How Does the Aquastat on a Boiler Work?

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The aquastat on a hot water boiler is essentially a combination heating control. The aquastat monitors the water temperature in the boiler through the use of a copper bulb that stays in contact with the water inside the boiler and that shuts down the boiler automatically if the low- or high-temperature set point is exceeded. Understanding how the controller works can help users who are uncertain of why their boiler is shutting down unexpectedly.


Low-Limit Setting

One of the features of a hot water boiler aquastat is the low-limit setting, which is either pre-set at the factory or adjusted by the homeowner on the controller. Once the water temperature reaches below the low-limit setting, the boiler burner turns on and begins to heat the water to maintain the desired water temperature.

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High-Limit Setting

The other primary control on the aquastat is the high limit, which is the highest allowable hot water temperature before the controller shuts down the boiler. This is also factory pre-set or adjusted by the homeowner. The boiler burner automatically shuts off when the high-limit temperature is reached. The burner will restart when the water temperature falls within a differential temperature, usually about 10 degrees Fahrenheit, below the high-limit setting. This maintains a general set temperature for the water inside the boiler.


In most cases, the high-limit temperature setting on an aquastat should be between 180 and 200 degrees Fahrenheit. If the aquastat is set at 200 degrees Fahrenheit as the high-limit setting and the pressure valve begins to leak, set the temperature back to 195 degrees Fahrenheit. Contact the aquastat manufacturer if the valve continues to leak at 195 degrees Fahrenheit or less.

Set the low-temperature limit at least 20 degrees below the high-temperature limit setting. The circulator pump will lock out if the low-limit and high-limit points are too close or are accidentally mixed up, which prevents the boiler from working properly. Many home appliances require hot water that is at least 120 degrees Fahrenheit, which can make 120 degrees a favorable low temperature setting.


The other setting on the aquastat is the differential, which is the temperature-degree difference between the low and high settings at which the boiler will continue to operate. The lowest differential is 10 but the recommended differential is 25, which will maintain higher boiler temperatures and offer more hot water than a differential of 10.



Meredith Jameson

Meredith Jameson writes early childhood parenting and family health articles for various online publications. She holds a Bachelor of Arts in history from San Francisco State University.