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What to do if…
your boiler pressure drops suddenly and the heating or hot water stops working

Short answer

If you smell gas or a carbon monoxide alarm is sounding, move everyone into fresh air immediately and call the gas emergency number. If there’s no gas/CO concern, check the boiler pressure and (only if you’re confident) top it up into the boiler’s normal/green range using the filling loop/key, then stop and call a Gas Safe engineer if the pressure won’t hold.

Do not do these things

  • Don’t stay inside to “just reset it” if you smell gas or a carbon monoxide alarm is sounding.
  • Don’t keep resetting the boiler repeatedly if it locks out again — stop and escalate.
  • Don’t keep adding water if the pressure drops again soon after — that suggests a leak or failed part.
  • Don’t open random valves or dismantle the boiler casing.
  • Don’t raise pressure above the normal/green range “to be safe”.

What to do now

  1. Rule out immediate danger first (takes 10 seconds).
    If you smell gas, suspect carbon monoxide, or a carbon monoxide alarm is sounding: open doors/windows on your way out if you can, get everyone outside to fresh air, and call the National Gas Emergency Service on 0800 111 999. If there’s immediate fire danger, call 999.
  2. Turn the boiler off at its controls and let it cool for a few minutes.
    This makes it easier to check pressure and reduces the risk of overfilling while it’s hot.
  3. Check the pressure reading on the boiler’s gauge/display.
    On many UK systems, when cool, normal pressure is often around 1–1.5 bar (or the boiler’s “green” area). If it’s very low, the boiler may shut down to protect itself.
  4. If you feel confident and your boiler has a filling loop/key, top up pressure slowly — then stop.
    • Find the filling loop (often a silver braided hose under the boiler) or an internal filling key/lever (varies by model).
    • Open the filling control slowly while watching the gauge.
    • Stop when it reaches the normal/green area (often around 1–1.5 bar when cool, unless your boiler indicates otherwise).
    • Close the control firmly. Remove/secure any key if your model uses one.
  5. Restart once, then observe.
    Turn the boiler back on and try heating/hot water. Watch the pressure for the next 10–15 minutes.
  6. If pressure won’t rise, drops again, or you see any leak: stop and escalate.
    • If the gauge doesn’t move while filling, close the filling controls immediately.
    • If you see drips under the boiler, wet patches by radiators/pipework, or repeated pressure loss, book a Gas Safe registered engineer (and keep the boiler off if you’re unsure).
  7. If you rent: report it as an urgent repair in writing (now).
    Say the boiler has no heating/hot water and pressure has dropped. Take a photo of the pressure gauge and any error code, and keep a dated record of messages.

What can wait

  • You do not need to bleed every radiator right now unless you already know trapped air is the cause and you’re comfortable doing it.
  • You do not need to decide on a new boiler, service plan, or upgrades today.
  • You do not need to try multiple “advanced” fixes — one safe top-up attempt is enough before calling for help.

Important reassurance

A sudden pressure drop is a common reason boilers stop delivering heat or hot water, and it’s often fixable. Your job right now is simply to rule out gas/CO danger, check the gauge, and either restore normal pressure safely or stop early and get the right help.

Scope note

This is first-step guidance to stabilise the situation and avoid damage. Persistent pressure loss, leaks, or repeated lockouts usually need a qualified engineer to diagnose safely.

Important note

This guide is general information, not professional advice. Boiler designs vary. If anything feels unsafe or unclear, leave the boiler off and contact a qualified professional.

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