Vaillant boiler fault codes F22, F28 and F75 explained

Vaillant ecoTEC boilers are some of the most common in London homes. Their fault codes are among the most searched boiler terms in the city. Here is what the three most common ones mean and what you can safely do.

F22: low water pressure

F22 is the boiler telling you there is not enough water pressure in the heating system. Vaillant describes it as a dry fire warning: the boiler will not run because there may not be enough water to heat safely.

What to do

1. Look at the pressure gauge or display. If it is below about 1 bar, pressure is low. Check your manual for the right figure.
2. Top up using the filling loop if your manual shows how. Close the loop afterwards.
3. Reset the boiler.
4. Watch the pressure over the next few days.

If F22 keeps coming back, water is leaving the system. Common causes are a leak, a pressure relief valve letting by, or a flat expansion vessel. Our post on boiler pressure keeps dropping explains each.

F28: failed to light on start up

F28 means the boiler tried to light, failed after several attempts, and locked out. It is one of the most common winter codes.

Common causes

Cause	What to check
Gas supply off or low	Is the gas on at the meter? Does the hob work? Prepayment meter in credit?
Frozen condensate pipe	Does the plastic pipe run outside? Is it frozen?
Blocked condensate trap	Needs an engineer
Worn ignition electrode	Needs an engineer
Gas valve fault	Needs an engineer

What to do

1. Check other gas appliances work and the meter has credit if prepayment.
2. In cold weather, check the condensate pipe outside. If frozen, thaw it with warm, not boiling, water. See our post on frozen condensate pipes.
3. Reset once.

If F28 returns, book a repair. Do not keep resetting.

F75: no pressure change detected

F75 means that when the pump started, the boiler’s pressure sensor did not see the change in pressure it expected. In plain words, the boiler cannot confirm water is being pumped round properly.

Common causes

• Low system pressure
• Air trapped in the boiler or pump
• A failing pump
• A faulty pressure sensor
• Sludge restricting the pump or sensor

What to do

1. Check the pressure and top up if low.
2. Bleed radiators if they are cold at the top, then check pressure again.
3. Reset once.

If F75 keeps returning, the pump or pressure sensor is the likely problem and needs an engineer. In older systems with sludge, the pump may be struggling. A magnetic filter clean can help prevent repeat faults.

Quick reference

Code	Meaning	First safe check
F22	Low water pressure	Check gauge, top up via filling loop
F28	Failed to light on start up	Gas supply on, condensate pipe not frozen
F75	No pressure change when pump starts	Check pressure, bleed radiators

These are general meanings. Your model’s manual is the final word.

Cold snaps in south east London

On frosty mornings, many F28 lockouts in places like Lewisham come down to frozen condensate pipes. Lewisham’s terraces often have the plastic condensate pipe running down an outside wall into a gully. In a cold snap it freezes, water backs up inside the boiler, and it will not light. Our Lewisham area page covers the housing there.

What you should not do

Do not remove the boiler case, touch the gas valve, or try to clean the condensate trap from inside. All of that is gas work and must be done by a Gas Safe registered engineer.

If you smell gas, call the National Gas Emergency Service on 0800 111 999, turn off the gas at the meter if safe, and open windows.

Warranty repairs

If your Vaillant is still in its guarantee period, contact Vaillant first. Vaillant’s guarantee terms require an annual service by a Vaillant or other Gas Safe registered engineer, following its instructions. Keep your service records ready. See our post on Vaillant boiler servicing.

Writing it down

Before you call, note the code exactly as shown, the time it appeared, and anything that happened just before, such as a power cut, a frost, or the heating coming on for the first time in autumn.

Booking a repair

When you book a boiler repair, give us:

• The boiler model, such as ecoTEC plus or ecoTEC pro
• The code shown
• When it started and what you have checked
• A photo of the display and the data badge if you can

Gas work in the UK must be done by a Gas Safe registered engineer. Check the ID card and the number on the Gas Safe Register.