Most homeowners purchase a variable speed furnace for comfort. The unit has a variable speed motor. It moves at different speeds to offer precise control of how much or how little heat or cold the system will generate.
A variable speed offers better airflow which equates to better temperature control.
These units used 2/3 less energy to run. They tend to be very quiet. You do not get that sudden air blast or at start-up of the furnace.
These furnaces reduce indoor air temperature fluctuations, thereby increasing your comfort. With mild outdoor temperatures the furnace operates on its low-fire stage, which is much more efficient. When it's cold outside the furnace will use the higher stage with increased airflow.
When properly sized for the property, a two-stage furnace will provide just enough heat on very cold days. On a very cold day a single-speed furnace would be cycling on an off, creating temperature fluctuations. The on and off increases your energy costs and decreases the evenness of the heat.
Annual Fuel Utilization Efficiency or AFUE is the standard rating for furnaces.
The less fuel required to produce your heat, the more efficient your furnace is.
78% AFUE is the minimum efficiency standard for furnace manufacturers.
The older your furnace is, the less efficient it is.
When looking at furnaces consider the AFUE rating, but also look to see how much electricity is required to power your furnace. Looking for Energy Star Labels and ratings is always a good strategy.
Things to consider are how much it costs to heat a house with an older furnace. Units made today are significantly more efficient than models from just a few years ago. The utility savings of your new high-efficiency furnace will often pay for itself within a few short years due to pure efficiency. HVAC, heating inspections and energy analysis can give you a good indication if its time.
According to the National Association of Homebuilders, a boiler will last
13 to 21 years. A gas furnace, 15 to 20 years. An oil furnace, 10 years or less.
A heat pump would last 15 years. These are averages; your results may vary based on how you maintain your equipment.
A home heating evaluation/audit is the only way to right-size the furnace you need. Every family's need is different, based on many factors that are too many to list here. Quality of windows, doors and overall insulation would factor into furnace size.
Airflow is restricted by dirty filters. This can increase your heating costs and lower the useful life of your unit. As a rule of thumb, you should have your filters changed annually.