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Fix a combi boiler that will not heat

We'll confirm the symptom, check pressure, flow, power, and gas, then isolate the cause—or tell you when to call a pro.

Troubleshooting · Home maintenance
15–45 min
What you'll need
  • Owner manual (for pressure range and error codes)
  • Filling loop key or screwdriver (if repressurizing)
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Steps

Goal: Confirm the symptom, check pressure, flow, power, and gas, then isolate the cause.

  • Confirm whether heating, hot water, or both have failed. A combi boiler provides both.
  • Check the system pressure gauge—typically 12–15 psi. If below 12 psi, the boiler may not fire.
  • Good: Pressure low—repressurize using the filling loop. Bad: Pressure OK—proceed to Check flow and thermostat.

Check pressure

Goal: Confirm system pressure is adequate. Low pressure is a common cause.

  • Check the pressure gauge on the boiler. Normal is 12–15 psi.
  • If low, use the filling loop per the manufacturer. Open the valves to add water until 12–15 psi, then close the valves.
  • Good: Pressure in range. Test heating and hot water. If the boiler fires, fix complete.
  • Bad: Pressure was low and repressurizing fixed it. Monitor for leaks.

Check flow and thermostat

Goal: Confirm the thermostat is calling (heating) or flow is present (hot water).

  • For heating: set the thermostat to heat and raise the setpoint above room temp. The thermostat must be calling.
  • For hot water: open a hot water faucet fully. The boiler senses flow and should fire.
  • Good: Thermostat calling or flow present—check power and gas.
  • Bad: Thermostat not calling or no flow—fix thermostat or bleed air.

When to get help

Call an HVAC professional if:

  • The pressure is correct but the boiler still does not fire.
  • The boiler displays an error code you cannot resolve.
  • You smell gas (evacuate and call 911 first).
  • You are not comfortable with gas or electrical work.

Verification

  • System pressure is 12–15 psi.
  • Radiators or baseboard heat when the thermostat calls.
  • Hot water reaches fixtures when you open a hot faucet.
  • No error codes on the display.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm symptom Verify whether heating, hot water, or both have failed.
  2. Check pressure System pressure 12–15 psi. Repressurize if low.
  3. Check flow and thermostat Thermostat calling for heat; hot water flow when faucet is open.
  4. Bleed air Bleed radiators and clear airlocks.
  5. Call a pro Pressure correct but no heat, error code, or gas smell—call an HVAC professional.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Whether heating, hot water, or both have failed
  • System pressure reading
  • Whether thermostat is calling
  • Error code (if displayed)
  • Steps already tried

Is the system pressure below 12 psi?

Combi boilers need 12–15 psi to operate. Low pressure prevents firing.

Check the pressure gauge on the boiler. Yes: repressurize using the filling loop. Add water until 12–15 psi, then close the valves. No: proceed to check flow and thermostat.

Yes No

You can change your answer later.

Repressurize the system

Use the filling loop per the manufacturer. Open the valves to add water until the gauge reads 12–15 psi. Close the valves. Test heating and hot water. If the boiler fires, fix complete.

Done

Is the thermostat calling for heat (heating) or is water flowing (hot water)?

Heating needs thermostat call. Hot water needs flow at a faucet.

Heating: set thermostat to heat, setpoint above room temp. Hot water: open a hot faucet fully. Yes: check power and gas. No: fix thermostat or flow first.

Yes No

You can change your answer later.

Fix thermostat or flow

Heating: check thermostat batteries, set to heat, setpoint above room temp. Hot water: open faucet fully, bleed air if needed.

Done

Is power on and gas supply on?

Check circuit breaker and boiler switch. Check gas valve to boiler. If you smell gas, evacuate and call 911. Yes: boiler may have internal fault—check error code, call a pro. No: fix power or gas.

Yes No

You can change your answer later.

Fix power or gas

Reset circuit breaker. Turn on boiler switch. Confirm gas valve to boiler is on. Do not work on gas lines yourself.

Done

Call an HVAC professional

Pressure is correct, thermostat is calling, power and gas are on, but the boiler does not fire. Check for error code. Call an HVAC professional for internal faults. If you smell gas, evacuate and call 911.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a combi boiler not heat?
Common causes: low system pressure, no flow (closed valves, airlock), no power, thermostat not calling, gas supply off, or a fault in the boiler. Combi boilers need adequate pressure and flow to fire.
Can I fix a combi boiler that will not heat myself?
Yes, for pressure, flow, power, and thermostat checks. Repressurizing the system and bleeding air are often DIY. Gas, ignition, and internal boiler faults require a professional.
When should I call a professional for a combi boiler?
Call an HVAC professional if you smell gas (evacuate and call 911 first), the pressure is correct but the boiler still does not fire, the boiler displays an error code, or you are not comfortable with gas or electrical work.

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