If you own an older furnace, you likely have a single-stage model. Single-stage furnaces use a relatively straightforward design that satisfies your thermostat's set point without varying heat output. Instead of adjusting the heat available based on need or demand, the furnace runs at full bore until it reaches the desired temperature, at which point it shuts down and waits for its next cycle.

Two-stage and modulating furnaces operate differently. Instead of blasting hot air whenever the interior temperature falls below your set point, they adjust their output to balance efficiency and comfort. Two-stage furnaces can run in "high" and "low" modes while modulating furnaces can vary their output continuously.

Are Single-Stage Furnaces Bad?

It's important to understand that there's nothing wrong with a single-stage furnace. Single-stage units are still the most common design found in many residential homes, and they will work to keep your house warm. Likewise, a properly-sized single-stage furnace will have enough capacity to deal with even very cold days.

Of course, there are several real downsides to this furnace design. Most importantly, a single-stage furnace may use more energy than advanced models. Since single-stage designs can't vary their heat output, they need to run in high-intensity bursts, burning more fuel even when your heating needs are relatively minor.

Comfort is another potential issue. Since the furnace can't vary its output, you may sometimes notice swings between hot and cold. Instead of maintaining your temperature, the furnace must wait for the temperature to fall low enough before running at full speed to heat your home. While a correctly-sized furnace will keep your home comfortable, this effect can be noticeable.

Will a Higher-End Model Save You Money?

A single-stage furnace can still achieve a high AFUE (annual fuel utilization efficiency), but this value doesn't tell the whole story. While these high-efficiency models will effectively convert most of their fuel into heat, they won't necessarily be able to maintain the temperature in your home as effectively. As a result, you may ultimately spend more money on your heating bills.

Since two-stage and modulating furnaces can adjust their output based on conditions, they can run for longer at much lower speeds. This design allows them to maintain your home's temperature better, avoiding the inefficient behavior that results from constantly allowing your interior to cool down before heating it again.

While it can be hard to measure how much money you'll save with these higher-end models, they can help you reduce your energy bills. These furnaces will also improve your home's comfort by maintaining a more consistent interior temperature. If you're looking for a way to upgrade your heating that's also cost-effective, a two-stage or modulating furnace may be a good option.

To explore more options for your new furnace installation, contact an HVAC contractor in your area.