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Combi Boiler Water Heaters & Space Heating

High efficiency space heating and domestic hot water combined into one appliance.

For over 80 years A. O. Smith has been a leader in the water heating industry, establishing a tradition of innovation that continues to this day. A. O. Smith provides energy-efficient solutions for water heating needs, including combination boiler water heaters that also provide space heating.

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Benefits of Combi Boiler Water Heaters

  • All-in-One Water & Space Heating

    Our high efficiency 95% AFUE combi boilers can replace both a water heater and space heater in your home to save you space, improve performance, and help you save money on operating costs.

  • Reliable Hot Water Supply

    The ProLine® XE Combi Boiler provides industry-leading continuous hot water delivery with its on-demand, high efficiency design.

  • Superior Space Heating Performance

    The ProLine® XE Combi Boiler offers up to 74% more heating capacity than other combination boilers.

  • Space Savings

    Easy to install and maintain, combi boilers feature a compact footprint and a wall-hung design that frees up space in your home.

  • Money Savings

    A. O. Smith combi boilers have a 95% AFUE (Annual Fuel Utilization Efficiency) and are ENERGY STAR® certified, which helps you save money on your energy bills.

  • Easy Installation

    Direct venting options up to 100 feet allow for flexibility and easy installation.

A Closer Look

Combi boiler water heater.

What is a Combination Boiler?

Combination, or Combi, boilers provide high efficiency space heating and domestic hot water (DHW) in a compact footprint that is easy to install and maintain. When a hot water tap is opened, incoming water flows through the heat exchanger. Heated boiler water, supplied from the fire tube heat exchanger, prioritizes heating the domestic water to the desired temperature. When space heating is needed, a diverter valve directs the heated boiler water into the external space heating loop.

Combi boilers combine space heating and hot water supply in one high efficiency appliance, providing a space-saving choice. The ProLine® XE Combi Boiler boasts industry-leading hot water delivery and up to 74% more space heating capacity than other combi boilers, ensuring that you are always comfortable. They are also easy to install and operate.

Although both can heat your home and provide hot water, there are key differences. Traditional boilers are designed mainly for space heating. Hot water from the boiler is circulated through radiators or a hydronic heating system to warm the ambient air. The traditional boiler water may also be used for domestic hot water by directing the water through a separate, integrated water heating unit before routing it to a fixture, such as a faucet or shower. Some traditional boilers are compatible with potable water, but others require additional water treatment.

Combi boilers are designed to accomplish both space and water heating with a single appliance. Like a tankless water heater, a combi boiler provides hot water on demand. It can also seamlessly switch between water heating and space heating to provide the best possible experience.

Combi boilers are highly efficient for both space and water heating, and they may contribute to lower energy costs in your home.

Yes! They are ENERGY STAR® certified and have a 95% AFUE rating. Because combi boilers provide both space and water heating, it is like having two high efficiency appliances in one. The ProLine® XE Combi Boiler also has a modulating combustion system with 10:1 turndown to ensure high efficiency operation even when heating demands are reduced.

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Contractor Resources

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    Installation Guide

    Download our combi boiler water heater installation instructions.

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