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.