What Is The Difference Between Single Stage and Two-Stage Furnaces?

December 10, 2022
Heating Installation in Roseville, MI

Single-Stage vs. Two-Stage Furnaces: What’s the Difference?

When choosing a new furnace, it is vital that you carefully consider all of your options to ensure that the new unit heats your home effectively without using too much energy. AFUE (Annual Fuel Utilization Efficiency) is one of the most important factors since it will directly impact your heating costs. If the unit has a lower AFUE, it won’t use energy as efficiently and thus will need to run longer to heat your home thoroughly. Another factor that will impact your heating costs is whether the furnace is single-stage or two-stage, and here is everything you need to know about each of these furnace types, how they work and what makes them different.

How Do Single-Stage and Two-Stage Furnaces Work?

Single-stage furnaces are the most basic type and only have two settings: On and Off. It will always run at 100% power whenever the furnace is heating. Two-stage furnaces are different in that they can run at total capacity and also at a lower setting. To run at lower power, two-stage furnaces use a three-way gas valve to regulate how much gas flows into the furnace simultaneously. The gas valve is always either fully open or fully closed in a single-stage furnace.

When a two-stage furnace runs at lower power, the gas valve is only partially open, reducing the gas flow to somewhere between 60 and 70% of its average rate. Switching to low power means that the furnace will produce less heat and use less energy. This is hugely useful for milder days when you never need your furnace to run at full power or when the furnace only needs to raise the indoor temperature by a degree or two.

Two-stage furnaces will always start and run at the lower setting. If the thermostat determines that much more heat is needed, it will signal the furnace to switch to run at full power. This typically only happens overnight or on much colder days. If you use your furnace during the late fall, early spring, or anytime when the outdoor temperature is above 40 degrees, it will generally only run at the lower setting.

Difference Between Two-Stage and Modulating Furnaces

There is also a third type known as a modulating or variable-speed furnace. These furnaces have a valve that can adjust the gas flow to anywhere from 40 to 100% based on the current heating needs. Modulating furnaces are paired with a variable-speed blower fan that can automatically adjust the rate at which air is circulated throughout the home.

Modulating furnaces are the most energy-efficient unit, but they are also far more expensive. Not only is a modulating furnace more expensive, but you will also need to install a new blower fan since most HVAC systems use a single-speed fan. The only exception is if you have a variable-speed air conditioner, as these also use variable-speed blower fans.

Why Stage and AFUE Are Not Directly Related

All gas furnaces burn natural gas or propane to heat the air inside a building. The gas is burned inside a combustion chamber, which produces scalding combustion fumes. These fumes travel from the combustion chamber through a metal heat exchanger and eventually into the exhaust flue. The combustion fumes flow through the heat exchanger, making it extremely hot. The blower fan draws in cold air and forces it past the heat exchanger. Since the air is much cooler than the heat exchanger, heat energy flows out of the exchanger to raise the air temperature quickly.

The heat exchanger can only ever absorb heat energy from the combustion fumes at a specific rate, which means that some heat energy always remains in the fumes when they flow out of the exchanger into the flue. Therefore, the amount of heat the furnace can absorb from the fumes before they exit through the flue determines its AFUE.

In an 80 AFUE furnace, 80% of the energy is absorbed from the fumes, and the remaining 20% is lost. Higher-efficiency condensing furnaces utilize a second heat exchanger that can absorb far more heat energy from the combustion fumes. These units can be anywhere from 90 to 98.5 AFUE.

A furnace with a higher AFUE will always produce more heat at a time and thus have shorting heating cycles. Two furnaces that are the same size with different AFUEs will always burn the same amount of gas at a time, but your heating costs will be lower with a higher AFUE furnace since it will use the gas more efficiently.

How many stages the furnace has doesn’t impact its AFUE whatsoever. For example, a two-stage 90 AFUE furnace will always capture 90% of the heat energy it creates, whether running at high or low power, and the same is true for a modulating furnace. The difference is simply that the unit will burn less gas when running at low capacity; thus, your heating costs will be further reduced.

Benefits of Two-Stage and Modulating Furnaces

The most significant benefit of a two-stage or modulating furnace is that it will significantly reduce your heating costs since the unit won’t always run at full power. Your heating costs will stay mostly the same on much colder days but should be around 30 to 40% lower most other times since the furnace will burn less gas at a time. Two-stage and modulating furnaces are also much quieter whenever running at lower speeds.

Another benefit is limiting temperature fluctuations and preventing hot and cold spots in some parts of the home. Single-stage furnaces produce lots of heat at one time, leading to the more central parts of the house becoming hot before the furnace shuts off. As a result, some rooms will often be a few degrees above your thermostat setting when the furnace does turn off. The issue is that the furnace will only run for a short time, so rooms furthest away from the furnace may never receive enough heat.

Two-stage and modulating furnaces will usually run for a longer time at a lower power, which should ensure that all rooms are sufficiently heated before the system shuts off. It also means that the temperature won’t fluctuate between hot and cold as much since the furnace will heat more slowly and run longer.

Top-Rated HVAC Services in Macomb County

At C & C Heating & Air Conditioning, we specialize in furnace installation and can assist you with choosing which type of furnace is the best fit for your home. We carry a wide range of furnaces and air conditioners from top brands like Carrier and Trane, and we also install ductless mini-splits and heat pumps. Our team also specializes in heating and cooling repairs and maintenance. We also offer various indoor air quality services and equipment, including duct cleaning, humidifiers, germicidal UV lights, and air filtration systems. We can also help with duct repair and installation, smart thermostats, and water heaters. Call us today if you have any questions about furnace installation or need any other HVAC service in Roseville or the Detroit Metro area.

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