Vaillant F22 Fault: What It Means and How to Fix It

F22 means your Vaillant boiler has shut itself down because the water pressure in your heating system has fallen too low. In most cases the heating comes back once you top the pressure up to between 1 and 1.5 bar using the filling loop. If it falls again within a few days, water is escaping somewhere — that needs an engineer, not another top-up.

What F22 actually means

F22 is one of the most common fault codes on Vaillant ecoTEC boilers (you will also see it on the ecoFIT range). A sensor inside the boiler keeps watch on the pressure of the water circulating through your radiators. When it sinks too far — often to around half a bar or less — the boiler refuses to fire and locks out, showing F22.

It is worth knowing the boiler is protecting itself here, not necessarily broken. A heat exchanger run without enough water flowing through it can be seriously damaged — engineers call it dry firing. Rather than risk that, the boiler stops, and your heating and hot water stop with it.

One worry is worth clearing up straight away: F22 is about water pressure inside your sealed heating system, nothing to do with your gas supply, and on its own it is not a dangerous fault.

Why the pressure dropped in the first place

A sealed system should hold its pressure for months, so a drop always has a cause, even if a small one. The usual culprits:

• A small leak. A weeping radiator valve, a tired joint or a slow drip inside the boiler itself can lose water so gradually you only notice when the gauge hits the bottom.
• Recently bled radiators. Letting air out lets a little pressure out too, so F22 often appears the day after a round of bleeding.
• A flat expansion vessel. The vessel holds a cushion of air that soaks up the extra volume as water heats and expands. When that charge fades, pressure climbs too high while the heating runs, the safety valve dumps the excess outside, and once everything cools the pressure crashes — setting up the next F22.
• A pressure relief valve that no longer seals. Once it has lifted, grit can lodge on its seat and let water trickle away through the discharge pipe outside, even at normal pressures.
• Nothing dramatic at all. On an older system it can be nothing more than tiny losses adding up over the year.

The checks you can safely do yourself

There is one proper fix you can do at home — the same one an engineer would start with: topping the system back up through the filling loop. It involves no gas work and nothing inside the boiler. Take it slowly, and stop if anything does not look right.

1. Switch the boiler off at its controls and let it cool — around half an hour is ideal, because pressure readings are only accurate on a cold system.
2. Check the pressure reading — most ecoTECs show it on the digital display; some models have a dial gauge. Below 1 bar confirms it is low. Our boiler pressure guide explains how to read the gauge and what the numbers mean.
3. Find the filling loop. On most Vaillant combis it is a short, flexible braided hose between two valves, usually directly underneath the boiler. Some models use a built-in filling key or lever instead — your manual shows which you have.
4. Open the valve (or valves) slowly. You should hear water begin to flow into the system — a gentle fill is a controlled fill.
5. Watch the gauge climb, and stop between 1 and 1.5 bar. Close both valves firmly. If your filling loop is the detachable type, remove it once finished.
6. Turn the boiler back on. The F22 code should clear and the boiler fire as normal. Overshot slightly? Letting a little water out of a radiator bleed valve brings the reading back down.

Safety, plainly put: only do this if you feel confident, and never remove the boiler’s outer case — everything behind it is engineer territory. Do not touch the gas pipework, the flue or the pressure relief valve itself. If you find heavy leaking or corrosion, stop and call an engineer. Anyone working on the boiler itself should be Gas Safe registered — you can check their details on the official Gas Safe Register.

When topping up is not the answer

One successful top-up that holds its pressure is a fix. A pattern is a symptom. Book a repair rather than reaching for the filling loop again if:

• The pressure falls again within a few days of topping up.
• You are topping up every few weeks, or more than two or three times a year.
• The needle swings wildly — very high while the heating runs, then near zero when it cools. That pattern points strongly at the expansion vessel.
• You spot damp patches, staining, or a drip from the small copper pipe that pokes through the wall outside (the relief valve discharge pipe).

Keep topping up a leaking system and two things happen: the leak quietly worsens — sometimes rotting floorboards or staining ceilings — and every fill of fresh water dilutes the corrosion inhibitor protecting the inside of your radiators. It buys a day of heating at the cost of a bigger problem later. If any of the signs above sound familiar, the sensible next step is a proper diagnosis — our boiler repair page explains how that works.

What an engineer will do, and what it typically costs

An engineer investigating a recurring F22 will work through the suspects in order: checking for leaks, testing the expansion vessel’s air charge with the boiler drained down (it can often be recharged; a failed vessel is replaced), and checking whether the pressure relief valve is passing water. If the gauge reads a healthy pressure while the code still shows, the pressure sensor itself joins the list.

The reassuring part is that the common F22 fixes are ordinary repairs, not worst-case scenarios. A vessel recharge, a replacement relief valve or a small leak repair will typically sit within the £70–£250 range that covers most standard boiler repairs. A major component failure can push a bill beyond that — all the more reason to agree a fixed price after diagnosis. Our guide to boiler repair costs in Walsall breaks down what drives a repair bill up or down.

A note on prevention: the vessel’s charge and the relief valve’s condition are checked during an annual service, long before they become a cold house and a fault code. If yours has not been looked at for a while, booking a boiler service is the cheapest way to keep F22 off the display.

Vaillant F22: your questions answered

Can I reset the boiler to clear F22 without topping up?
You can press reset, but if the pressure is still low the code will come straight back — the boiler is refusing to run for a reason. Restore the pressure first; reset afterwards only if the code has not cleared on its own.

How often should a Vaillant need topping up?
Rarely. A healthy sealed system holds its pressure; once or twice a year on an older system is common enough. Needing the filling loop every few weeks means water is escaping — a fault to find, not a routine to accept.

Is F22 dangerous?
No — the code is a safety lockout doing its job, and the underlying issue is low water, not gas. What would be unsafe is opening the boiler up to investigate: anything inside the case, or involving the gas supply or flue, belongs to a Gas Safe registered engineer.