Furnace technology has advanced rapidly in recent decades. This has resulted in many modern furnaces being much more effective and also far more energy efficient. The improved energy efficiency is especially a great thing for consumers since it directly translates to lower heating costs. When you’re in the market for a new gas furnace, you now have many more options including single-stage, two-stage and variable-speed units. We’ve designed this guide to help you understand what exactly a variable-speed furnace is and how it compares to the other types so that you can more easily decide if this type of unit is right for your home.

Comparing Single-Stage and Variable-Speed Furnaces

Traditional furnaces are single-stage units, which means they always operate at the same speed. When a single-stage furnace turns on, the gas valve opens all the way so that the unit runs at 100% of its heating capacity. This isn’t necessarily always a bad thing as it allows the furnace to produce lots of heat quickly. However, it typically leads to the furnace continually cycling on and off in quick succession. The issue with this is that furnaces are under more strain when they first turn on, which means that this rapid cycling on and off leads to much greater wear and tear.

There are also two-stage furnaces that have two settings and can operate at 100% capacity or around 60 to 70% capacity. Two-stage furnaces have many advantages over single-stage units, but variable-speed units are even better.

As the name suggests, a variable-speed furnace can operate at a variety of different speeds or power settings so that it doesn’t always have to produce as much heat as quickly. Most variable-speed units have at least a few dozen settings and they will automatically adjust the speed based on how much heat is currently required. This type of unit will still sometimes operate at 100% of its total capacity on extremely cold days when lots more heat is needed to keep the home sufficiently warm.

Variable-speed furnaces have a microprocessor that measures the rate of heating and how quickly the temperature inside the building is increasing. They will always start out on a lower setting and then only switch to a higher setting if the target thermostat temperature isn’t reached within 10 minutes or so. Once it has brought the temperature back up to where it should be, it will then switch back to a lower speed and continually adjust the rate of heating based on the current conditions and heating requirements.

Variable-speed furnaces can typically run at anywhere from around 30 to 100% capacity. This type of unit is often also referred to as a modulating furnace as it has a modulating gas valve that can partially open or close as needed to regulate how much gas flows at one time. The other difference is that the furnace is paired with a variable-speed blower that can also adjust its speed to regulate how much cool air comes into the furnace at one time and how much warm air circulates through the air ducts and blows out of the vents. The variable-speed blower and the modulating gas valve work together so that the gas pressure and the airflow are always perfectly in sync to ensure the heating system works as energy efficiently as possible.

Benefits of Variable-Speed Heating

Variable-speed furnaces are quite a bit more expensive than single-stage or two-stage units so they’re not always the best option if you’re on a more limited budget. That said, variable-speed units will typically be the best long-term investment as they are more energy efficient, have a longer average lifespan and usually don’t need to be repaired as often. Let’s now look at the benefits a variable-speed furnace offers in more detail to help you understand why this type of unit can be such a great choice.

Improved Energy Efficiency

In most cases, a variable-speed furnace will only run at high speed for maybe 20% of the time. This provides a major benefit in terms of energy efficiency and heating costs. On average, a variable-speed unit will use around two-thirds less energy than a single-stage furnace so your heating bills will be greatly reduced. This improved energy efficiency is important as it helps to balance out the higher initial cost of installing a modulating furnace and variable-speed blower.

One slightly confusing thing about variable-speed furnaces is that they tend to run almost nonstop throughout the day and night and will typically only ever shut off on much warmer days when almost no heat is needed. On the face of it, this may seem like a bad thing but the fact is that variable-speed furnaces will still use far less energy despite running so much more.

More Consistent Temperature Regulation

The fact that variable-speed furnaces run almost nonstop also provides many benefits in terms of temperature regulation. By running continuously at lower speeds, a variable-speed furnace ensures that the temperature throughout the entire home stays much more consistent.

One major issue with single-stage furnaces is that they put out so much heat that they will typically only ever run for 10 to 15 minutes at the most. A single-stage furnace will always shut off when the room where the thermostat is located reaches the desired temperature. The problem is that this type of unit often doesn’t run long enough to fully heat every room. This often leads to the parts of the home that are furthest away from the furnace or rooms that have exterior walls consistently staying colder than the more central areas of the home.

Single-stage furnaces also result in the temperature frequently fluctuating between warmer and cooler as they will only turn back on once the temperature drops below a set level. Instead of waiting for the temperature to drop, variable-speed furnaces just reduce how much heat they produce so that the temperature throughout the whole home always stays consistently warm and comfortable.

Longer Life Expectancy

Again, cycling on and off all the time puts lots of extra strain on a furnace, leading to more wear and tear and shortening the unit’s lifespan. The fact that variable-speed furnaces almost never cycle on and off enables them to last much longer and also typically reduces their repair needs.

Quieter Operation

Variable-speed furnaces are also much quieter and can be almost silent when running at much lower speeds. With a single-stage unit, you’ll normally hear a loud rush of air when the blower motor starts up, and then you’ll also hear the noise of the gas flowing to the unit and from the flames produced by the burners. With a variable-speed blower, the motor will always start slowly and then gradually ramp up the speed as needed so it won’t make nearly as much noise at first. The noise level will be the same whenever the unit has to run at high speed, but this only happens very rarely.

Improved Air Quality

Variable-speed furnaces will also help to improve the air quality in the home. The longer the furnace runs, the more the air in the home recirculates. This results in the air passing through the furnace filter more often so that more allergens and contaminants are removed and the air stays much cleaner. You will need to replace the filter more often, but that’s a small price to pay for vastly improved air quality.

AJ Danboise Plumbing, Heating, Cooling & Electrical is a family-owned company, and we have nearly 100 years of experience providing home services to the Farmington Hills and Greater Detroit Metro area. Whether you’re looking to replace your furnace or you need any other HVAC, plumbing or electrical service, our expert team is always ready to help. To learn more about variable-speed furnaces and your other furnace replacement options, give us a call today.

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