We can identify several types of furnaces, according to the fuel they use (natural gas, electricity, oil, pellets), their technology (single-stage, dual stage or multi-stage) or the distribution system (ductless, central heating furnaces).
Single, two and multi-Stage furnaces
Old furnaces were single-stage.
They have only a full-speed option, when they are On, and they are very inefficient and outdated.
Two-stage furnaces are a little smarter than the single stage ones. They haven’t only a simple On/Off operation. They have also an On/High low speed and an On/Low speed... They are quieter and do not cost much more.
Multi-stage furnaces adjust the flame incrementally; the temperature can vary in small steps according to the thermostat's target temperature. They are the best for comfort and energy savings, but they are also the most expensive.
Central gas furnaces: Low and High-Efficient Units
Low-efficiency furnaces: AFUEs (energy coefficient) of 80% or less;
Mid-efficiency furnaces: AFUEs of 80-89%;
High-efficiency furnaces (condensing furnaces): AFUEs of 90% or more...
Top new gas furnaces are multi-stage and multi-speed variable units, with air-sealed combustion. They are safer, and provide more comfort and larger energy savings. But they are also a lot more expensive.
AFUE - Annual Fuel Utilization Efficiency
The AFUE is a percentage expressing how much fuel the furnace converts into heat: say, 90% or 80%. A furnace with a AFUE of 75% is a low-efficient unit. A unit with an AFUE of 97% is a very high-efficient furnace. Most modern furnaces have AFUEs between 80% and 97%.
Low-energy efficient furnaces (with AFUEs below 80%) are outdated. Some contractors argue they are the best choice for mild climates, since the energy bills and the energy savings in these climates are not high enough to offset the higher cost of the best units.
But that's a rather spurious argument; if the energy bills and the heating needs are small or relatively small, then it is also difficult to justify the use of a central furnace… In such climates, reasonable insulation levels and advanced windows combined with ductless gas furnaces or other space heating strategies can replace with advantage a central furnace system…
See: Furnaces Prices & Payback.
New top furnaces are of condensing type (they recapture and reuse the water vapor and the combustion gases) and have When buying, prefer a top 90%+ furnace. In homes with high heating loads, it’s not difficult to recover the higher initial investment in energy savings.
New oil furnaces can be fairly efficient and relatively clean and safe. There are now oil furnaces mimicking close-sealed combustion high efficient gas furnaces, able to provide high levels of efficiency.
See: Gas vs. Oil furnaces
Electric heating is typically expensive, and traditional electrical furnaces are big energy wasters.
Manufacturers are now offering new electric heat pump “furnaces”: these furnaces have electric resistance elements, but in their core they are air-source heat pumps (they are air conditioners, able to work in reverse). This type of new “furnaces” can be a good choice for climates without long periods of freezing temperatures.
Pellet furnaces have little in common with traditional Midwest wood furnaces. The best units are sophisticated equipment, with many of the features of the best gas furnaces. They are relatively easy to maintain and to use, and with a relatively low carbon footprint (their fuel is largely made from recycled wood wastes).
But the most sophisticated models are more expensive than gas furnaces, and are designed to provide high amounts of heat, which contradicts the low-energy home approach, based on very high levels of insulation and air sealing, and small heating systems.
See: Wood Pellet Furnaces
Wall Ductless furnaces
Ductless (direct-vent) furnaces are more than just an heating option for hard-to-heat areas like basements, bonus rooms, carriage houses or in-law suites.
They can meet all the heating needs of most homes in a moderate climate; or even the heating requirements of small or relatively small homes with high levels of air sealing and insulation, in colder climates.
The installation of this type of furnace is easy, since the venting only requires a 3” wall hole...