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How to Replace an Aquastat

Claire Gillespie

An aquastat has an important function, keeping the water temperature in a hydronic heating system under control. If your aquastat stops working, it's easy to replace yourself.

How to Replace an Aquastat

An aquastat is a switch located on the side of a hydronic heating system where water is heated in a boiler or a water heater. It controls water temperature, continually monitoring the temperature and reacting if it goes too high or too low. An aquastat is similar to a thermostat, in that it keeps hydronic heating systems safe. While a thermostat controls air, an aquastat monitors and controls water temperature. It keeps the water hot but below boiling temperature. Both thermostats and aquastats are used in hydronic heating systems. As well as a high limit temperature, aquastats have a lower limit temperature, which allows them to turn the boiler on again if the water temperature goes below this point. This ensures the water temperature is maintained at one level.

If an aquastat burns out or stops working it might need to be replaced, but in most cases, this is a simple task that can be carried out in about 30 minutes. These steps should work for all brands of aquastats but refer to the manufacturer's directions provided with your switch to ensure you follow the correct procedure.

    Test for Power

  1. Switch off the breaker switch for the heating system and test for power with a noncontact voltage tester before replacing the aquastat.

  2. Unscrew the Aquastat Switch

  3. Remove the screws on the old aquastat switch and remove the switch from the boiler unit. Set the screws to one side and leave the wires in place.

  4. Move the Wires

  5. Take the new aquastat out of its packaging. Disconnect the wires and move them to the new switch, one at a time. This ensures you keep track of where each wire should go. If you put a wire in the wrong place, the switch won't work.

  6. Attach the New Aquastat

  7. Attach the new aquastat switch to the boiler unit with screws. Turn the breaker switch for the boiler system back on.

  8. Tip

    Before replacing your aquastat, check if the solder connections on the back of the switch are loose by dismantling the switch and unhooking all the wires. To get a replacement aquastat, visit the website for the manufacturer of your old switch. If replacing the aquastat doesn't solve the problem, seek advice from a professional heating specialist.


    To avoid electric shock when replacing the aquastat, always turn the breaker switch off to the boiler at the breaker box.