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Single-Stage Furnace Vs Two-Stage Furnace


Is it time for you to replace your furnace? Do you wonder how furnaces differ? Furnaces come in a few different variations. Single-stage and two-stage are the two basic variations of furnaces. Furnace AFUE, or annual fuel utilization efficiency according to Energy Star, is the amount of fuel that is actually converted to heat, i.e. a standard single-stage furnace with 80% AFUE will only produce 80% of the heat for the fuel that it uses. There are also variable speed blower motors and modulating gas valves that can modify a furnace’s functions and improve comfort as well. Below we will go over the differences between these furnaces and how they operate.

Single-Stage Furnace

The single-stage furnace is the most inexpensive kind of furnace, but also the least efficient. A single-stage furnace works in one setting, full blast.  It is sized to handle heating your home on the coldest of winter days, however on mild winter days it will still blow at maximum output.

Two-Stage Furnace

A two-stage furnace is a step above single-stage in terms of comfort. A two-stage furnace, as the name implies, is broken up into two stages. The first stage is equivalent to about 60% of the furnace’s heating capacity, if the first stage is not enough to meet the heating requirement, the furnace turns up to the second stage which is the full capacity of the furnace. The first stage of heat will be enough to be used most of the time and the higher stage will get used on colder days of the year. A two-stage furnace will run more than a single-stage furnace, but will provide you with a more stable, more comfortable experience.

Variable Speed Blower Motor

A variable speed blower motor increases the comfort provided by your two-stage furnace even more so by allowing the fan on your furnace to operate at different speeds. A variable speed blower motor comes equipped with technology that adjusts the fan to the optimal speed to maximize comfort in your home. A variable speed motor draws less electric than a standard motor providing you some minor energy savings, especially if you operate your furnace in the fan “on” position.

Modulating Gas Valve

The most efficient and comfortable furnaces come with a modulating gas valve and can adjust the heat in minor increments to maximize your comfort. The burner flame can change incrementally from 40% output up to full capacity to maintain an unsurpassed level of comfort. These units run quieter with lower fan speeds. A two-stage furnace with a modulating gas valve and variable speed blower will be your most comfortable and efficient option available. For example, the Carrier Infinity 98 Furnace has an efficiency of 98%!


If you’re looking to replace a furnace give us a call at 630-896-1560 or reach out to us via email at info@artlipandsons.com.  We look forward to working with you and providing you with a range of options from a single-stage furnace all the way up to a high efficiency two-stage furnace with a modulating gas valve