Natural Gas Hot Water Boilers

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Natural gas hot water boilers are one of the most popular types of boilers in the country. We carry a full selection of natural gas hot water boilers from the top brands in the industry. Find yours today.

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Weil-McLain (139)

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Crown Boiler Co. (47)

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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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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.

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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.

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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

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93 - 94 (14)

95 - 96 (92)

Efficiency (%)

94 (1)

95 (2)

96 (5)

97 (1)

Vent Type

Products that use combustion (burning) of fuel to create heat also create potentially dangerous gasses that need to be exhausted (AKA "Vented"). Depending on the product, venting can be done via a chimney, through a side wall, or use a second vent to provide fresh air to the appliance.

Chimney Vent (140)

Direct Vent (133)

Power Vent (47)

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 (123)

Single Stage (190)

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

No (200)

Yes (113)

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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Free Shipping & Lift Gate

*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.