PanicStation.org
UK USA
us Home & property emergencies boiler pressure dropped • low boiler pressure • hydronic boiler low pressure • no heat from boiler • no hot water from boiler • baseboards cold • radiators cold • boiler gauge reads low • boiler shut off low pressure • boiler needs water added • add water to boiler system • feed valve on boiler • auto fill valve boiler • boiler pressure 12 psi cold • boiler pressure too low psi • pressure relief valve discharge • boiler leaking water • heating stopped suddenly • boiler lockout • natural gas safety • carbon monoxide alarm

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 going off, leave to fresh air immediately and call 911 or your gas utility’s emergency number from a safe location. If there’s no gas/CO concern, check the boiler pressure gauge and (only if you’re confident) add water via the system’s fill/feed setup toward its normal cold-fill pressure, then stop and call an HVAC/plumbing pro 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 CO alarm is sounding.
  • Don’t operate light switches, appliances, or anything that could spark if you suspect a gas leak.
  • Don’t keep flipping the reset switch if the boiler locks out again.
  • Don’t crank the system pressure high to “force it to run”.
  • Don’t open unfamiliar valves or remove boiler panels.
  • Don’t keep adding water if pressure drops again soon — that often means a leak or failed component.

What to do now

  1. Check for gas/CO danger first.
    • If a carbon monoxide alarm is sounding: leave immediately to fresh air and call 911.
    • If you smell gas (or hear hissing): leave the area immediately and, from a safe location, call 911 or your natural gas utility’s emergency line (follow the utility’s instructions). Avoid doing anything that could create a spark.
  2. Turn the boiler off at its switch/thermostat and let it cool briefly.
    This helps you read the gauge and reduces the chance of overfilling while hot.
  3. Read the boiler pressure gauge (usually in PSI).
    Many residential hot-water (hydronic) systems are commonly around 12 psi when cold, but the right number depends on building height and system design. If your gauge is far below the system’s usual cold pressure, the boiler may shut down.
  4. If you’re confident, add water slowly using the system’s fill method — then stop.
    What you may have:
    • An automatic pressure-reducing fill valve (often set around 12 psi) with a fast-fill lever, or
    • A manual feed valve you open briefly to add water.
      While watching the gauge, add water slowly toward the system’s normal cold-fill pressure (often around 12 psi, but not universal). Close any manual valve fully when done.
  5. Turn the boiler back on once and monitor.
    Give it a few minutes. Watch for: pressure dropping again, error lights, water dripping, or the relief valve discharging.
  6. If any of these happen, stop and call for service (don’t keep topping up).
    Call an HVAC technician or plumber if:
    • You can’t get the pressure to rise,
    • Pressure falls again soon after,
    • You see leaks anywhere (near boiler, piping, radiators/baseboards), or
    • The pressure relief valve is releasing water.
  7. If you rent: notify your landlord/property manager immediately and document it.
    Send a message stating no heat / no hot water and boiler pressure is low, and include a photo of the gauge and any error code.

What can wait

  • You do not need to drain the system, “flush” it, or adjust an expansion tank right now.
  • You do not need to diagnose the root cause today — a single safe refill attempt is enough before you escalate.
  • You do not need to decide on replacement equipment or upgrades during the emergency.

Important reassurance

A low-pressure shutdown is a common reason hydronic boilers stop producing heat, and sometimes restoring the normal cold fill pressure gets things running again. If it won’t hold pressure, that’s useful information — it helps a professional fix the real problem faster, without you taking risky steps.

Scope note

This is immediate triage only. Repeated pressure loss, relief valve discharge, or any suspected leak typically needs in-person diagnosis and repair.

Important note

This guide is general information, not professional advice. Heating systems vary by home and fuel type. If you’re unsure at any point, leave the boiler off and contact a qualified professional.

Additional Resources
Support us