Boiler Manuals & Owner Guides

Boiler efficiency and AFUE

AFUE (Annual Fuel Utilization Efficiency) is the share of the fuel burned over a heating season that becomes useful heat. ENERGY STAR certified boilers must reach at least 90% AFUE for gas and 87% for oil, compared with federal minimums of 84% and 86% for gas- and oil-fired hot-water boilers.

Condensing boilers and water temperature

A condensing boiler recovers extra heat by condensing water vapor out of its exhaust, but only when the water returning from the system is cool — below roughly 130 °F with natural gas. Radiant floors, panel radiators and properly sized baseboard with an outdoor-reset control keep return water cool most of the season. Systems designed for 180 °F water all winter will condense less and gain less from a condensing boiler.

Outdoor reset

A control that lowers supply-water temperature as outdoor temperature rises.

Turndown ratio

Maximum input divided by minimum input. A 10:1 boiler can fire at one-tenth of its full rating, reducing short-cycling in mild weather.

Condensate neutralizer

A cartridge of mineral media that raises the pH of acidic condensate before it goes down the drain.

Boilers and domestic hot water

Boilers can supply domestic hot water in two common ways: a combination (combi) boiler heats tap water on demand through a built-in heat exchanger, while a conventional boiler can heat an indirect storage tank as a priority zone. Indirect tanks handle simultaneous showers well; combis save space but are limited by their flow rating.

Boiler FAQ