What air purge mode does and why it starts
Air purge mode is a built-in routine that pushes trapped air out of the boiler before the burner is allowed to light. Worcester Bosch's support page "What do the error symbols mean on my boiler?" describes it plainly: the boiler will not fire, but the pump pulses on and off. Air in the heat exchanger stops water flowing properly past the hot surfaces, so firing with air inside could cause overheating and noisy running. Purging first protects the boiler.
According to that FAQ page, air purge should normally only happen when the boiler is switched on for the first time after installation, or after a circuit board has been changed. Worcester's technical bulletin TB 0171 (September 2023) for the Greenstar 1000 combi adds more triggers on that model: the full purge starts whenever the boiler is powered on or switched off and on again, and also after the system is repressurised following a low pressure lockout.
So if you have just had a new boiler fitted, had a repair, had a power cut or topped up the pressure, a purge is expected.
How to tell your boiler is in air purge
What you see depends on the model. On the Greenstar i (the 25i and 30i ErP combi and the 9i to 24i System ErP), the user instructions say a flashing "A" on the text display means the boiler is in air purge mode. The same manuals list status code 357 as the heat exchanger air venting programme. Worcester's FAQ page shows air purge as a symbol in its list of display icons, alongside siphon fill and service symbols, which our guide to Worcester boiler display symbols explains.
The TB 0171 bulletin for the Greenstar 1000 does not say what the display shows during the purge. Because the boiler will not fire, the practical signs are the same on any model: you hear the pump starting and stopping, the radiators stay cold, the hot tap runs cold and there is no flame symbol on the display.
How long air purge mode lasts
Worcester gives different timings in different documents, because each one describes a different model or a different stage of the routine.
| Source | Model | What it says |
|---|---|---|
| Worcester FAQ page on error symbols | Not model-specific | About 15 minutes; call Worcester if it carries on for more than half an hour |
| Greenstar i user instructions (6 720 806 946 and 6 720 806 947) | Greenstar i combi and System ErP | Status 357, heat exchanger air venting, approximately 100 seconds |
| Technical bulletin TB 0171 (09/2023) | Greenstar 1000 combi | Initial purge of 12 cycles, about 9 minutes; then one cycle of about 45 seconds after each of the next ten heating or hot water demands |
The Greenstar 1000 pattern explains why some owners think the purge has "come back". After the 9 minute first run, the boiler does a short 45 second purge each time heating or hot water is next called for, ten times over. Each one briefly delays the burner, which is normal behaviour, not a fault.
Do not confuse air purge with siphon fill mode. Worcester's FAQ page describes siphon fill as the boiler running at low burn for about 15 minutes to fill the condensate trap. In air purge the burner stays off; in siphon fill it is lit but at low output.
Why the boiler will not fire, and why it can seem stuck
The burner is held off on purpose until the purge is complete. If the routine keeps restarting, the boiler can appear stuck. TB 0171 lists three Greenstar 1000 codes tied to air and poor circulation:
- 213: the flow and return sensors see a temperature difference above 32°C, and the boiler restarts the full purge cycle.
- 341: the flow temperature rises too fast after the burner starts, which can happen when circulation through the heat exchanger is poor.
- 2964: no temperature rise after the burner fires, which points to poor or no circulation and can also mean trapped air.
For 341 and 2964, the bulletin's advice is to make sure water can flow freely round the system, then put the boiler back into air purge by powering it off and on. For a homeowner, the safe version is to check the pressure, make sure radiator valves are open, then switch the boiler off at the fused spur, wait a few seconds and switch it back on once. If the code returns, our page on Worcester A1 281 covers the closely related pump and air fault on other Greenstar models.
Can you skip or turn off air purge mode?
You should not try to. Some websites describe button combinations or service menu settings that cut the purge short, and owners of the Greenstar 4000 and 8000 often go looking for them. Those are installer functions, and bypassing the purge lets the boiler fire with air still in the heat exchanger, which is exactly what the routine exists to prevent. The safe options are to let the cycle finish, or to have a Gas Safe registered engineer look at why it will not.
The same goes for the automatic air vent on the pump. It sits inside the casing, so venting it is an engineer's job, not a homeowner's. Our guide to the automatic air vent explains what it does.
Low pressure and bleeding radiators as causes of repeat purging
Air gets into heating systems in a few common ways, and low pressure is the one you can do something about. When the pressure falls far enough to lock the boiler out, topping it up and restarting triggers a fresh full purge on the Greenstar 1000. If the pressure keeps dropping, you will see the purge again and again.
- Check the gauge with the system cold. The Greenstar i user instructions ask for a reading between 1 and 1.5 bar and recharging if it falls below 1 bar. Our Worcester boiler pressure guide shows how.
- If radiators are cold at the top or gurgle, bleed them with the heating off, then recheck the pressure, as bleeding lets out water as well as air.
- If you need to top up more than once every few weeks, there is probably a leak or a failing component, and repeated topping up adds fresh, oxygen-rich water that brings more air and corrosion with it.
When to call a Gas Safe registered engineer
Worcester's FAQ page sets the threshold: if air purge continues for more than half an hour, call to arrange an appointment. You should also get help if the boiler goes into purge every day without a power cut or top up, if codes 213, 341 or 2964 keep returning on a Greenstar 1000, or if the pressure keeps falling.
Do not remove the casing to vent the pump or heat exchanger, and do not use service menus to bypass the purge. Both are jobs for a Gas Safe registered engineer.
The engineer will usually check the system pressure and expansion vessel, vent air from the boiler and pump, and look for sludge or a failing pump restricting flow. Typical UK costs are a call-out and diagnosis at £60 to £150, an expansion vessel recharge at £80 to £150 and a pump replacement at £150 to £400.