How to Adjust the Flame on a Natural Gas Boiler

Updated February 21, 2017

Boilers, such as those that use natural gas, have a burner with a flame that will heat the water in the tank. To adjust the flame to control the temperature of the water, you must locate the aquastat. This has a sensor that determines the water temperature and controls the flame for the burner that heats the water. Change the settings on the aquastat to adjust the flame on the boiler.

Open the cover to the aquastat. The aquastat may be built into the boiler or it may be a metal box that is attached to it.

Find the knob for the high-limit controller and set it to the highest temperature you would like to have the water. The maximum setting for the high differential is typically 87.8 degrees C. When the water temperature reaches the high differential setting, the burner will shut off.

Set the low low-limit knob, located next to the high-limit controller knob, to the lowest setting at which the burner should reignite to heat the water. This can be from 76.6 to 85.1 degrees C.

Set the knob for the differential control for the unit using a screwdriver. Insert the screwdriver into the screw head under the window opening in the cover.

Turn the screwdriver to the left or right to set the temperature scale for the differential. The differential can be set anywhere from -15 degrees C to -1.11 degrees C, but should be set at -9.44 degrees Cor normal operation. This is the temperature difference that the system will allow before the hot and cold water begin to circulate.


An aquastat is similar to a thermostat. There should be no more than a 20-degree difference between the high-limit and low-limit controller settings. Some aquastats have a knob to set the differential and do not require the use of a screwdriver.


If you set the low-limit controller knob at too low a temperature, the boiler will use more energy to heat the water in the tank.

Things You'll Need

  • Screwdriver
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About the Author

Cameron Easey has over 15 years customer service experience, with eight of those years in the insurance industry. He has earned various designations from organizations like the Insurance Institute of America and LOMA. Easey earned his Bachelor of Arts degree in political science and history from Western Michigan University.