What does an aquastat or aquastat relay do on a gas fired hot water boiler?

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Boiler Aquastat

The above image is of a simple gas fired boiler aquastat, what an aquastat does is read and controls the water temperature in a hot water boiler, It has a probe that sits in a well that sits in the top of the boiler water. When the aquastat is set at 180 degrees F it will stop the boiler from running when the water temperature reaches 180 degrees. In most cases the circulator or pump will continue to run, as long as there is a call for heat. This should not be set above 200 degrees F. The boiler will fire again once the water temperature drops about 20 degrees below the set temperature.

Most of the time, when an aquastat fails it will cause the boiler to get hotter then the set temperature rather then cause the boiler not to run, but sometimes it will cause a boiler not to fire.
First remove one wire from the aquastat. Then use a tester to see if you have continuity between the 2 terminals, if you do the aquastat is working, the problem is else ware. (Remember we are working on a cold boiler, if your boiler is hot, this does not apply)

Aquastat relay

This is an aquastat relay yours may be different please note: this control box has both high voltage 120 volts and low voltage 24 volts. Touching the wrong thing can hurt you and burn out the control, causing you additional problems, it only takes a split second to burn out this control, and they are not cheap to replace, in addition to this other components on the boiler can be damaged or destroyed. I don't want to scare you I just want you to know what you are getting in to, it is best to

Please note the aquastat is similar to the aquastat in the above image, turn the power to the boiler, remove 1 of the wires and do a continuity test on the 2 terminals if you have continuity the aquastat part of the control is working. Don't for get to put the wire back after the test. The relay part will be covered in the transformer relay page

If you do not have continuity between the 2 terminals you may have a bad aquastat, but have a pro check it out to be sure.

To get an accurate understanding to your problem, please use the Open Dialog Service.


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