Outside Air Reset For Boilers

If your facility is heated by a hot water boiler, you can save money and improve comfort by installing an outside air reset control. The controls are relatively inexpensive to install and typically pay for themselves in one year.

How It Works

Your hot water boiler is designed to provide water to radiators or hot water coils at a constant temperature, usually 180°. The gas burner cycles on and off to maintain this temperature. A temperature sensor turns on the gas burner when the water temperature falls below a low limit, usually 170°. During cold weather, this type of water temperature control works well. However, during mild weather this high water temperature may overheat the space, causing discomfort to occupants and wasting energy dollars.

Outside air reset control reduces the maximum boiler water temperature depending on the outside air temperature. For instance, if the outside air temperature is 0°, the hot water will need to be 180°. However, if the outside air temperature is 40°, the hot water will need to be only 130°. The controls should also be set to shut off the heating system when the outside air reaches a preset temperature, usually 65°. Caution: This may not apply if you have special hot water uses, such as a dishwasher, etc. on the system. In such a case, the temperature should not be reset below the temperature requirements of this equipment.


For a steel fire tube boiler, the outside air reset can directly control boiler water temperature, and installation costs are low. Cast iron boilers will require a three-way mixing valve with actuator, both of which will add to the installation cost.

Things to Remember

  • Have a licensed heating contractor install the controls.
  • If you now have pneumatic controls, you may want to upgrade to electronic controls before installing the outside air reset.
  • Resetting the hot water temperature during mild weather conditions may save air conditioning costs as well as heating costs.