Overheating of a Combi Boiler


Combination boilers, also known as combi boilers, are usually high-efficiency units that provide both heating and hot water for the home or business. Combi boilers are higher in efficiency because the unit pulls more heat from the flue gases than traditional water heaters or furnaces and are also a space saver since there is only one unit necessary for both hot water and heat. The boiler heats water from the cold mains and has a secondary heating circuit that provides unlimited hot water whenever the boiler pilot is firing. However, if the combi boiler is overheating, some troubleshooting will be needed.


  • If the combi boiler is connected to radiators in the home or business and the temperature in the room is too high, adjust the central thermostat. Turn the thermostat temperature control dial down to a lower temperature or use the down button to lower the temperature setting. Do not try to adjust the radiator valves, as overheating is not usually a symptom of closed radiator valves.

Gas Supply

  • Locate the gas supply valve on the gas tank and look to make sure there is sufficient gas in the tank, if the combi boiler uses a gas tank. Other combi boilers are attached to a main gas valve. If the gas valve is turned off, the boiler will overheat when the unit tries to operate. Turn the gas valve counterclockwise fully until positioned at the “Open” or “On” setting. Stop immediately if you smell gas, turn off the boiler and leave the house. Call emergency services or your gas company for help.

Water Supply

  • As with the gas supply, if the water supply is limited or cut off, the combi boiler will not be able to function correctly and may overheat. Turn the water valve counterclockwise until completely open and make sure the water line is not bent, crushed or otherwise obstructed.

Overheating Lockout

  • When the combi boiler shuts down automatically due to overly high temperatures, turn off the boiler and allow the boiler to cool. Once cool, press the boiler reset button. This button is typically located on the control panel and is usually red or green in color. Then restart and relight the boiler to resume normal operation. Repeat the reset operation several times if the pilot light does not ignite at first, as there may be air in the pipes. Stop if the boiler does not operate normally after several tries and contact the manufacturer or another qualified service professional.

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