Williamson-Thermoflo builds dependable cast iron boilers for both gas and oil applications, with a strong presence in the Northeast market. Their residential lineup covers hot water, steam, and oil-fired models that are common in Long Island homes. We install and service the full Williamson line across Nassau and Suffolk County.
Williamson-Thermoflo is one of those brands that may not have the name recognition of Weil-McLain or Burnham, but their boilers are well represented in Long Island basements. The company manufactures a wide range of cast iron boilers for gas and oil applications, and their products have earned a solid reputation among Northeast heating contractors for reliable performance and straightforward serviceability.
What makes Williamson practical for the Long Island market is the breadth of their lineup. They build gas hot water boilers (GWA and GWI series), gas steam boilers (GSA series), oil hot water boilers with and without tankless coils (OWB and OWT series), and oil steam boilers (OSB series). Whether your home runs on gas or oil, whether you have hot water baseboards or steam radiators, Williamson likely makes a direct replacement for your current system.
Williamson boilers are factory assembled and factory tested, which simplifies installation and reduces the chance of issues on startup day. The cast iron sections use steel push nipples, and the GWI induced draft models offer the flexibility of sidewall or chimney venting. For homeowners looking for a straightforward, dependable cast iron replacement at a competitive price point, Williamson is a strong option. Schedule a free estimate and we can show you how it compares to other brands on our brand comparison page.
GWA atmospheric and GWI induced draft hot water models from 38 to 233 MBH. GSA steam models from 75 to 300 MBH. Covers small condos to large homes.
OWB and OWT cast iron oil boilers from 98 to 196 MBH with up to 87% AFUE. OWT includes tankless coil for domestic hot water. Bioheat (B20 and B100) compatible.
The induced draft GWI series allows sidewall or chimney venting. Ideal when the chimney needs work or for installations where chimney access is limited.
All Williamson residential boilers use cast iron sections with built-in air separators and insulated steel jackets. Proven construction for Northeast cold.
One-piece access door, swing-away burner door on oil models, top-access cleanout, and plug-in wiring harness. Controls are accessible and color-coded.
The average Williamson boiler installation on Long Island falls between $7,500 and $10,500 for a typical residential replacement. Gas models (GWA and GWI) tend to be on the lower end, while oil installations may cost more depending on burner setup, oil line routing, and chimney or venting requirements.
One advantage of Williamson is competitive equipment pricing. Their cast iron boilers deliver dependable performance at a lower cost than premium brands like Buderus or integrated systems like System 2000. For homeowners who want a reliable, no-frills cast iron boiler that will run for 15 to 25 years, Williamson is hard to beat on value.
GWI models with qualifying AFUE ratings may be eligible for National Grid rebates. Oil models meeting DOE standards can qualify for federal tax credits. We also offer flexible financing through Wisetack. For a full breakdown by system type, visit our cost guide.
Many Long Island homes have a Williamson oil boiler that has been running for 20 years or more. When the time comes for replacement, the most straightforward path is a direct Williamson-to-Williamson swap using the current OWB or OWT series. The newer Series 3 models offer improved efficiency, updated controls, and are compatible with bioheat blends up to B100.
If your Williamson is an oil-fired model and natural gas is now available on your street, replacement time is an ideal opportunity for an oil to gas conversion. You can move to a high-efficiency condensing gas unit from brands like Navien or Bosch and potentially cut your heating costs by 30% or more.
For homeowners who want to stay with cast iron gas, the Williamson GWA and GWI series are dependable replacements. The GWI models are particularly useful when the chimney is in poor condition, because the induced draft design allows sidewall venting through PVC pipe instead of relying on the existing chimney. We can evaluate all these options during a free in-home estimate.
From the first phone call to your first warm morning, here is what to expect when you replace your boiler with a Williamson.
We evaluate your current system, measure your heating load, and recommend the right Williamson model. We explain all rebates and financing options before you decide.
Once approved, we schedule at your convenience. We stock popular Williamson models for fast turnaround. Emergency service is also available during the heating season.
Our crew removes the old boiler, installs the new Williamson, connects all piping and venting, and tests the entire system. Most residential installations are completed in one day.
We register your warranty with Williamson-Thermoflo, submit rebate paperwork, and provide documentation for any applicable tax credits. We are here for any questions after the job.
The complete Williamson residential boiler lineup we install on Long Island. Gas hot water, gas steam, and oil-fired models available.
| Model | Type | AFUE | Sizes (MBH) | Application |
|---|---|---|---|---|
| GWA Series | Atmospheric | 82-84% | 38, 52, 67, 100, 133, 166, 200, 233 | Gas hot water atmospheric boiler. Cast iron sections with built-in air separator. Stainless steel burners. Chimney vented. Natural gas or propane. Limited lifetime heat exchanger warranty. |
| GWI Series | Induced Draft | 83-84% | 48, 63, 95, 127, 158, 190 | Gas hot water induced draft boiler. Sidewall or chimney venting for added flexibility. Hot surface ignition. Cast iron. Natural gas or propane. Limited lifetime warranty. |
| GSA Series | Steam | 82-83% | 75, 100, 125, 150, 200, 300 | Gas steam atmospheric boiler. Generous heating surface. Automatic vent damper. Cast iron sections. Chimney vented. 10-year limited warranty on boiler sections. |
| Model | Type | AFUE | Sizes (MBH) | Application |
|---|---|---|---|---|
| OWB Series | Hot Water | 85-87% | 98, 140, 168, 196 | Oil-fired hot water boiler. Cast iron with swing-away burner door and top-access cleanout. Flame retention burner. Bioheat B20/B100 compatible. Limited lifetime heat exchanger warranty. |
| OWT Series | Hot Water + Tankless | 85-87% | 98, 140, 168, 196 | Same as OWB with integrated tankless coil for domestic hot water. Good option when space does not allow a separate water heater. Bioheat compatible. |
| OSB Series | Steam | 85% | 98, 144, 192 | Oil-fired steam boiler. Cast iron construction for homes with steam radiator systems. Chimney vented. Burner sold separately. |
Long Island homeowners share their experience with Williamson boiler installations by our team.
"Our old Williamson oil boiler ran for 24 years before a section cracked. Domino's replaced it with a new Williamson OWB in the same footprint, so the installation was quick and clean. The new one is noticeably quieter and the oil consumption seems to have dropped by about 15%. Good honest crew."
"Our chimney had cracks and relining would have cost over $3,000. Domino's recommended a Williamson GWI instead, which vents through the sidewall. Saved us the chimney expense entirely and the installation was completed in one day. The boiler has been running perfectly through two winters now."
"We had an old steam boiler that was leaking badly. Domino's put in a Williamson GSA-125 and the steam heat has never been more even throughout the house. They were upfront about pricing, showed up on time, and cleaned up everything when they were done. Very professional from start to finish."
We install and service Williamson boilers across every town in Nassau County and Suffolk County. Based in Melville for fast response.
Every town from the Queens border to the Suffolk line. Garden City, Levittown, Massapequa, Hempstead, and 60+ more communities.
Nassau County DetailsOur home base. Huntington to Montauk, including the North Fork, South Fork, and everything in between.
Suffolk County DetailsCommon questions Long Island homeowners ask about Williamson boiler replacement and installation.
Get a free in-home estimate. We will evaluate your current system, recommend the right Williamson model for your home, and provide a complete quote with all available rebates factored in.