The gas valve controls the flow of gas from the supply pipework to the boiler burner. It is a modulating valve capable of varying the gas flow rate to match the boiler’s current output demand. This modulation is what allows modern condensing boilers to achieve high efficiency at part load.
Gas valve replacement must be carried out by a Gas Safe registered engineer. This is a legal requirement, not a recommendation.
Fault Codes Indicating Gas Valve Failure
F27 — Flame Loss After Ignition: F27 indicates that the burner flame has been successfully established but has then been lost during operation. One of the more serious fault scenarios — a gas valve that opens for ignition then fails to maintain flow can indicate a faulty modulating actuator within the valve.
F28 — Ignition Failure: F28 indicates failure to ignite after multiple attempts. The gas valve is one of several components to investigate — alongside ignition electrode, ignition lead, and PCB. If electrode and lead are confirmed functional and gas supply is confirmed at adequate pressure, the gas valve is the likely remaining cause.
Before Condemning the Gas Valve
Gas valves are expensive. Before replacing the gas valve, confirm:
Gas supply is adequate. Check that other gas appliances in the property are functioning. Use a manometer to check the gas inlet pressure at the boiler — it should be approximately 20 mbar for natural gas at the boiler inlet.
Ignition components are functional. If F28, check the electrode for cracking or carbon tracking and the ignition lead for damage.
The gas valve is receiving the correct control signal from the PCB. If the PCB is not sending the correct signal, the valve cannot open.
The gas valve is not blocked. In rare cases, foreign material in the gas supply can block the valve inlet filter.
Vaillant Gas Valve Details
The Vaillant ecoTEC uses a modulating gas valve manufactured to Vaillant’s specification. The valve is calibrated to the boiler’s specific output range. Gas valves are specific to ecoTEC Pro, Plus, and Exclusive generations — not interchangeable between generations.
Post-Installation Requirements
After fitting a new gas valve, a full combustion analysis must be carried out by the Gas Safe engineer:
- Setting the minimum and maximum gas rate
- Checking CO and CO₂ levels in the flue gas
- Confirming gas inlet pressure and working pressure
- Verifying correct modulation across the output range
This is not optional. An incorrectly set gas valve produces unsafe combustion and can produce excess carbon monoxide.
Need a Recommended Installer?
Gas valve replacement and post-installation combustion testing requires a Gas Safe registered engineer.
- HGB Heating Kent — Gas Safe engineers, Vaillant specialists, Kent and Medway
- Boiler Sense — Certified boiler specialists, Kent and South East London
We supply the genuine Vaillant gas valve. They fit it safely and carry out the combustion check.