Single-Stage, Two-Stage, and Modulating Furnaces

A furnace may seem at first sight to be a simple thing that warms the house. However when we are shopping for a new one, furnaces turn out to be quite a complex topic. Their thermal efficiency has been evolving for over a century, through natural draft, forced air, forced draft, and condensing phases. However, in this article we speak more about managing their heat output.

Cost-Benefits to Consider When Shopping for a Furnace

Purchasing a new furnace can be an unplanned event, after ‘old faithful’ lets us down in the middle of the winter. There are good reasons why some are cheaper, and others more expensive. If you are planning to stay put in your home for five years, then a modulating furnace should break even in that time. But we are getting ahead of ourselves. We haven’t even begun to explain what a modulating furnace is.

The Differences between Single Stage, Two Stage, and Modulating Furnaces

The three main types of furnaces vary in the way we can control the temperature. Single Stage ones either blast out heat, or do nothing. Two Stage ones, as the name suggests, have dual operating modes. Modulating ones allow us to make detailed settings.

Single Stage Furnaces – The Basic Model

Single stage furnaces are dumb devices that only take instructions from their thermostats. When these turn them on, they run at full capacity blasting air into the house. They are therefore the noisiest type of furnaces. However, they are also the cheapest because they use old technology.

Once the thermostat reaches its setpoint, the furnace shuts down. While the room closest to it may be nice and warm, the rooms furthest away may still be a little chilly. This uneven heat comes at a cost saving, but you have to wait until the house has cooled down before the next recharge of warm air.

Two Stage Furnaces – A Bit More Sophistication

Two stage furnaces offer two operating modes after the thermostat turns them on. We can preset them to run at full speed when they perform similarly to the basic model. Alternatively, we can choose the reduced-speed option. In this case, they run quieter and move their air more gently.

There are several advantages to the latter, gentler operating mode. The house warms a little slower, but the longer cycle moves it more evenly through the rooms. The air is also cleaner because it passes through the furnace filter more frequently. Finally, there are energy savings from greater efficiency.

Modulating Furnaces – Pay More for the Best

While single-stage furnaces are dumb devices, modulating ones are smart ones with adjustments. In fact, they can modify their performance continuously depending on temperature demand. They only work when necessary and this can achieve noteworthy energy savings. They represent the pinnacle of furnace design.

The best modulating furnaces step their output up and down in increments as small as 5%. Because they tick over continuously, the temperature in the house remains consistent throughout. There is little noise, less fuss, and no blasts of hot air to assail nostrils. Moreover, their thermal efficiency is 98% and they payback in five years.

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