Vacuum breaker stuck

The small valve on top of the steam boiler that's meant to vent air on startup and close once pressure builds — but instead stays open, allowing continuous steam to escape from the top of the machine.

Why it matters — Symptom is usually a constant hiss from inside the casing, sometimes a small puddle of condensate on top of the boiler. Steam is escaping where it shouldn't.

Good to know

• Vacuum breakers gum with scale. Common failure point in hard-water areas.
• Replacement is straightforward and inexpensive — the part is widely stocked.

Related: Vacuum Breakers