SupplyHouse Blog Banner 2

The Power of Aquastats: Controlling Boiler Temperatures


BoilerWhat’s that random rectangular product mounted to the side of your boiler? What does it do? All of these are valid questions homeowners have probably thought and asked their installer.


A temperature control—also known as an “aquastat,” due to Honeywell’s catchy branding name-senses and adjusts water temperature, as well as operates a circulator pump. Makes sense, right? It’s in the name, after all. They are made up of three components: a switch, a temperature-sensing element (called a “bulb”, too), and a capillary tube that connects the two. The bulb contains a temperature-sensitive liquid called “fill.” This allows the product to not only gauge the how hot the water is getting, but also tell the pressure switch if it needs to open or close in order to turn off or on the boiler. It will shut off once it reaches the set maximum temperature.


Aquastats are safety features and dependent on the type of system they’re connected to. A few common applications include hydronic heating systems (such as boilers or storage tanks) and other systems where water temperature control is required. Several manufacturers we carry for this type of product is Honeywell, White Rodgers, and Johnson Controls.


Why is it so difficult to search for one I need? It sounds simple. Well, the term “aquastat” will most likely pull up dozens of products, and that’s because there are several types based on their functionality. While dual function aquastats are the products I’m typically asked about in Product Support, there are ones considered single function (meaning there’s a sensor bulb and capillary tube drive a single pole/single throw electrical switch). Let’s go through the basics of single function types:
Honeywell Aquastat

  • High Limit: This refers to the maximum water temperature, and this function really demonstrates the aquastat’s purpose as a safety mechanism. It specifies the cut-off temperature for the boiler’s burner; the contact breaks when the water temperature rises to the chosen setpoint on the dial.
  • Low Limit: Although this is less common outside of residential boiler systems, there is an aquastat to control the minimum water temperature. Generally, such a function would be utilized once it is no longer the heating season. This prevents your water from getting too cold. (For those living in places that reach extremely cold temperatures, this is most likely the main—or one—of the functions for your home’s aquastat.)



Yet it is possible your aquastat is dual function, meaning it is a high/low limit aquastat! From my experience with sourcing replacement parts as well as speaking to homeowners and installers, I’d say this is the most popular type. (And it doesn’t hurt to cover both bases with having a maximum and minimum temperature controller.)


Should you need to replace your temperature control/aquastat, feel free to call in. Our staff can find an updated part number if you’re having trouble locating it. Sometimes, we can even look up this replacement part for a boiler on the unit’s actual parts list. Contact us with your boiler’s brand, model, and serial numbers; we’ll do our best to assist you!


Posted in All Posts, Products | Tagged , , ,

Leave a Reply

Your email address will not be published.


Join Our Mailing List - Sign up and receive special offers and coupons