Keeping your home heated throughout the long, cold winter can be expensive. Luckily, today’s modern furnaces are far more energy efficient than units produced even a decade ago. Replacing or upgrading your furnace is the easiest way to lower your heating costs, and here is a full overview of which furnace options are the most energy efficient.

How Furnace Efficiency Is Measured

The energy efficiency of a gas furnace is measured in terms of AFUE, or annual fuel utilization efficiency. AFUE is a measurement of what percentage of energy the furnace consumes that it converts into heat. Burning natural gas or propane to produce heat always works in a one-to-one ratio. For every unit of gas burned, a unit of heat energy is created.

The issue is that even the most efficient gas furnace can’t ever utilize 100% of the heat that it creates. No matter how efficient the unit is, some heat will always remain in the combustion fumes when they are vented out through the exhaust flue. The percentage of heat that remains in the fumes and is wasted is what determines a furnace’s AFUE. All new furnaces in the U.S. are required by law to be at least 80 AFUE, which means that they utilize 80% of the heat energy that they create.

It is important to understand that AFUE has nothing to do with how much gas a furnace burns at one time. An 80 AFUE furnace will burn exactly the same amount of gas at the same rate as a 90 AFUE furnace of the same size. The difference is that the 90 AFUE unit will utilize the heat it creates more efficiently, which means it will produce more heat at one time so that each heating cycle is shorter.

High-Efficiency Condensing Furnaces

A new conventional furnace will always be more energy efficient than an older model as many old furnaces have an AFUE that is in the 60s or 70s. Nonetheless, conventional furnaces still aren’t that efficient as their heat exchanger can only capture around 80% to 89% of the heat from the combustion fumes. High-efficiency condensing furnaces are much better and can be anywhere between 90 and 98.5 AFUE.

A condensing furnace has two heat exchangers, which enables these units to capture much more heat energy from the combustion fumes. The combustion fumes first flow out of the combustion chamber into the primary heat exchanger just like in a conventional furnace. The metal heat exchanger absorbs most of the heat from the fumes as they pass through it. However, instead of the fumes then flowing directly out of the exhaust flue, they first flow into the secondary heat exchanger so that the majority of the remaining heat can be absorbed.

This process removes so much heat from the combustion fumes that they cool to the point where any moisture condenses into water, hence the name condensing furnace. In order to handle this water, all condensing furnaces have a drain pan, which leads to a condensate drain system that carries the water away from the furnace.

Two-Stage and Modulating Furnaces

If you’re looking for the most efficient furnace option, you may want to consider either a two-stage or modulating furnace. Most two-stage and modulating units are condensing furnaces, but you can still find some that are conventional units with only a single heat exchanger. The difference between a normal furnace and a two-stage or modulating unit is that these more advanced units can switch to run on lower power settings, unlike single-stage furnaces that always run at full speed. These units do so by reducing the amount of gas that flows into the furnace at one time.

Whether a furnace is single-stage, two-stage, or modulating does not affect its AFUE as it has no impact on the percentage of heat that the unit can capture. No matter whether the unit is running at full power or on a lower setting, it will still absorb the same percentage of heat energy. The difference is that it will burn less gas at a time and thus further decrease your energy costs.

Two-stage furnaces have three settings — High, Low, and Off. The gas valve is fully open on the high setting so that the unit produces more heat. On milder days or whenever not as much heat is needed, the unit will slightly close the gas valve so that the flow is reduced to around 60% to 70% of normal. These units always start on the low setting and then will only ever switch to full power on colder days or when the indoor temperature is much further below the thermostat setting.

Modulating furnaces are even more efficient as they have multiple settings and can adjust the gas flow anywhere from 40% to 100%. These units are paired with a modulating blower that can also slow down to reduce the rate at which air circulates throughout the system.

If you have any questions about furnace efficiency, Zark Heating & Cooling is here to help. We specialize in furnace installation and can assist you with choosing the best unit for your home. We also install air conditioners, heat pumps, and ductless systems and offer a full range of heating and cooling repair and maintenance services. Our team can help improve your indoor air quality as well. Contact us today to learn more about your options for a new furnace or if you need any HVAC service in the New Lenox area.

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