Understanding the Different Types of Gas Furnaces

Gas furnaces are probably the most common heating source in the U.S. This is even true in places like Modesto, where the winters are usually fairly mild. If you plan to install or replace a furnace, multiple options are available. The one you choose will play a major role in how effectively it heats your home and how high or low your heating costs are. To help you make a more informed choice, here is everything you need to know about the various gas furnaces on the market and how they work.

Conventional and High-Efficiency Condensing Furnaces

No matter which type of gas furnace you’re looking at, they all function in a similar way. The unit first burns gas inside its combustion chamber, producing extremely hot combustion fumes. The fumes travel out of the chamber through what is known as a heat exchanger. The heat exchanger is a series of metal tubes that run from the combustion chamber to the exhaust flue.

As the combustion fumes flow through the heat exchanger, they quickly cause the metal to become extremely hot. Cold air is blown over the heat exchanger, releasing all the stored heat to raise the air temperature to somewhere between 120 and 140 degrees. The hot air circulates through the ductwork and out into the home to increase the indoor temperature.

Conventional furnaces only have one heat exchanger, which limits their efficiency. With only one heat exchanger, much of the combustion fumes retain a lot of latent heat when they exit the exchanger and flow out of the exhaust flue. This typically results in between 10% to 20% of the energy the furnace uses being wasted. High-efficiency condensing furnaces overcome this problem by using a second heat exchanger to capture the vast majority of the heat that remains when the combustion fumes exit the primary heat exchanger.

The energy efficiency of gas furnaces is expressed in terms of AFUE or Annual Fuel Utilization Efficiency. AFUE shows the total percentage of the energy that the unit fully utilizes. You can also think about it as an expression of how much energy goes to waste as you operate your furnace.

Conventional furnaces are always somewhere between 80 and 89 AFUE. An 80 AFUE furnace converts 80% of the gas it burns into heat, and the remaining 20% is wasted due to heat escaping through the flue. Condensing furnaces range between 90 and 98.5 AFUE, which means they only waste anywhere from 10% to as little as 1.5% of the energy they consume.

Single-Stage, Two-Stage, and Modulating Furnaces

Virtually all conventional furnaces are single-stage units, which means they always run at full power. Condensing furnaces can also be single-stage, but most are two-stage or modulating furnaces. These designations don’t have an effect on AFUE and instead refer to how many power settings the unit has.

A two-stage furnace uses a specific type of gas valve that can partially close to allow the unit to run at a lower power. When the valve closes, the gas flow is reduced to between 60 and 70% of its regular rate. This gas flow reduction results in less heat created at one time and the furnace using less energy.

Two-stage furnaces are an excellent choice for California and other warmer climates since there are very few times when it is cold enough to need a furnace to run at full power. Unless the weather is extremely cold, the furnace will always run at the lower setting and use less energy.

It is important to note that changing between the two power settings does not have an effect on the unit’s AFUE. Whether running at low or full power, a 90 AFUE two-stage furnace will always convert 90% of the gas it burns into heat. The only difference is in terms of how much gas the unit burns at one time.

Modulating furnaces are even more efficient as they can regulate the gas flow anywhere between 1 and 100%. These units also require a modulating blower fan that can adjust how slowly or quickly air is circulated into the furnace and through the duct system. These furnaces are typically best for colder climates or places prone to extreme temperature fluctuations. They can still be a decent choice in California, but they are typically not worth the higher cost if you live in a warmer climate.

If you’re looking to install or replace your furnace, the experts at Honey's Air & Solar can help you choose the best fit for your home and budget. We carry an extensive selection of furnaces and heat pumps, and our team also specializes in heating repairs and maintenance. We also install and service ductless HVAC systems, ventilation systems, indoor air quality equipment, whole-house fans, standby generators, water heaters, and solar power systems for customers in Modesto and the surrounding areas. To learn more about your furnace options or schedule any of our services, contact Honey's Air & Solar today.

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