What’s an Aquastat?

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An aquastat regulates the temperature of fluid in hydronic heating systems, which circulate heated fluid through pipes to maintain a desired temperature. Thermostats monitor air temperature, while the aquastat monitors fluid temperature and activates the boiler or circulator as needed. An aquastat relay can recirculate hot fluid without heating it for greater efficiency. A modulating water switch adjusts fluid temperature based on outside temperature for even greater efficiency.

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 the heated fluid through pipes or tubes, typically located under the floor. This circulating fluid must be kept within a particular temperature range to prevent the environment from becoming too cold or too hot. Its temperature is regulated by a thermostat which signals to the system when the water needs to be heated or circulated.

Also known as radiant heating, hydronic heating systems circulate heated fluid from a boiler through pipes or tubes throughout a building. As this heated fluid circulates, heat radiates from the pipes or tubes into the surrounding space. This raises the air temperature to the desired level. The fluid then travels back through the system to the boiler. When it returns to this starting point, it is fresher than when it initially set off.

One or more thermostats are used to monitor the air temperature in the building to be heated. When the air temperature drops below the desired level set on a thermostat, the thermostat signals to the system that more heat is needed in a particular location. The aquastat monitors the fluid temperature 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.

Working fluid temperature monitoring 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 the desired range, too high or too low. In this case the boiler is activated to keep the temperature of the working fluid within the desired limits.

This device can activate two different functions. It can activate the boiler to heat the working fluid or it can activate 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 recirculated without being heated first. A combination of controls, known as an aquastat relay, can perform the heating and circulation functions independently. This device consists of an aquastat, relay and transformer all contained within a common housing. The sensor part of the aquastat is located outside the housing. By recirculating sufficiently hot return fluid without heating it, the system can achieve greater 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 water switch, senses the temperature outside and adjusts the working fluid temperature accordingly for more efficient operation. This device saves up to 10% on fuel costs compared to a normal device.

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