Vaillant F22 Fault Code: Low Water Pressure / Dry Fire

What does F22 mean on a Vaillant boiler?

On Vaillant ecoTEC and ecoFIT boilers, F22 is a low water pressure / dry-fire protection fault. The boiler has detected that there is too little water circulating through the system. Rather than fire the burner against an empty or near-empty heat exchanger — which could damage it — the boiler shuts down and shows the F22 code.

In plain terms: your system pressure has dropped too low. The fix is usually simple and something most homeowners can do safely in a couple of minutes, because it doesn't involve the burner, gas, or any sealed part of the boiler.

Why has my pressure dropped?

Heating systems lose a little pressure naturally over time, but a sudden or repeated drop usually points to one of these:

• A slow leak somewhere on the pipework, a radiator valve, or a joint — sometimes too small to see.
• Air recently released — for example after you've bled radiators, which lowers pressure.
• A failed expansion vessel inside the boiler, which can cause pressure to swing or drop.
• A weeping pressure-relief valve (PRV), often spotted as a drip from the outside copper overflow pipe.

If you top up and the pressure holds, it was likely a one-off. If it keeps falling, you have an underlying fault that needs investigating.

What normal pressure looks like

Check the pressure gauge on the front of your boiler (a dial or a digital readout). As a UK guide:

Reading	What it means
Below ~1 bar (cold)	Too low — likely to trigger F22. Top up.
1–1.5 bar (cold)	Normal resting pressure for most homes.
Around 2 bar (hot)	Normal once the heating is running and the water has expanded.
Above ~2.5–3 bar	Over-pressurised — don't add more; you may need to bleed a little out.

How to fix F22: repressurise the system (homeowner-safe)

This is the standard fix and it's safe to do yourself. The filling loop is the silver braided hose (or a built-in key/lever) connecting the cold mains to your heating pipework, usually beneath the boiler.

1. Turn the boiler off and let it cool. Filling a cold system gives an accurate reading.
2. Find the filling loop — a flexible braided hose with a valve (or two) at each end, or an internal filling key on some ecoTEC models.
3. Open the valve(s) slowly. You'll hear water flowing in. Watch the pressure gauge.
4. Stop at 1–1.5 bar. Close the valve(s) firmly as soon as you reach this range. Don't rush.
5. Close everything off and, if your filling loop is the removable type, detach it once you're done.
6. Reset the boiler and turn it back on. The F22 code should clear and the boiler should fire normally.

If the pressure keeps dropping

Topping up is a fix for the symptom, not always the cause. If F22 returns within days or hours, stop repeatedly refilling — that just masks the problem and can introduce fresh oxygen into the system, encouraging corrosion. Instead:

• Check visible pipework and radiator valves for damp patches or staining.
• Look at the external overflow/condensate area for any drip suggesting a weeping PRV.
• Book a Gas Safe registered engineer to trace the leak, test the expansion vessel, or replace a failed PRV. These are not DIY jobs.

You can confirm an engineer's registration on the Gas Safe Register. (CORGI registration was replaced by Gas Safe back in 2009, so always check the Gas Safe number.)

Related Vaillant codes worth knowing

F22 is one of a family of Vaillant fault codes. If you see something different, these are the most common neighbours:

• F75 — a pump or pressure-sensor fault, often confused with low pressure because the symptoms overlap. See our guide to the Vaillant F75 fault code.
• F28 — an ignition/no-flame fault on start-up. Read more in our Vaillant F28 fault code guide.

Will boiler cover pay for an F22 repair?

If the cause is a faulty expansion vessel, a failed PRV or a leak on the system, a boiler cover policy with parts and labour will typically handle the diagnosis and repair, subject to your excess and any pre-existing-fault exclusions. A simple top-up you can do yourself for free, so it's rarely worth a callout on its own. If your boiler is ageing and these niggles keep cropping up, cover can take the cost uncertainty out of the next breakdown — see whether boiler cover is worth it for your situation, or read up on what boiler cover actually includes.

This article is general information, not gas-safety or financial advice. Always have gas appliances checked and repaired by a Gas Safe registered engineer. In a gas emergency, call the National Gas Emergency line on 0800 111 999. Costs and pressure figures are indicative UK guides for 2026.