New furnaces can provide significant energy savings .
Do not forget the long lifetime of furnaces, often 20 years or more. Savings of, say, $200 or $300 per year, will amount to $4.000-$6.000 over the lifetime of the furnace.
Top gas furnaces have an energy efficiency (AFUE: Annual Fuel Utilization Efficiency) of 90% or more, and features like: 1) sealed combustion, 2) multi-stage and variable speed motors and 3) zone heating capabilities.
Multi-Stage variable-Speed furnaces
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Buying Efficient Furnaces
Older furnace have no output controls. Only Off and Full Blast... And that doesn't make sense! Even two-velocity furnaces make little sense. Prefer a variable-speed, multi-stage furnace. They work more smoothly and allow more comfort, and are a lot more efficient.
Multi-stage furnaces run on different speeds rather than cycling repeatedly from Off to On/High velocity as in single-stage furnaces. They adjust the flame incrementally, according to the thermostat's target temperature.
This feature reduces drafts, fluctuations in temperature and noise (new fan motors use less electricity than a common 100 W bulb, and are great for homes with artificial ventilation systems); and provides higher energy savings.
Multi-stage furnaces are combustion-sealed units; they draw the combustion air from outside through a dedicated pipe, and vent exhaust gases to the outdoors through another pipe.
And that means more than just energy-efficiency; it means also higher safety. Furnaces emit dangerous pollutants, and they should vent them directly to the outside. It’s your life and health, and those of your family, that are at stake.
Zoning distribution capability
Zoning (room-by-room heating) is also a feature that you should look for in a new furnace. Properly used, zoning can provide huge energy savings.
Why heat all the rooms, when you aren't using all of them? Why not have sensors and thermostats in each room, and ductwork equipped with special dampers, to distribute the heat in the right amounts and almost instantaneously only when and where it is needed?
See: What's Home Zoning
Other features that you should take into account
The heat exchanger is a critical part of any furnace, and your new furnace should have a dual heat exchanger, that is, a primary and also a secondary heat exchanger - usually a robust stainless steel part of the furnace responsible for capturing waste heat, allowing higher efficiency levels.
Look for a manufacturer offering a limited lifetime warranty for the heat exchanger, or a 20 years warranty - a telltale sign of quality.
Some furnaces have enhanced air filtering capabilities, to help trap particles and to reduce the amount of dust circulating through the ductwork. That feature is particularly useful for people with asthma or other respiratory problems.
The best units are extremely quiet - several times quieter than other models, due to their variable-speed high-quality motors and well insulated and noise-reducing cabinets.