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.