter, or the new pump will fail the same way.
How an engineer fixes F75
- Checks the pressure and fault history on the boiler’s diagnostic menu.
- Releases air from the pump and boiler, and checks the automatic air vent.
- Tests the pump: supply voltage, rotation and flow.
- Tests the pressure sensor reading against an independent gauge, and checks its wiring and connector.
- Checks the bypass and expansion vessel, which affect how pressure behaves when the pump starts.
- Replaces the faulty part, refills, vents and tests the boiler through several pump starts to confirm the fault has cleared.
The cost depends on which part has failed, the boiler model and whether a system clean is needed, so ask for a diagnosis before agreeing to a price. Our guide to boiler repair cost factors explains what drives the bill.
Is it safe to use the boiler with F75 showing?
When F75 is showing, the boiler has locked itself out, so it will not run. The lockout protects the heat exchanger from overheating with no water moving through it. The code is not a gas leak warning.
The risk is in what happens next. Do not bypass sensors or keep resetting a boiler that keeps faulting. If you smell gas at any time, open windows, do not use electrical switches, turn off the gas at the meter if you can, leave the property and call the National Gas Emergency Service on 0800 111 999.
If your heating fails in a cold spell and someone in the house is vulnerable, treat it as urgent. Our emergency boiler repair service covers same-day call-outs where we can.
Get your Vaillant running again
Check the pressure, top up if it is below 1 bar, and try one reset. If F75 comes back, the boiler is telling you the pump, sensor or system needs attention, and that is a job for an engineer with the right test equipment.
Gasmaster’s Gas Safe registered engineers repair Vaillant boilers across Stoke-on-Trent, Staffordshire, Cheshire and Derbyshire. Contact us to book a repair. If you have seen ignition codes too, our guide to Vaillant F28 and F29 errors explains those.
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Frequently asked questions
Can low pressure cause a Vaillant F75 error?
Yes. Vaillant lists insufficient water in the product as one of the causes. With too little water, the pump cannot create the pressure change the boiler is looking for. If the gauge reads below 1 bar, topping up to 1 to 1.5 bar and resetting once may clear it. If not, an engineer needs to check the pump and sensor.
Why does my Vaillant show F75 after bleeding radiators?
Bleeding lowers the pressure and can push air towards the boiler. Air around the pump stops it moving water properly, so the boiler sees no pressure rise. Check the pressure and top up if needed. If F75 continues, an engineer can vent the pump and boiler properly and make sure the automatic air vent is working.
Is F75 the pump or the pressure sensor?
It can be either, and the code alone cannot tell you which. That is why an engineer tests both: checking the pump turns and moves water, and comparing the sensor reading with an independent gauge. Replacing parts without testing can mean paying for a pump when the sensor was at fault, or the reverse.
How can I prevent F75 coming back?
Keep pressure in the normal range, have the boiler serviced every year, and ask your engineer to check the condition of the system water. Clean water with inhibitor and a working magnetic filter protects the pump and sensor. Our Vaillant boiler service guide covers what an annual service includes.
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Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
