Hot Water Boilers

Select Product Type

A hot water boiler, also called a hydronic boiler, can provide reliable and affordable heat throughout your home or office. View our hydronic boiler offerings below and read our boiler buying guide for more information.

Shop by Brand

Shop by Fuel Type

Shop by Feature

Shop by Vent Type

Shop by Brand

Shop by Fuel Type

Shop by Feature

Shop by Vent Type

Filter By

Best Selling▼

Best Selling
Price: Low to High
Price: High to Low
Name: A-Z
Name: Z-A
Recommends Only
Closeouts Only
In-stock Only
Next Day Air Only
Availability

Filters

Brand

Noritz (4)

Weil-McLain (201)

Triangle Tube (18)

Bosch Thermotechnology (8)

Crown Boiler Co. (85)

Buderus (27)

Burnham (103)

Peerless (7)

ProSelect Force Boilers (15)

Rinnai (10)

Williamson-Thermoflo (4)

+ Show More

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.

50 - 50049 (46)

50050 - 100049 (141)

100050 - 150049 (128)

150050 - 200049 (76)

200050 - 250049 (33)

250050 - 300049 (10)

300050 - 350049 (7)

350050 - 400049 (1)

400050 - 450049 (2)

450050 - 500049 (1)

500050 - 550049 (2)

600050 - 650049 (3)

650050 - 700049 (1)

700050 - 750049 (1)

750050 - 800049 (1)

800050 - 850049 (4)

900050 - 950049 (2)

950050 - 1000049 (1)

1000050 - 1050049 (1)

1050050 - 1100049 (1)

1150050 - 1200049 (1)

1200050 - 1250049 (2)

1250050 - 1300049 (2)

1350050 - 1400049 (1)

1450050 - 1500049 (1)

1550050 - 1600049 (1)

1600050 - 1650049 (1)

1650050 - 1700049 (3)

1700050 - 1750049 (1)

1800050 - 1850049 (1)

1900050 - 1950049 (1)

2000050 - 2050049 (1)

2100050 - 2150049 (1)

2450050+ (1)

+ Show More

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 - 40056 (14)

40057 - 80056 (76)

80057 - 120056 (116)

120057 - 160056 (82)

160057 - 200056 (73)

200057 - 240056 (44)

240057 - 280056 (18)

280057 - 320056 (10)

320057 - 360056 (2)

360057 - 400056 (6)

400057 - 440056 (1)

440057 - 480056 (1)

480057 - 520056 (1)

520057 - 560056 (1)

560057 - 600056 (1)

600057 - 640056 (1)

680057 - 720056 (1)

720057 - 760056 (3)

840057 - 880056 (1)

880057 - 920056 (1)

920057 - 960056 (3)

960057 - 1000056 (1)

1040057 - 1080056 (1)

1080057 - 1120056 (2)

1120057 - 1160056 (1)

1240057 - 1280056 (1)

1360057 - 1400056 (1)

1400057 - 1440056 (2)

1440057 - 1480056 (2)

1560057 - 1600056 (1)

1680057 - 1720056 (1)

1760057 - 1800056 (1)

1880057 - 1920056 (3)

1920057 - 1960056 (1)

2000057 - 2040056 (1)

2080057 - 2120056 (1)

2200057 - 2240056 (1)

2280057 - 2320056 (1)

2400057 - 2440056 (1)

2840057+ (1)

+ Show More

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.

30000 - 129999 (159)

130000 - 229999 (166)

230000 - 329999 (60)

330000 - 429999 (11)

430000 - 529999 (2)

530000 - 629999 (2)

630000 - 729999 (2)

730000 - 829999 (3)

830000 - 929999 (2)

930000 - 1029999 (3)

1030000 - 1129999 (2)

1130000 - 1229999 (2)

1230000 - 1329999 (2)

1430000 - 1529999 (4)

1530000 - 1629999 (1)

1630000 - 1729999 (1)

1730000 - 1829999 (1)

1930000 - 2029999 (4)

2030000 - 2129999 (1)

2130000 - 2229999 (1)

2330000 - 2429999 (1)

2430000 - 2529999 (2)

2530000 - 2629999 (1)

2730000 - 2829999 (1)

2830000 - 2929999 (1)

2930000+ (1)

+ Show More

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

81 - 82 (33)

83 - 84 (222)

85 - 86 (65)

87 - 88 (21)

91 - 92 (10)

93 - 94 (15)

95 - 96 (94)

+ Show More

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 or Direct Vent (3)

Chimney Vent (259)

Direct Vent (136)

Power Vent (82)

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 (102)

Natural Gas (320)

Oil (52)

Oil/Natural Gas (6)

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 (126)

Single Stage (343)

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.

(3)

Closeout

$1,999.00

Was

$2,439.00

SAVE $440.00

Only 1 In-Stock

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.

2% Check Discount

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.