Single-Stage Heating vs. Two-Stage Heating – What’s the Difference?

Published on: October 18, 2013

One of the more recent developments in home heating systems has given homeowners another option besides single-stage heating. Two-stage heating is now available, and it’s a solid option for heating homes in the milder winter climates found in the southeastern U.S.

A gas furnace that provides single-stage heat operates on high whenever it runs. When temperatures are over 40 degrees, the furnace won’t run long to heat a home, which means that the air and contents of the home don’t have time to absorb the warmth as thoroughly. On the other hand, a furnace equipped with two-stage heating adjusts its running speed based on the amount of heat needed indoors.

The ability to calculate how much heat your home needs and the best speed to run at has many benefits that include:

Improved Indoor Comfort

When the furnace senses that the temperature differential between your home and the outside air isn’t extreme, it runs more slowly. This gives the furnace more time to run and it’s able to circulate the air better, resulting in fewer cold spots in your home. You might find rooms that are further from the air handler become warmer without the areas closer to the air handler becoming overly heated.

Quiet Operation

Unlike a single-stage heating system, the two-stage furnaces start out slowly on their lower speed. This means that the air handler’s fan doesn’t run as fast, creating less noise. It’ll slowly ramp up if your home needs more heat, and as it does, the noise the fan makes is less noticeable. Starting in the slower mode also means you don’t feel that cold blast of air coming from the ducts. If the furnace needs to run on high, the air coming from the ducts will already be warmed.

Better Indoor Air Quality

The slow start-up and longer running speed increase the time the furnace runs, helping the air filter inside the air handler trap more airborne particulates. This is a key benefit because pollen can be a year-round problem in many parts of the southeastern U.S. since they have a 12-month growing season.

Households who have family members with allergies, asthma or other respiratory issues can benefit from better air filtration, especially if they equip the air handler with filters that have a higher capacity to trap particles. The efficiency of air filters is known as MERV (minimum efficiency reporting value), and runs from 1 to 16 for residential uses. Most furnaces can use an air filter with a MERV of 8, which is capable of removing mold spores, dust, dust mites, pet dander and pollen.

Longer System Life

Because a two-stage heating system runs at a slower speed, especially at start-up, it doesn’t run into the short-cycling issues that a single-speed heating system encounters. Short-cycling happens when systems are oversized for homes and they run in short spurts. This adds a lot of wear to the equipment because the frequent starts create the most wear on the parts and increase energy consumption.

In our climate, furnaces need to be carefully sized to avoid this problem since the heating load is relatively low. Contractors use software called Manual J for load calculations for both air conditioning and heating to accurately determine the proper size HVAC system you need. Having a two-stage heating system reduces the risk that single-stage heating systems pose in mild climates when the temperature differentials are low between indoor and outdoor air.


Choosing a two-speed furnace also makes it possible for you to more easily install a zoning system in your home. Zoning systems are a simple way to reduce your energy consumption if you don’t occupy all parts of your home each day. HVAC contractors install dampers inside the ductwork and a thermostat for each zone connected to the main unit.

You can set the temperature in each zone separately. If you have a two-story home, you can adjust the temperature according to the heat you need upstairs. If you have several rooms you seldom use, you can turn the zone off. Zoning your home won’t make the heating system operate more efficiently, but it will reduce the length of time it needs to run, which lowers energy bills.

If your whole house needs more heating, the furnace can put out more heat on the high setting. If you need just a little heat in one zone, it’ll run at its slower setting.

To learn more about two-stage heating versus single-stage heating, contact us to find a contractor in your area.  Our team of professionals at AC Southeast® is prepared to answer all your HVAC questions.