Vaillant F22 Error: Low Pressure Fix MK

F22 is probably the most common Vaillant fault we see across Milton Keynes — and the good news is, it's also one of the easiest to fix yourself. In about half the F22 cases we deal with, the homeowner could have sorted it in five minutes without a callout if they'd known what to do.

This guide walks you through exactly what F22 means, how to repressurise your Vaillant boiler step by step, how to spot a hidden leak that keeps causing F22 to return, and what it'll cost if you do need an engineer. As local Vaillant specialists in MK, we've talked hundreds of homeowners through this fix over the phone — most of the time at no charge.

What the F22 error means

Your central heating system runs at a pressure of around 1 to 1.5 bar when cold, rising to about 1.5-2 bar when hot. This pressure keeps water flowing properly around the radiators and ensures the boiler has enough water to heat safely.

F22 means the pressure has dropped below the minimum safe threshold — typically around 0.5 bar on most Vaillant ecoTEC models. Below this level, the boiler can't fire safely because there isn't enough water in the system to absorb the heat from the burner. So it locks out and shows F22 to tell you.

You'll usually see F22 alongside a pressure gauge reading well below 1 bar — sometimes the needle is right down on the zero stop. Vaillant publish a full fault code reference at vaillant.co.uk/service/boiler-fault-codes.

Most common causes of F22 in Milton Keynes

Across MK, here's what we find when we investigate F22 callouts, ranked by frequency:

1. Slow leak in the system (most common). A pinhole leak in a pipe, a weeping radiator valve, or a damp patch under the floor. Often invisible at first — the water evaporates before you notice it.
2. Recently bled radiators. Bleeding releases air and lowers the system pressure. If you've bled multiple radiators without topping back up, F22 is almost guaranteed.
3. Faulty pressure relief valve. The PRV is meant to release excess pressure but sometimes weeps continuously. Look for a small copper pipe outside the property dripping water.
4. Expansion vessel needs recharging. The expansion vessel inside the boiler loses air pressure over time. When this happens, system pressure swings wildly and can trip F22.
5. Faulty filling loop. If the filling loop has been left slightly open, water can flow back the wrong way and cause pressure loss.
6. Faulty pressure sensor. Less common — the sensor reports incorrect pressure even though the actual system pressure is fine.

F22 cause and fix at a glance

Likely cause	Symptom you might notice	Fix	DIY or engineer?
Recent radiator bleed	One-off F22, system was bled recently	Repressurise	DIY
Slow system leak	Pressure drops every few days	Find and repair leak	Gas Safe engineer
Faulty PRV (dripping outside)	Copper pipe outside dripping water	Replace PRV	Gas Safe engineer
Expansion vessel	Pressure swings high then low	Recharge or replace	Gas Safe engineer
Filling loop left open	Pressure climbs too high then PRV releases	Close filling loop	DIY
Faulty pressure sensor	F22 with healthy gauge reading	Replace sensor	Gas Safe engineer

How to repressurise a Vaillant boiler (step by step)

This is a safe DIY job that takes about five minutes. You don't need any tools, and you're not touching any gas components — you're just adding water to the heating system.

Step 1: Find the filling loop

The filling loop is a flexible silver braided hose with two black valves, usually mounted under the boiler or in the airing cupboard nearby. On most Vaillants installed in the last 15 years, it's a permanent fixture under the boiler. The two valves should both be closed (handle perpendicular to the pipe).

Step 2: Turn the boiler off

Switch the boiler off at the front panel — not the mains, just the on/off button. This isn't strictly required but makes pressure changes easier to read on the gauge.

Step 3: Open both valves slowly

Open the first valve a quarter turn, then the second valve a quarter turn. You should hear water flowing into the system. Watch the pressure gauge on the front of the boiler — the needle should start climbing.

Step 4: Watch the gauge climb to 1.2 bar

Keep an eye on the gauge as it climbs. When it reaches 1.2 bar (or just slightly below 1.5 bar), close both valves firmly. Close the second one first, then the first.

Step 5: Reset the boiler

Switch the boiler back on and hold the reset button for 3 seconds. F22 should clear and the boiler should fire normally. If the pressure gauge stays at 1.2 bar over the next few hours, the job is done.

Step 6: Check pressure 24 hours later

This is the important bit most people skip. Check the gauge a day later. If it's still around 1.2 bar, brilliant — you're done. If it's dropped significantly, you have a leak somewhere and need a Gas Safe engineer to find it.

What to try yourself if F22 keeps returning

1. Hunt for visible leaks

Check around every radiator valve, every visible pipe joint, and the boiler itself for damp patches, white limescale marks (a sign of slow weeping), or actual drips. Pay particular attention to bedroom radiators — those are usually upstairs above bedrooms, and a slow drip can soak into a carpet without anyone noticing for months.

