An emerging idea in boiler technology that should adapt well to the tighter spaces in older homes are condensing combination (or “combi”) boilers. In a technology similar in concept to tankless water heaters, these high-efficiency boilers heat water on demand and also separate the lower-temperature water for residential use from the super-heated water used for space heating (through a hot-water baseboard system, for example). Although the technology is proven in institutional and commercial settings, the industry has experienced maintenance issues with units intended for the residential market. As part of the next generation of combi units, Lochinvar just introduced the noble Fire Tube Combi. The noble places critical components like the built-in circulator in an easily accessible location within the unit to simplify maintenance.
As with boilers, furnaces are rated for efficiency with an AFUE. Minimum efficiency furnaces generally have an AFUE rating of about 80 percent. Fine for milder climates, the best offer electronic ignition instead of standing pilots, better heat exchangers, and internal vent dampers that reduce the loss of heated or cooled air when the unit isn’t cycling. Mid-efficiency furnaces (83 to 87 AFUE) offer more precise control of combustion and venting. If you are burning oil, look for a furnace that has a high-static burner; it will extract more heat from the fuel. High-efficiency furnaces—recommended for large houses or homes in areas with extreme heating or cooling demands—have AFUEs in the 90 to 96 percent range. High-efficiency furnaces incorporate a second heat exchanger to reclaim some of the heat lost through vaporization.
For boilers and furnaces, specify a sealed combustion unit, which brings outside air directly into the burner and exhausts combustion gases directly to the outside, eliminating the need for a draft hood or damper. Also, take electricity use into consideration. Boilers use electricity to power circulating pumps; furnaces use it to run the fan motor. For a boiler, look for a unit with high-efficiency pump. Similarly, for furnaces, variable-speed or multi-speed fan motors are usually more efficient than single-speed motors.