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All KN-Series boilers are available with the NEW Virtuoso₂ high turndown system. Working off the legendary Tru-Flow air/fuel coupling system; the engineers at ATH have developed an air shutter-driven system expanding the KN’s turndown ratio to 10:1. The uniquely designed air shutter system is positioned between the venturi and the blower inlet. The air shutter opens and closes electronically depending on the firing rate. The air shutter and blower design protect against intake and flue pressure fluctuations by maintaining blower RPM; providing complete combustion stability across the firing rate without sacrificing performance and reliability at any turndown ratio.


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ATH KN Series KN2

Condensing, High Efficiency: 200,000 BTUs

The Energy Star-rated KN2 cast iron boiler is ideally suited for residential and light commercial boiler applications, including new construction and retrofits. With a maximum output of 200,000 BTUs and capable of modulating down to a 40,000-BTU minimum, the compact footprint of the KN2 makes this cast iron condensing boiler the ideal choice to for hydronic space heat, radiant heat and snow melting applications.

And don’t let its size fool you, the KN2 is built with the same quality, durability and levels of efficiency as larger KN Series units. Constructed with the signature KN Series cast iron heat exchanger, the KN2 offers unrivaled heat retention, resulting in lower operation costs.


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Condensing, High Efficiency: 400,000 BTUs

Featuring large Delta Ts and fully modulating 400,000 BTUs, the KN4 cast iron boiler is the ideal condensing boiler unit to address the challenges of the growing radiant heat and snow melt markets, as well as other new construction and retrofit boiler applications.

KN4’s small vent size also allows the utilization of existing vent work as a chase for new equipment in retrofit applications, and its small footprint makes incorporating its power into new constructions simple as ever. With 92.7% thermal efficiency at full input and 99% combustion efficiency at minimum input (5:1 turndown), the KN4 is a high efficiency condensing gas-fired boiler option. And the durable cast iron engineering behind the KN Series, when coupled with HeatNet 3.0 boiler control technology, makes for optimal fuel utilization and increased boiler performance.


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ATH KN Series KN6

Condensing, High Efficiency: 600,000 BTUs

With 600,000 BTUs in a compact footprint, the KN6 features a fully modulating, cast iron design and boasts increased condensing efficiency when compared to common condensing, gas-fired boilers. Built strong to handle the increasing demands of today’s commercial boiler applications, the KN6 is an ideal choice for boiler operations within corporate office facilities and educational centers.

Designed for increased boiler performance and longevity, the KN6 was engineered with an ultra-efficient Tru-Flow symmetric air/fuel coupling design that burns clean while ensuring proper combustion at all firing rates, as well as 5:1 modulation for maximum efficiencies at all levels of heat demand.


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ATH KN Series KN10

Condensing, High Efficiency: 1,000,000 BTUs

Producing 1,000,000 BTUs, the KN10 is engineered to meet the low fuel use, low installation cost and small footprint needs of today’s commercial boiler market. Like all KN Series boilers, the KN10 combines the condition-tolerant and heat retaining characteristics of cast iron with the fuel savings of full-modulating condensing boiler performance.

As one of the most flexible boilers on the market, the KN10 boasts 5:1 continuous modulation and is capable of handling variable volume flows, resulting in one of the most innovative and efficient boilers on the market.


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Condensing, High Efficiency: 1,600,000 BTUs

The latest addition to the KN Series line of condensing boilers, the KN16 produces 1,600,000 BTUs, while maintaining the features that have made KN Series boilers the most recognized line of condensing, gas-fired boilers in the commercial boiler industry long lasting durability and ultra-high condensing efficiencies ‘ all in a compact footprint.

Capable of rising to the challenge of any commercial boiler application, the KN16 was engineered on the solid foundation of a high-efficiency, condensing boiler with a cast iron heat exchanger. Paired with the state-of-the-art HeatNet 3.0 boiler control platform, Tru-Flow “whirlwind” blower and gas valve package that replace traditional VFDs, the KN16 delivers pinpoint modulation, ensuring maximum performance at any firing rate.


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Condensing, High Efficiency: 2,000,000 BTUs

With an output of 2,000,000 BTUs, the KN20 is built to withstand any harsh environmental conditions that come its way. Engineered with the signature KN Series cast iron heat exchanger and full-modulation condensing boiler performance, the KN20’s durability and reliability is second to none.

Regardless of the boiler application size, the KN20 enables low installation cost alongside optimal fuel utilization in a small footprint. And when integrated with HeatNet 3.0 boiler control software, the KN20 can be used in conjunction with other KN Series boilers in a master/member network of up to 16 units and other units using HeatNet 3.0 software, creating a custom boiler network to meet the heating demands of the largest commercial boiler operations.


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Condensing, High Efficiency: 2,600,000 BTUs

The KN26 keeps the KN Series’ tradition of excellence alive with its cast iron heat exchanger and full-modulation capabilities. Producing 2,600,000 BTUs within such a reduced footprint makes the KN26 a versatile condensing gas fired boiler sized to meet the most demanding operations within the commercial boiler industry.

From single boiler applications to multiple boiler master/member networks controlled by HeatNet 3.0 software, the KN26 is built for the long haul. Developed for maximum efficiency, the KN26’s cast iron heat exchanger retains heat longer, allowing for shorter cycling and, most importantly, lower costs of operation.


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ATH KN Series KN30

Condensing, High Efficiency: 3,000,000 BTUs

At 3,000,000 BTUs, the KN30 is the largest boiler in the KN Series line. Without sacrificing reliability for size, the KN30 is designed to conquer the largest, most challenging demands of the modern commercial boiler market, using a historical favorite, cast iron.

Condition-tolerant and heat retentive, cast iron heat exchangers make the KN30 stronger and allow for short cycling ‘ reducing boiler installation and operation costs over its lifetime. And when paired with full-modulation capabilities and HeatNet 3.0 boiler control software, the KN30’s high-efficiency condensing boiler technology continues to exceed industry standards and expectations.


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In today’s commercial boiler market, efficiency, durability and the environment are everything. The KN-Series has stood the test of time and has outperformed the competition in all facets.

A staple in the condensing boiler market for over 15 years and over 26,000 installations the KN-Series uses the durability and heat retention capabilities of cast-iron heat exchangers with state-of-the-art air/fuel delivery systems and the HeatNet control platform to outpace the competition keeping KN at the head of the class.

Rugged and long lasting, cast-iron is the ideal material for condensing boiler systems. KNSeries boilers are built to last the long haul and backed by the industry’s best heat exchanger warranty. All heat exchangers carry a 25-Year non-prorated warranty* covering materials and workmanship against failure due to condensate corrosion and thermal shock.
*See KN-Series written Warranty for specific requirements.


  • Industry’s best 25-year heat exchanger warranty
  • KN-Series boilers have an installation base of over 26,000 units since its introduction.
  • KN-Series boilers have not had one documented failure associated with corrosion.
  • Cast-Iron boilers are an acceptable material for use in condensing boiler applications by ASHRAE.

  • Cast-iron has been the material of choice in the boiler industry for over 150 years.