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Aquastats are used in hydronic heating systems to control water temperature. Hyrdonic systems use water as the transfer medium for both heating and cooling and are primarily used in residential applications. The aquastat has a sensing bulb that's placed in a well in the side or front of the boiler. The bulb senses the temperature of the water inside the boiler and triggers the boiler to maintain the temperature between a low and high end set by the user. In a water heating system, your decision to turn your thermostat up--or it merely being triggered at your preset designation--makes hot water leave the boiler to circulate through the pipes. Naturally, some heat is lost in the process, so when the return water is sensed at its now-cooler temperature, the bulb on the aquastat calls the boiler into action to reheat the water for when the thermostat calls on it again.

Types Of Aquastats

An aquastat may be designed to control the temperature limit or to switch on the circulator. If used as a limiter, the aquastat closes at the low extreme and opens at the high end. But when used to operate the circulator, it closes on both temperature extremes. The former is referred to as direct action. For energy purposes, the high setting should be as low as possible to ensure proper heating. For a gravity hot-water system, a setting of 170 degrees is recommended; for a forced hot-water system, the recommendation is 200 degrees. Operating as the circulator, the aquastat is referred to as reverse action and should be mounted on the largest riser from the boiler. The temperature settings in either setup are used to prevent the boiler from firing too often.

Note also that the method for attaching aquastats can be different. Strap-on models are universal and can be used for circulators, hydro-air fan controlling or hot water makers. Clip-ons are normally used in hydro-air heating air handlers, while well aquastats are for boilers and, occasionally, hot-water makers.

Replacing Or Repairing An Aquastat

Before replacing, it's important to test the aquastat. Remove the cover from the device, usually mounted on the outside of the furnace. Turn the main wall thermostat to its highest setting. This should trigger the furnace. Now drop the high temperature setting to less than 100 degrees. The furnace should go off if the aquastat is functioning properly. Return the aquastat to its original high point. If the burner doesn't respond to the setting drop, or go back on within 10 minutes of returning the aquastat to its high point, then the device needs replacing.

If the aquastat needs to be replaced, before doing anything else the circuit breaker box and emergency cutoff should be used to cut the power. Next, disconnect the wires leading to the aquastat, being sure to label each wire with its respective terminal. Take out the screws holding the aquastat in place, as well as remove it from the furnace. Take the whole part with you to a dealer, and buy a matching replacement. Then simply remount the new aquastat, using your labeled wires to ensure proper connection.

Who Makes Replacement Aquastats?

Honeywell is the most prominent Aquastat manufacturers, though other prominent HVAC companies such as White-Rodgers and Carrier also make models.

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