2. Check the PRV discharge pipe outside

Find the small copper pipe that exits the wall behind your boiler — usually 15mm copper at low level outside. If it's dripping water, your pressure relief valve is faulty and the system is losing water out of this pipe every time pressure builds. This is a common cause of recurring F22 in MK.

3. Check radiator valves

Run a tissue along the bottom of each radiator valve (both the thermostatic valve and the lockshield on the other end). Even a tiny weep that's invisible to the eye will leave a mark on the tissue.

When to call a Gas Safe engineer

Call us out if:

• You've repressurised the system but F22 returns within a few days
• You can see water dripping from the PRV pipe outside
• You've spotted a damp patch but can't find the source
• The pressure gauge keeps climbing well above 1.5 bar when the system is hot
• You've topped up multiple times in a month — that's not normal and indicates a leak

Typical repair costs in Milton Keynes

Rough F22 repair costs across MK:

• Repressurise system (DIY): Free
• Engineer visit to repressurise + check: £60-£90
• Leak detection visit: £80-£120 depending on time taken
• Repair small pipe leak: £80-£150 typical
• Replace radiator valve: £40-£80 parts + £60-£90 labour
• Replace PRV: £40-£70 parts + £60-£90 labour
• Recharge or replace expansion vessel: £180-£280 (some need full replacement, some just air pressure)
• Replace pressure sensor: £90-£150 parts + £60-£90 labour

Most F22 fixes in MK come in well under £200. The exceptions are hidden leaks under floors (which can sometimes need a bit of detective work) and expansion vessel issues on older boilers. Our boiler repair team always diagnoses first and confirms costs before any work.

How to prevent F22 happening again

• Annual servicing. A yearly boiler service includes a pressure check, an expansion vessel test, and a visual inspection for any signs of weeping joints.
• Top up when you bleed radiators. Always check the gauge after bleeding and top up if it's dropped.
• Don't over-pressurise. Keep the gauge between 1 and 1.5 bar when cold. Going much above this puts strain on the PRV and joints.
• Watch the PRV pipe outside. A monthly glance at this is enough to catch issues early.
• Fit a magnetic system filter. Prevents debris reaching the pressure sensor.

For more on common Vaillant faults, see our error codes guide or our Vaillant installation guide. If your boiler keeps breaking down regardless of pressure issues, our breakdown guide has a wider checklist.

Get F22 sorted today

Plumbline MK is a Gas Safe registered local plumber covering all of Milton Keynes — including Bletchley, Newport Pagnell, and Wolverton. We're happy to talk you through repressurising over the phone at no charge — and if you do need a callout, we usually fix F22 in a single visit.

Need expert help? Contact Plumbline MK for a free, no-obligation quote. Call 07805 844 016 for same-day response across Milton Keynes and surrounding areas.

Frequently Asked Questions

How do I repressurise a Vaillant boiler myself?

Find the filling loop (a silver braided hose with two black valves, usually under the boiler), turn the boiler off, open both valves slowly a quarter turn each, and watch the pressure gauge climb. When it reaches 1.2 bar, close both valves firmly — second valve first, then the first. Turn the boiler back on and reset it. The whole job takes about five minutes. Always check the pressure again 24 hours later — if it's dropped significantly, you have a leak that needs an engineer.

Will an F22 fault damage my boiler?

No — F22 is a safety lockout that protects the boiler from running with insufficient water. The boiler won't fire while F22 is showing, so there's no risk of damage from heat. The risk is the underlying cause: if a leak is dripping into woodwork or onto electrical components, that can cause secondary damage. A persistent F22 also points to either a leak or a failing expansion vessel — both worth getting sorted promptly to avoid bigger problems later.

Why does F22 keep returning even after I repressurise?

If F22 returns within a week of repressurising, you have a leak somewhere — that's the only realistic explanation. The leak might be tiny and invisible (a pinhole in a pipe, a weeping radiator valve, or a slow drip from the PRV outside). Less commonly, the expansion vessel inside the boiler has lost its air charge, which causes pressure to swing widely. Either way, a Gas Safe engineer needs to track down the cause — repressurising repeatedly just delays the fix.

How often should I top up my boiler's pressure?

A healthy system should hold its pressure for months at a time. Topping up once or twice a year (typically after bleeding radiators in autumn) is normal. If you find yourself topping up every few weeks, something is wrong — most likely a leak or a faulty pressure relief valve. Get it diagnosed before the underlying issue gets worse. Many MK homeowners only spot a hidden leak when it's caused damage to floorboards or ceilings.

Is 0.5 bar pressure dangerous on my boiler?

It's not dangerous in the sense of risking an explosion or anything dramatic — the boiler will simply refuse to fire, which is exactly what you want it to do. Running a boiler at below 0.5 bar would risk the heat exchanger overheating without enough water to absorb the heat, so the F22 lockout is doing its job. The fix is just to top the pressure back up to 1.2 bar and find any leak that caused it to drop.