Do Boilers For Home Heating Actually Boil Water?

Homes in the United States are constantly undergoing construction with new and improved technologies for heating and cooling. But, there still remains a vast infrastructure of homes that were built using older tech. You may own such a home with a hot water boiler used for heating and wonder if the boiler actually (as the name implies) boils the water.

What's in a Name?

Contrary to the name, most boilers (especially those installed since the mid-20th century) do not boil water. Instead, they heat the water to just below the boiling point. From there, the hot water either heats air as part of a forced-air system, or it is pumped through the home through a distribution network. It is because of this that most boilers in the United States are referred to as furnaces, to make the distinction between a system that uses boiling water and one that doesn't.

So, Some Boilers Do Boil?

Earlier furnaces in colder climates, especially in the New England and Mid-Atlantic areas of the United States, used steam as a source of heat. Steam, of course, is only produced by boiling water. That means that there are some boilers that do actually boil water to generate steam for heating. According to Technical Heating in Long Island, NY, about 65% of 1 and 2 family homes in New York City have steam heat.

A Caveat

If you do have a hot water system that warms forced air (called a hydronic system), your system may sometimes boil water. It's not designed to, and this fault is called kettling. This is caused by sedimentary deposits left by hard water on the heat exchanger. Water flow is restricted by the deposits so it can no longer move freely and begins to overheat. The trapped water boils, forming steam, which puts pressure on the heat exchanger, causing it to rumble like a kettle. This is extremely dangerous and should be corrected immediately. If you notice your system making strange rumbling sounds, contact a plumber immediately.

Now, you know that in most of the United States a boiler is really just an old term for a water heater. However, if you live in the northeast, chances are your boiler really does boil and cause steam. If you're really in doubt, the next time you have your furnace serviced, talk to the technician. They will let you know the definitive answer about your heating system. You can also visit websites like