high efficient furnaces
If your heating needs are large, replacing your old furnace with a new high efficient furnace will reduce significantly your energy bills and cause much less pollution. But you should also consider other alternatives.
Old furnaces efficiency
Old furnaces are inefficient appliances and a major cause of high energy bills. Their AFUE is often 70% or less, which means that their energy efficiency is very low (modern condensing furnaces AFUE is 90% or more, that is, they turn into heat more than 90% of the fuel they use).
Note: AFUE = Annual Fuel Utilization Efficiency
High efficient (condensing) furnaces
Many modern furnaces are condensing (gas or oil fired). If the energy efficiency (AFUE) of a furnace is above 90%, then it is "condensing" or "high efficient" – one that recaptures most of the water vapor wasted in traditional systems, condensing that vapor and producing extra-heat.
Alternatives to old furnaces
Modern high efficient gas and oil fired condensing furnaces are a direct alternative to old central furnaces.
Wood-pellet and corn furnaces are also environmental and economical alternatives, easy to implement, though less common and involving old or non-tested technologies.
Space heaters like direct-vent wall gas furnaces, gas stoves and gas fireplaces inserts are also excellent alternatives to central furnaces in well insulated homes, even in very cold climates. And they are much more environmentally-friendly. Future heating strategies will be based in space heaters and zone heating. Central heating based on furnaces, will decrease in its importance. But excellent home insulation is a pre-requisite to such strategies.
Other more radical alternatives include highly renewable heating choices, namely solar-powered systems, solar passive principles (for new homes) or geothermal heat-pumps, whenever their implementation is possible.
Furnaces, Duct sealing and home insulation
Before buying a new furnace or implementing any other heating solution, make sure that your house is properly insulated and that the ducts are properly sealed. A well insulated house will demand a smaller heating system and will save you energy and money. Sealing any air leaks and ducts, adding insulation and replacing inefficient windows and doors, can make small miracles.
For more information on insulation, sealing and advanced doors and windows:
Home insulation basics.
Wall insulation
Attic insulation
Duct sealing
High efficient windows
High efficient doors
Weatherstripping
Caulking
High efficient forced-air furnaces and gravity furnaces
Gravity furnaces are a thing of the past. New furnaces are forced-air appliances. The difference between the two systems is on the blower: both use ductwork to circulate room air through the house, but just forced-air furnaces have a blower to assist that circulation (gravity furnaces use natural convection, which makes them far more inefficient in terms of delivering the heated air).
Wall and floor furnaces
Wall and floor furnaces do not use ductwork, and are typically single-room heating appliances (unlike common furnaces, providing heated air to the whole house or a significant part of it).
Wall furnaces are installed on walls (or recessed), and the heated air is circulated by gravity or by fan. These furnaces are available either in direct-vent models and conventional venting. See: Direct-vent wall gas furnaces
Old floor furnaces are commonly installed within an unconditioned crawl space and draw combustion air from outside. This type of furnaces is naturally-vented and doesn’t use fan-assisted combustion or blower. Avoid them: they aren't safe...
Modern direct-vent wall gas furnaces is a solution your may want to consider, though rather similar to gas stoves and gas fireplaces inserts, often more advantageous and environmentally-friendly.
See:
Gas stoves
Inserts
Furnaces vs. stoves and fireplaces
Electric, wood-pellet, oil and gas furnaces
New furnaces can run on natural gas, propane, fuel oil, wood-pellet, or corn. Gas is by far the more common choice. Wood pellet and corn fired furnaces are new environmental and economical choices. Electric furnaces are a bad alternative unless for sporadic and small heating needs.
See also: Direct-vent wall gas furnaces
Furnaces and electrical efficiency
Even gas and oil furnaces may demand a reasonable amount of electricity, to power the fan motor and the blower. To save electricity and money use variable speed fan motors. They are typically more high efficient than the standard PSC motors.
Efficiency
The overall performance of a furnace (or a boiler) is given by its Annual Fuel Utilization Efficiency (AFUE). AFUE is a very important rating, but you should also consider Energy Star labels, or certifications by UL, Gas Appliance Manufacturers Association and other qualified organizations.
Energy Star labeled products meet strict energy efficiency guidelines set by EPA, and are a guarantee of a higher efficiency than standard models.
Sizing
When replacing an old furnace, make sure that the furnace heating capacity isn’t too high for your needs. There are many oversized furnace systems, causing higher energy bills and an discomfort. Sizing a furnace is crucial, and a certified contractor is the best way to establish the proper size.
Furnace Manufacturers and Brands
For a list of popular high efficient furnace manufacturers, and some tips on this issue, see: Furnace Manufacturers
Furnace Manufacturers and Brands