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How do combination gas boilers work?

Updated February 21, 2017

A combination gas boiler is a relatively efficient type of condensing boiler or water heater. While "boiler" typically refers to a water heating device in the U.K,. known as a water heater in the U.S., and to a device used for home heating in the U.S. and Canada, the combination gas boiler may serve either purpose. Primarily, though, the term is used to refer to boilers in the British sense of the word.

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Condensing Technology

The combination gas boiler is a type of condensing boiler. This means that instead of allowing some of the water vapour produced through combustion to escape, it makes full use of the vapour to use as heat, letting none escape through the flue. As a result, you can heat more water with a smaller amount of input energy. To extract the maximum heat possible from combustion, a combination boiler uses a dual heat exchanger, which is larger than that found on a normal unit.

No Tank

Combination boilers are often known as "tankless" boilers, as they do not have a tank in which to store hot water, as usual boilers would require. As a result, you can immediately use hot water from a combination boiler. In a traditional boiler system, the heated water sits in copper cylinders once it is heated. By contrast, the combination boiler draws cold water and heats it directly as you turn on the hot water faucet.


Because the combination gas heater doesn't require a tank, it takes up less space than a traditional boiler system, a boon for smaller homes or apartments, where space is limited. In addition, the combination gas boiler requires fewer components, making it relatively inexpensive to maintain in good operating condition. Finally, the energy savings of a combination gas boiler mean that it will incur lower fuel costs over time, for the same amount of heating. In addition to the savings, a combination heater provides the convenience of hot water immediately available, on demand.


The principal drawback of the combination gas boiler system is the initial expense of installing a new system. Because the combination gas boiler can require significant changes to your gas, water and venting systems, switching from regular boilers to tankless combination boilers usually costs more than installing a new boiler with a tank. Nonetheless, the savings of using an energy-efficient combination boiler over time may outweigh the cost of the initial investment. To determine the financial outlook for your home's boiler system, have a contractor provide you with estimates for either boiler type. In addition to the initial expense, another drawback of combination boilers is the chance of slightly lower water pressure. As water comes directly from the main water supply and does not sit in a holding tank, it may not arrive through your taps with the same force as a standard boiler system.

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About the Author

Danielle Hill has been writing, editing and translating since 2005. She has contributed to "Globe Pequot" Barcelona travel guide, "Gulfshore Business Magazine," "Connecting Lines: New Poetry from Mexico" and "The Barcelona Review." She has trained in neuro-linguistic programming and holds a Bachelor of Arts in comparative literature and literary translation from Brown University.

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