Vaillant F75 fault code: what it means and how to fix it
F75 is a pressure fault. When the pump starts, a Vaillant boiler expects its water pressure sensor to register a change. If it does not, the boiler locks out and shows F75. Low system pressure is a common cause, and that one you can usually sort yourself.
What F75 means
Your boiler has a water pressure sensor inside it. Every time the pump starts, the pressure in the system should change slightly, and the sensor should pick that up. F75 means the sensor did not detect that change, so the boiler locks out to protect itself.
That points to one of two things: water is not moving round the system the way it should, or the sensor reading it is faulty.
Before you do anything
Never remove the boiler casing. Everything behind the front panel is gas work, and only a Gas Safe registered engineer should touch it. Every check below is done from outside the boiler.
If you can smell gas, turn it off at the meter, open the windows, do not touch electrical switches, and ring the National Gas Emergency Service on 0800 111 999.
Safe checks you can do yourself
Work through these in order. Keep your boiler’s user manual to hand for the filling loop.
Look at the pressure gauge
With the heating off and the system cold, the pressure should read about 1 to 1.5 bar. Below that is low, and low pressure is enough to trigger F75.
If it is low, top up with the filling loop
The filling loop is the valve, or braided hose with valves, on the pipework under the boiler. Follow the instructions in your manual: open it slowly, watch the gauge, and close it again at around 1.5 bar. If your loop is a removable hose, disconnect it afterwards as the manual describes.
Bleed the radiators if there is air
Radiators that are cold at the top, or gurgling in the pipes, mean air in the system. Bleed them with the heating off, then check the gauge again. Bleeding drops the pressure, so you may need to top up a second time.
Reset the boiler
Press reset as described in the manual and let it run. If the heating comes on and the pressure holds steady over the next few days, you are done.
If F75 keeps coming back, or you keep having to top up the pressure, call an engineer. A system that keeps losing pressure has a leak somewhere, and topping up only hides it.
What an engineer will check
If the pressure is right and F75 still appears, or the pressure will not hold, these are the usual causes:
- Low pressure or a leak. Pressure that keeps dropping means water is escaping, from a radiator valve, a pipe joint, or the boiler itself.
- Air in the system. Air trapped in the pump or the boiler can stop water circulating even after the radiators have been bled.
- Failed pump. If the pump is seized or not running properly, there is no pressure change for the sensor to detect.
- Faulty water pressure sensor. The sensor itself can fail and give a false reading even when the system is fine.
- Sludge or magnetite. Black sludge from inside the radiators and pipes restricts flow and can clog the sensor. The fix may be a system flush and a magnetic filter.
When you ring, tell us the code on the display, what the pressure gauge reads, and whether you have had to top it up recently.
Common questions
What does F75 mean on a Vaillant boiler?
The boiler’s water pressure sensor did not detect a pressure change when the pump started, so the boiler locked out.
Can I fix a Vaillant F75 fault myself?
Sometimes. If the pressure gauge reads low, topping up with the filling loop as your manual describes, bleeding any air from the radiators and then resetting can clear it. If it comes back, call a Gas Safe registered engineer.
What pressure should my Vaillant boiler be?
About 1 to 1.5 bar when the system is cold.
Why does my boiler keep losing pressure?
Usually a leak somewhere in the system or the boiler. Topping up keeps it running but does not fix the cause, so it needs an engineer to find it.
Can I take the boiler cover off to check the pump?
No. Never remove the boiler casing. The parts behind it are gas parts, and only a Gas Safe registered engineer should work on them.