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An aquastat on a heating system.

What is an Aquastat?

An aquastat monitors and controls the temperature of the water inside your heating system, much like a thermostat would do for the air inside your home. It is typically located next to your boiler, switching it on or off as necessary to maintain the desired temperature. The aquastat is also used to regulate the water temperature in hot water heaters.

Why is An Aquastat Important?

While it’s called a boiler, most boilers are not actually meant to boil water. Doing so would result in steam, and an unsafe buildup of pressure in your system that it is not designed for. The aquastats high limit setting is designed to switch off the burner when the water temperature reaches this point. 

The low limit settings purpose is to not allow the water temperature to drop too low. After reaching the high limit setting your boiler or hot water heater will continue to circulate water as long as it’s being called for. But when the burner is off and water is still circulating through the system, the water temperature will cool down. When the temperature reaches the low limit, your aquastat will switch the burner back on. 

Without an aquastat your boiler or hot water heater would overheat or not heat enough to meet your home’s needs.

What Causes an Aquastat to Malfunction?

  • Sensor issues: The temperature sensing element (bulb) on the aquastat may be faulty or dirty, which can lead to incorrect temperature readings and improper functioning.
  • Faulty wiring: If the wiring connections to the aquastat are loose or damaged.
  • Corrosion: Over time, corrosion can build up on the contacts or terminals of the aquastat.

How Do I Know if It’s Broken? How Will it Affect My System?

The two most common ways you might be alerted that your aquastat is not working as intended are: Your burner on the boiler or hot water heater continues to run but it is unable to satisfy the thermostat or the relief valve has discharged water.

Not only might this cause you to be uncomfortable with the temperature in your home, but your equipment might be overworking itself, causing unnecessary stress on the system and wasting energy. If you do not have a bucket under the relief valve to catch any discharge you may also cause damage to surrounding items in your home like carpet flooring or other objects stored near the heating equipment.