Vaillant F22 & F75 Fault Codes: What They Mean & How to Fix Them
By Mark White, Gas Safe Registered Engineer (638919) & Vaillant Accredited Installer · 6 min read
If your Vaillant is showing F22 or F75, the short version is that both codes are about the water inside your heating system, not the gas. F22 means there isn't enough water pressure for the boiler to run safely. F75 means the boiler started its pump and didn't see the pressure rise it expected. They're related, which is why I've covered them together, and the first check for both is the same: look at the pressure.
I'm a Vaillant Accredited Installer based in Addlestone, and I see both codes most weeks across Surrey, especially in October and November when heating that's sat idle all summer gets switched back on. Here's what each one means, what you can safely check yourself, and what's going on when it won't stay fixed.
What does F22 mean on a Vaillant boiler?
F22 is Vaillant's low water pressure fault, sometimes described as "dry fire". The boiler has a pressure sensor, and if the system pressure falls too low it locks out rather than heat a boiler that may not have enough water in it. It's one of the most common codes on the ecoTEC pro, ecoTEC plus and ecoFIT ranges.
What does F75 mean on a Vaillant boiler?
Vaillant describes F75 as the pressure change being too low when the heating pump starts. Every time the pump kicks in, the boiler watches for a small jump in pressure to confirm water is actually moving. If it doesn't see one after several tries, it shuts down. The usual causes are low system pressure, air in the boiler, a pump that's seized or failing, a faulty pressure sensor, or an expansion vessel that's lost its charge.
Safe checks you can do yourself
1. Read the pressure. Newer Vaillants show the pressure on the display; older ones have a dial on the front. With the heating off and the system cold, it should read roughly 1 to 1.5 bar. If it's well below 1 bar, low pressure is almost certainly behind F22 and is a common trigger for F75 too.
2. Top it up using the filling loop. Most Vaillant boilers have a filling loop underneath, either a braided hose with a small valve at each end or, on some combis, a built-in lever or key. Your user manual shows which you have. Open the valves slowly, watch the gauge climb to around 1.2 to 1.5 bar, then close both valves fully. Take your time; overfilling causes its own problems.
3. Reset once. Press the reset button (or hold it, depending on model) once. If the boiler fires up and runs normally, that may be the end of it.
4. Watch the pressure over the next few days. This is the step people skip. If it holds steady, you're done. If it creeps back down, something is letting water out, and topping up again only hides the problem while adding fresh, oxygen-rich water that speeds up corrosion inside the system.
That's the full list of safe DIY checks. Anything inside the boiler casing, such as the pump, pressure sensor or expansion vessel, is work for a Gas Safe registered engineer.
Why F22 keeps coming back
A leak somewhere. It's often small: a weeping radiator valve, a joint under the floor, or something inside the boiler. Check around radiator valves and under the boiler for drips, staining or damp patches.
The pressure relief valve. Look outside for a small copper pipe near the boiler, usually poking through the wall. If it's dripping, or there's a wet patch or limescale stain underneath, the boiler is dumping water through its safety valve. That usually happens because the pressure is shooting up when the heating gets hot, which points at the next cause.
A flat expansion vessel. The expansion vessel absorbs the extra volume when the water heats up. When it loses its air charge, pressure spikes when hot, the relief valve opens, and the pressure drops below normal when it cools down. A classic sign is a gauge that swings a long way between cold and hot. I can recharge or replace the vessel, and I check it on every annual service.
Recently bled radiators. Bleeding radiators lets air out, and the pressure drops with it. That's normal, and a one-off top-up is all it needs.
Why F75 keeps coming back
If the pressure is fine and F75 still returns, the cause is almost always inside the boiler:
A seized or failing pump. This is very common after summer. Pumps that have barely run for months can stick, especially in systems carrying sludge. Sometimes the pump can be freed; sometimes it needs replacing.
Air in the boiler. This often follows recent work on the system, or a system that's been topped up a lot. The pump can't build pressure while it's pushing air.
A faulty pressure sensor. The sensor is what reports the pressure rise in the first place. If it's failing, the boiler can't tell the pump is working even when it is.
Sludge. Dirty system water wears pumps, blocks sensors and restricts flow. If your water comes out of a radiator black, the code is a symptom and a power flush plus a magnetic filter is the longer-term fix.
What I do on an F22 or F75 callout
I test before replacing anything. That means checking the pressure cold and hot, testing the expansion vessel charge, checking the relief valve, testing the pump and the pressure sensor, venting air from the boiler, and checking the system water quality. That tells me whether you need a quick fix or a part, or whether the system itself needs attention. I carry common Vaillant parts on the van, so most of these are sorted on the first visit.
There's a diagnostic fee that's included in the repair cost if you go ahead, and you'll always get a clear price before any work starts.
Still under warranty? If your Vaillant is within its manufacturer's warranty, contact Vaillant or your original installer first, because the repair may be covered. If I installed it, just call me.
Related Vaillant fault codes
F28 is an ignition failure on start-up (check the gas supply and, in freezing weather, the condensate pipe), and F29 is the flame going out while it's running. I've covered the full list in my Vaillant error codes guide, and you can read more about my work on Vaillant boilers.
Frequently asked questions
Vaillant showing F22 or F75 in Surrey?
I repair Vaillant boilers across Addlestone, Weybridge, Chertsey, Walton-on-Thames, Byfleet, Woking, Shepperton and the surrounding areas. Call me on 07311 811574 and I'll usually get to you the same or next day, or send me the fault details if you can't talk right now.