What Are Aquastat Settings?


An aquastat is a device used on hot water heating systems to control the water temperature. To prevent the boiler from firing too often, the aquastat is used, which has a high and low limit. The aquastat uses a sensing bulb, placed inside the boiler, to measure the water temperature.

High Temperature Limit

  • When the high limit is reached, regardless of the position of the thermostat, the aquastat will tell the boiler to stop. Generally, keeping the boiler at a higher temperature is more efficient than letting it cool down and heat all the way back up. The high limit is typically set between 180 and 200 degrees Fahrenheit.

Low Temperature Limit

  • When the water temperature gets too cool, the aquastat will tell the boiler to fire to keep the water above a certain temperature. This keeps the water close to the normal running temperature so that even if the thermostat is turned down, the boiler doesn't have to work as hard to heat it back up when heat is called for. The low limit should usually be set about 20 degrees Fahrenheit lower than the high limit.

Differential Setting

  • Some aquastats have a differential control, which controls the temperature allowance around the high and low limits. For example, if the differential control was set at 10 degrees, the temperature would raise to the high limit and the burner would shut down and not fire again until the temperature drops 10 degrees. In this instance, the low limit would be used to shut down the boiler in the event something malfunctioned.

Reset Button

  • Aquastats have a reset button, usually red in color and found on the front of the device. This is used to reset the aquastat in the event of a malfunction in the device itself, or a low limit shutdown. If the boiler will not run and everything else has been checked, simply pressing the reset button will kick the aquastat back on.

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