Kettling And Hot Water Boilers — A Quick Guide

1 June 2022
 Categories: , Blog


"Kettle" might seem like an old-fashioned word for a teapot, but it describes a genuine condition that can occur with your boiler. When you boil water for tea, you can usually hear the water roiling in the pot before you hear the whistle. If your boiler makes a similar noise, you have a problem commonly known as "kettling." 

What Is Kettling?

Kettling typically refers to a boiling noise, although it won't always be the only sound you hear. You may also hear popping, snapping, or even banging sounds. You might expect that boiling water shouldn't be an uncommon occurrence in an appliance called a boiler, but a hot water boiler should never become warm enough to convert water into steam.

Instead, modern hot water boilers work by continuously circulating and heating water as it moves through your home. As a result, you shouldn't hear any boilings from the tank. If you can hear the water boiling off, your boiler is heating it beyond its design capacity. A kettling boiler will quickly increase the internal pressure in your heating system.

Why Should You Care?

Hot water boilers aren't just less powerful versions of steam boilers. A hot water boiler specifically takes advantage of the lower energy required to heat water to temperatures below boiling. This design, along with circulating pumps that move water throughout the system, makes hot water boilers a highly efficient option for heating many structures.

In addition to requiring more energy, converting water to steam also rapidly increases pressure inside the system. This excess pressure can damage your boiler or parts of your heat plumbing, although the temperature and pressure safety valve should prevent catastrophic failures. Still, operating a kettling boiler will reduce your heating efficiency and wastewater.

Why Does Kettling Occur?

There are essentially three reasons why a boiler will kettle:

  • Poor heat transfer
  • Inadequate water flow
  • High temperatures

Your boiler uses a thermostat to maintain the correct temperature at the heat exchanger. Meanwhile, the heat exchanger must be able to transfer heat energy to your household water supply. Water must also continuously move through the system to prevent the water near the burner from becoming too hot and boiling off.

In other words, kettling always indicates an underlying problem that requires repair. Common issues include sludge buildup, limescale, and faulty pumps, any of which can impact water flow or prevent heat transfer through the heat exchanger. Improper boiler thermostat settings or a damaged thermostat may also cause your boiler to overheat.

Whatever the case, you should always contact an HVAC professional if you notice your boiler kettling. Allowing your boiler to continue to kettle will waste money and may even cause components in your heating system to wear out more quickly or fail.

Reaching out to a professional for boiler repair can help.