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boiler circulator pump

Aquastat is the gray box on the left while the circulator is the black pump on the right.

Aquastat Temperature Control - The boiler aquastat controls the temperature of the water inside the boiler jacket. The boiler aquastat is a device to a boiler like a thermostat is to an air conditioner, heat pump, or gas/oil furnace. A boiler aquastat controls the temperature inside the boiler just as the thermostat controls the temperature for an air conditioner or heat pump. The boiler aquastat will turn the burners on and off and also controls the circulator pump on a hot water boiler system.

Aquastat Temperature Control | Boiler Components - How it Works

The house thermostat on the wall that is connected to the boiler is connected to the boiler aquastat. When the thermostat calls for heat it sends a signal to the boiler aquastat to energize the circulator pump. The circulator pumps kicks on and pulls hot water through the hot water loop to provide the house or dwelling with heat. When the temperature inside the boiler jacket falls below the setpoint of the boiler aquastat the boiler aquastat engages the burners of the boiler and the boiler fires the burners to heat the water inside the boiler jacket.
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Aquastat Temperature Control | Limit Control

  1. High Limit - The aquastat will limit the temperature in the boiler so the boiler does not overheat. Most aquastats have a sensor that is inserted in an immersion well. The immersion well is located inside the boiler where the water circulates. The sensor sense the temperature. Based on the aquastat setting, this will limit the high temperature in the boiler to prevent overheating. The cutoff temperature for most hot water boilers is 180° F. There is a differential setting to make sure the boiler does not short cycle or run unnecessarily.  Additionally, if your boiler is equipped with an outdoor temperature rest the 180° F. set point will change. This set point change will be based on the outside ambient air temperature. The cutout temperature is when the boiler reaches the high setpoint and turns off the burners. The circulator pump may still be running with the burners off. The cutout temperature should never exceed 200° F. This prevents making steam as we are discussing hot water boilers here. Most Aquastats have a fixed differential setting of 10° F.
  2. Low Limit - The low limit has two purposes. If you use the boiler for domestic hot water you will have a low limit setting on your aquastat. This will keep the water in the boiler at a minimum temperature. This ensures hot water is available when you need it for a shower or washing the dishes. It will also prevent the boiler water from getting too cold. This low setting also makes domestic water heating a priority (on some aquastats). Always read the owner’s manual for proper settings.
  3. Differential Setting - this applies to the Low Limit setting. To better understand these settings read the following article by clicking here.

Related Link - See our boiler sequence of operation article and learn more about aquastats.

Warning About Aquastat Settings

Aquastat settings can be difficult for people to understand. Always make sure you understand what you are doing before making any adjustments. Or consult a professional.

Aquastat Temperature Control | Boiler Components - Conclusion

When the aquastat is tied in with an outdoor air temperature reset control the set point temperature of the boiler will change. Change according to what the temperature is outside. In warmer temperatures, the set point is lowered on a linear scale as the temperature outdoors rises. Using an outdoor reset control in the manner can save money on the heating bill as it requires less work from the boiler when the weather or temperatures outdoors are higher. Why maintain 180° F. temperature for the boiler set point when the outside air temperature is 60° F? The boiler can efficiently and adequately heat the house with a set point of 140º F. using less energy. Less energy to maintain a lower set point?

If you get the settings wrong on the aquastat you can and will cause problems with heating. It can also cost you more for heating by making your gas bill higher. Therefore, it is important to know what you are doing when finalizing the settings. When in doubt, consult a professional.

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Aquastat Temperature Control | Boiler Components

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