Tuckers Air Conditioning, Heating & Plumbing Blog

Do Boilers Really Boil Water?

It may seem like an obvious thing, but you would probably be surprised at the answer to this question. Most boilers do not, in fact, boil water. Ironically, if your boiler is boiling water then you have a very big problem. There are some boilers, called “steam boilers,” that are specially designed to boil water and use the steam to provide heat. We’re going to focus on the much more common hydronic boiler, though. Let’s take a look at how a hydronic boiler works, and why you really don’t ever want it to actually boil water.

Boiler Construction

A hydronic boiler is constructed in a similar way to a gas furnace, with a couple of exceptions. There is a burner assembly, a heat exchanger, and often a pilot light. The difference is that in a boiler, the heat exchanger is a tube over the burner flames, through which water flows. As the water flows over the flames, it is heated to the proper temperature before moving on to heat the house. Because of the way the heat exchanger is constructed, and the method hydronic boilers use to heat homes, the water is not supposed to stay over the flame long enough to boil.


Though hydronic boiler systems are not designed to actually boil water, there is a condition where it happens. That condition is called “kettling” and is called such because of the deep rumbling noise that emanates from boilers that have it. Kettling is caused by hard water moving through the boiler over many years. Hard water is called that when it has high mineral content.

Over years of flowing through the boiler, the hard water deposits small amounts of minerals on the walls of the heat exchanger. Eventually, these deposits can become large enough to restrict or completely block the flow of water through the heat exchanger. When that happens, the trapped water begins to boil inside the heat exchanger. The evaporation of water into steam puts an enormous amount of pressure on the heat exchanger, causing it to rumble like an enormous kettle. This can be quite dangerous, both to the boiler and any nearby people. It is highly recommended that you call a professional if you notice your boiler kettling.

If you’d like to know more, call Tuckers Air Conditioning, Heating & Plumbing. We professionally install boilers throughout Bethesda.