An aquastat is a device commonly used on hot-water space boilers and some steam boilers to control temperature limit or to operate the circular (hot-water heating system pump). It is similar in function to the pressure control on a steam boiler or the fan and limit control on a forced warm-air furnace. Basically, an aquastat is an automatic switching device consisting of a metal or liquid filled heat sensitive element designed to detect temperature drop or rise of the boiler water. Aquastats can be strapped to the hot water supply riser or mounted so that the heat sensitive element is immersed in a boiler well.

The type of aquastat used in a heating installation generally depends upon whether it is designed to control temperature limit or to switch on the circulator. If the former is the case, the aquastat will close on temperature drop and open on temperature rise. The aquastat will close on temperature rise if it is used to operate the circulator.

An example of a strap-on type aquastat is the ITT General Controls L-53 Strap-On Hot Water Control. This type of aquastat responds to water temperature changes as conducted through the supply riser pipe wall to the temperature responsive base of the device. The enclosed switching is supplied in direct action, reverse action, and double action models. Consequently, these strap-on type aquastats can be divided into three basic types depending upon their operating principles: direct action types, reverse action types, and double action types.

A direct action type aquastat used as a high limit control must always be located on the supply riser where it will be subjected to the maximum boiler water temperature. Its location on the riser will therefore have to be as close to the boiler as possible, but ahead of any line shutoff valves.

Direct action type aquastats will open the circuit on temperature rise. Setting the adjustable scale pointer to a position on the scale will result in breaking the circuit. The "cut-out" setting when used as a high limit control should be as low as possible and still insure proper heating in cold weather. An initial "cut-out" setting of 170 degrees Fahrenheit is recommended for a gravity hot-water heating system. An initial setting of 200 degrees Fahrenheit is suggested for a forced hot-water (hydronic) heating system. The mechanical differential of the control is nonadjustable and approximately 15 degrees Fahrenheit.

A reverse action type aquastat should be mounted ahead of any valves or traps on the return line when it is used on a unit heater installation. It should be mounted on the largest riser from the boiler if it is use to prevent circulator operation when boiler water temperature is low. A reverse action type aquastat closes the circuit on temperature rise.

A double action type aquastat is used to start circulator operation with a single switch actuation and to function as an operating control to maintain boiler water temperature. This type of aquastat should be mounted on the largest riser ahead of any valve, but at a point where it will be subjected to maximum boiler water temperatures.

Another example of a strap-on aquastat is the Honeywell LA409A aquastat. This is a nonimmersion, surface mounted aquastat which functions as a high limit or safety control device in hot-water and steam heating systems, and which requires no tapping of the boiler or draining of the system. This aquastat is also available in a reverse action model to operate unit heater fans.

A strap-on aquastat can be mounted in any position. When mounting these aquastats, make absolutely certain that the pipe surface is clean and free of rust and corrosion. All rough and high spots should be filed smooth. Nothing should be allowed to interfere with the operation of the temperature responsive base of the control. The Honeywell L6081A aquastat is used to control boiler water temperature in gas- and oil-fired hydronic heating systems. It has an immersion type liquid filled heat-sensing element, which actuates two snap switches. One switch operates as a high limit control, and other as a low limit and/or circulator control.