High Efficiency Gas Boilers

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Get more heat for less energy with a high efficiency gas boiler. We carry a wide selection of gas boilers from the top manufacturers in the industry. Shop now and find your perfect high efficiency gas boiler.

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Greater than 383K BTU

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Greater than 383K BTU

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Noritz (4)

Weil-McLain (49)

Triangle Tube (18)

Bosch Thermotechnology (8)

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Williamson-Thermoflo (2)

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DOE Heating Capacity (BTU)

Also referred to as Gross Output, this is the output of a boiler after accounting for combustion efficiency. We recommend sizing hot water boilers based on the DOE Heating Capacity.

57 - 30056 (2)

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1890057 - 1920056 (2)

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Net I=B=R Capacity (BTU)

I=B=R, also referred to as Net Output or AHRI Capacity, is the rated heat output of a boiler after accounting for combustion efficiency, piping loss, and an extra pickup factor. We recommend sizing steam boilers based on the I=B=R rating.

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930050 - 960049 (1)

1230050 - 1260049 (3)

1650050 - 1680049 (3)

2100050 - 2130049 (1)

2490050+ (1)

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Maximum Input Capacity (BTU)

The Maximum Input Capacity describes how quickly the appliance will use fuel when delivering it's maximum capacity. The units may change depending on the fuel being used.

50000 - 89999 (19)

90000 - 129999 (33)

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DHW Production

DHW (Domestic Hot Water) is potable water for uses other than space heat. Some boilers are able to provide DHW "out of the box", while others can do so with the purchase of optional accessories.

Available (2)

No (95)

Yes (31)

AFUE (%)

AFUE (Annual Fuel Utilization Efficiency) measures the efficiency of combustion equipment over the entire heating season, and is calculated by dividing the amount of energy that is actually heating your home by the total energy contained in the fuel used by the boiler/furnace. Learn More

91 (5)

92 (5)

93 (9)

94 (6)

95 (87)

96 (7)

Efficiency (%)

94 (1)

95 (2)

96 (5)

97 (1)

Fuel Type

To create heat, some form of fuel is required. Fuel Type specifies what the product uses as an input to create heat.

Liquid Propane (3)

Natural Gas (125)

Modulating/Staging

Stages are unique speeds or levels that equipment can operate at. More stages allow for better fine-tuned control of output, which improves comfort and efficiency. Modulating equipment can operate across a range of levels, giving the highest level of fine-tuned control. Learn More

Modulating (124)

Single Stage (4)

Ultra Low NOx Emissions

NOx is a byproduct of the combustion process, and it's output is regulated in some jurisdictions. Many regulators mirror SCAQMD rules, which regulate air quality for some parts of Southern California. Ultra Low NOx appliances emit less than 10-14 ng/J (nanogram per Joule) of NOx. Learn More

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Yes (109)

Learn More About Williamson-Thermoflo FS-120 COMBI

Williamson-Thermoflo FS-120 - 109K BTU - 92.2% AFUE - Combi Gas Boiler - Direct Vent

Model: FS-120 COMBI

Wall Mounted, Condensing Boiler with Modulating Gas Valve. Ships Packaged. Ratings: 120K BTU Gross Input, 109K BTU DOE Capacity, 94K BTU Net IBR Capacity. Includes Circulator.

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$1,999.00

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$2,439.00

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*This product qualifies for Free Shipping anywhere in the contiguous US. Accessories may ship separately. Items over 100lbs shipped via semi-truck, be sure to select "Lift Gate" to lower the product to the ground curbside. Remote area surcharges may apply, see our full shipping policy for more details.

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Save 2% instantly by using our convenient "e-check" option or by simply mailing us a paper check. Yes, your order will be delayed slightly (5 business days). But, if you can wait, the savings are significant.