Boilers & Hydronic Heating

Cast-iron boilers burn oil and vent up a chimney; because they don't condense the flue gas, their efficiency tops out well below the gas condensing designs. Condensing gas and propane boilers pull extra heat out of the exhaust before it leaves the building, which is also why they vent through ordinary plastic pipe instead of a chimney. Combi boilers are condensing gas or propane boilers built to also heat domestic hot water on demand, without a separate storage tank.

Weil-McLain

Weil-McLain's flagship residential condensing line is the Evergreen Pro, built in five sizes from 110,000 to 399,000 Btu/hr input. The manufacturer's own boiler manual rates the two smallest models, the 110 and 155, at 95.0% and 95.1% AFUE, both certified by AHRI. Evergreen boilers must be direct-vented through 3-inch PVC pipe as a Category IV appliance — a chimney cannot be used with this line.

Burnham

Burnham is a boiler brand of U.S. Boiler Company. Its flagship residential condensing line, the Alpine, is rated at 95% AFUE across its six sizes (80,000 to 399,000 Btu/hr input) per the manufacturer's product data sheet, and vents through PVC or CPVC pipe with a CPVC-to-PVC adapter kit. U.S. Boiler Company still sells cast-iron oil boilers for homes staying on oil — its EMP and MPO-IQ lines are rated at 87% AFUE and the MegaSteam steam boiler at 86%, per the same product line-up.

Buderus

Buderus boilers are still common in Maine basements, but Bosch, which owns the Buderus brand, now lists Buderus residential boilers — including the Logamax plus GB142 — on its own "discontinued products" documentation page and sells its current condensing lineup under the Bosch name. The GB142's own service manual rates its thermal efficiency at 88% at full firing rate and up to 97% at part load with low return-water temperature. That isn't the same test as AFUE, but it illustrates how much a condensing boiler's real-world efficiency depends on the water temperature it's actually piped to run. The GB142 vents through PVC or CPVC pipe.

Navien

Navien's NCB-H series is a combi boiler — it heats the house and supplies domestic hot water from one unit, in five sizes handling 11,000 to 150,000 Btu/hr of space-heating input. The manufacturer's installation manual lists every NCB-H model at 95.0% AFUE, tested to U.S. Department of Energy procedures. Like the other condensing boilers on this page, it's a Category IV appliance, vented with 2- or 3-inch PVC, CPVC, or approved polypropylene pipe.

Viessmann

Viessmann's Vitodens 100-W, the company's entry-level residential condensing line, carries a 95% AFUE rating on every model and switches between natural gas and propane at the control, without a separate conversion kit, per the manufacturer's product page. Sizes run from about 8,500 to 199,000 Btu/hr input.

AFUE is measured under a standard laboratory test cycle, using Department of Energy procedures, not your specific home's piping or water temperature. That's why two condensing boilers with the same 95% rating can perform differently once installed: a condensing boiler only condenses, and only earns its rated efficiency, when the water coming back to it is cool enough. Piping, radiation type, and control settings all affect that return temperature in practice — the installation guide on this site covers how outdoor reset controls are used to keep it low through the season.

These five are large, multi-line manufacturers with published AFUE data available directly from their own manuals and spec sheets. We didn't find a Maine-specific study ranking residential boiler brands by installed base, so this page doesn't claim any one of them is "the most common" — they're widely installed nationally, and a technician recommending a brand not listed here isn't necessarily steering you wrong.