What is an Aquastat?


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 the desired temperature in an enclosed space by circulating the heated fluid through tubes or pipes, usually located below the floor. This circulating fluid must be kept within a certain temperature range to prevent 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 radiant heating, hydronic heating systems circulate the 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 raises the air temperature to a desired level. The fluid then circulates back through the system to the boiler. When he returns to this starting point, it is colder than when he initially set out.

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 at 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 at the desired location.

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

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

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

A typical aquastat will keep 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 weather. A special type of device, known as a modulating aquastat, senses the outdoor temperature and adjusts the working fluid temperature accordingly for more efficient operation. This device can save up to 10% on fuel costs compared to a regular device.

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