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What Is an Aquastat?

By Kirsten C. Tynan
Updated May 17, 2024
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An aquastat is a device used in hydronic heating systems to control the temperature of the fluid flowing through the system. Hydronic heating systems maintain a desired temperature in an enclosed space by circulating heated fluid through tubes or pipes, typically located beneath the floor. This circulating fluid must be maintained within a particular temperature range in order to keep the space from getting too cold or too hot. Its temperature is regulated by an aquastat that signals the system when the water needs to be heated or circulated.

Also known as a radiant heating, hydronic heating systems circulate fluid heated by a boiler through tubes or pipes in a building. As this heated fluid circulates, heat radiates from the tubes or pipes into the surrounding space. This increases the temperature of the air to a desired level. The fluid then circulates back through the system to the boiler. When it arrives back at this starting point, it is cooler than it was when it initially left.

One or more thermostats are used to monitor the temperature of the air in the building being heated. When the air temperature cools below the desired level as set on a thermostat, the thermostat signals the system that more heat is needed in a particular location. The aquastat monitors the temperature of the fluid in the system and signals the system when the working fluid needs to be heated or circulated to raise the air temperature in the desired location.

Monitoring the temperature of the working fluid involves a sensor bulb that is part of the device. This bulb is placed in a well in the boiler where it can sense the temperature of the fluid. At some point, the sensor may detect that the fluid temperature is outside a desired range, either too high or too low. When this is the case, the boiler is triggered to maintain the temperature of the working fluid within the desired limits.

This device may trigger two different functions. It may trigger the boiler to heat the working fluid, or it may trigger the circulator in the system to circulate fluid through the system. Generally, both the boiler and the circulator are triggered when heat is needed.

If the water is still hot enough when it returns to the boiler, it can be circulated again without first being reheated. A combination of controls, known as an aquastat relay, can accomplish the heating and circulating functions independently. This device consists of an aquastat, a relay, and a transformer, all contained within a common housing. The sensor portion of the aquastat is located outside the housing. By circulating sufficiently hot returned fluid again without reheating it, the system can achieve higher fuel efficiency and cost savings.

A typical aquastat will maintain the working fluid at a single constant temperature. Sometimes, however, it is more efficient to vary the temperature of the fluid depending on the current climate. In mild weather, the working fluid does not need to be heated as much as in very cold conditions. A special type of device, known as a modulating aquastat, senses the temperature outdoors and adjusts the working fluid temperature accordingly for more efficient operation. This device can save up to 10% off fuel costs as compared to a regular device.

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