Furnace Buying Guide

In most cold climates, forced air heating has replaced steam and hot water systems as the predominant method for heating a home.

The heart of nearly every forced air heating system is the furnace.
Gas furnaces come in a variety of sizes, types, and efficiencies to satisfy almost any application.

Basic Design

A furnace consists of two or three main components depending on the fuel source.

Oil, propane, and natural gas furnaces use a burner to combine fuel and air and then burn the mixture to produce heat. In all furnaces, this heat is then transferred to the air via a heat exchanger. In electric furnaces, the heat exchanger also converts the electricity into heat. Finally, a blower distributes this warmed air throughout the house.


Burner Options

Furnace burners are often designed to operate in one of three ways.


furnace burner


The first and most common is a single-stage burner. A furnace using a single-stage burner has only two operating modes, on or off. Single-stage furnaces will always operate at the rated capacity regardless of conditions inside or outside of the home. This operation often leads to inefficiency when conditions are mild.



To address the inefficiency with traditional burners, two-stage burners were introduced. A furnace using a two-stage burner has a total of three operating modes, off, low-fire, and high-fire.


High-fire is exactly what it sounds like and in this mode, identical to the single-stage furnace, producing its full rated capacity.


Low-fire is an intermediate step that allows the furnace to operate at some factory set level less than the maximum capacity. This level is typically 40%-60% of the rated capacity and will allow the furnace to better match a home’s heating requirements in mild weather. This leads to a more comfortable home, and a lower fuel bill.



two-stage furnaceExpanding on the benefits of two-stage operation, modulating burners can operate at almost any output. Modulating furnaces have a minimum and maximum heating capacity but can operate at almost any point within this range.


By constantly monitoring conditions and adjusting output to account for any changes, modulating furnaces are able to eliminate large temperature swings sometimes present with other types of furnaces. They also maximize efficiency by using the minimum amount of fuel to satisfy heating demands.


Comparison Charts

The charts below show a simulation of average temperature and fuel usage for single-stage, two-stage and modulating furnaces. As you can see, the 2-stage and modulating furnaces maintain a more stable temperature than does the single-stage unit.


The 2-stage and modulating options also offer savings of approximately 8% and 10% respectively when compared to a single-stage model, even when all three have the same rated efficiency.