Aquastats have numerous applications. If mounted on a boiler, it will control the temperature range that the boiler operates and the working temperature for the circulators. A thermostat transformer, relay, and circulator switch will sometimes be encased within the aquastat control on a boiler. Hydro-air systems use an aquastat to turn the fan on during heating mode. Some hot water heaters use an aquastat to regulate the water temperature.
Designs differ by application and manufacturer. Clip-on aquastats are common in air handlers for hydro-air heating. Well type aquastats are used on boilers and some hot water makers. Strap-on aquastats have universal application and can be used for circulators, hydro-air fan controlling, and hot water makers.
Most Aquastats use a bulb sensor and capillary tube. A seal copper bulb about the size of a pencil is attached to a diaphragm by a thin copper tube. The bulb is kept in contact with the heat source in order to function. As the bulb warms up, the gas inside expands through the tubing and into the diaphragm. Expansion of the diaphragm triggers a switch to control the intended device. The switch can be double acting. It can open a circuit to shut off the appliance (e.g. stop the heating action in a boiler or hot water maker) or close a circuit and turn an appliance on (turn the blower on for a fan coil heater). The switching contacts can be fixed at a predetermined temperature setting or adjustable, depending on the application. When handling the sensor bulb and capillary tubing caution must be taken not to kink or rupture the copper, or it will fail to operate. Some clip-on aquastats use a bi-metal strip and contacts that functions the same as a thermostat. They have a fixed setting, and are limited in application.