Sound Comparison Explanation

When you ask for a sound rating of a gas furnace today, you're liable to get just about anything. That's because there's no industry-wide standard for rating the sound of an operating gas furnace.

A manufacturer might claim to have "the quietest furnace" when comparing high-fire sound levels on two-stage furnaces, despite the fact that the furnace spends less than 20% of its operating time at high-fire. The rest of the time the furnace is operating in low-fire, where it might not necessarily have an advantage. How can you break through all the clutter about sound claims? This tool provides a relative sound comparison of a number of different furnaces in the same operating environment and operating at different firing rates—also known as "capacities" or "output levels."

This tool should only be used as a comparison device that means that it should be used to compare how different furnaces sound in this simulated home installation in comparison to one another. Further, this tool won't give you a precise idea of the sound level of a furnace in your home as the sound level is dependent on the volume level during the playback on your computer. In addition, the actual sound level of a furnace and how it will sound in your home will vary based upon where it's installed, what kind of ductwork you have, and a variety of other operating conditions.

The sound levels of the furnaces tested vary from the level of a whisper, around 40 dBA during the lowest firing rates, up to 60 dBA, or the sound level of conversation, during the highest firing rates.

The height of the bar indicates the relative sound level, measured in decibels, produced by the furnaces tested in the mode selected - either the lowest or the highest firing rate of the furnace.

The minimum level of the bars is the normal ambient sound level of a room, or approximately 35 dBA. The maximum level is 70 dBA, or a bit above the highest level measured of any of the furnaces tested. Compare the height of the bars to compare the measured sound levels. In some cases, the bars will be similar heights, but the furnaces may sound quite different, perhaps with the furnace with the shorter bar sounding louder to your ear.

Based on the volume level during the playback on your computer, some recordings may be difficult to hear. The "Start Up" and "Low Fire" sequences can be especially quiet.

What do we mean by Low and High fire? Furnaces are grouped into three categories those that have three or more firing rates, also called "modulating" furnaces, dual firing rate furnaces, also known as two-stage furnaces, and single firing rate furnaces, also known as single-stage furnaces. Modulating and two-stage furnaces have their lowest and highest firing rates compared. The single-stage furnaces are only compared at high fire.

The low fire rates of the modulating furnaces tested are 35% to 40% of the full capacity of the furnace, while the low fire rates of the two-stage furnaces tested are 65% to 70% of the full capacity of the furnace. The low fire rates of each modulating and two-stage gas furnace tested are shown as a percentage of their full gas input rate. The high fire rate recordings for modulating and two-stage furnaces are done at their full gas input. Single-stage furnaces are operated and compared at their full gas input capacity.

We chose to test furnaces from a variety of manufacturers that have high fire input gas rates of 65 to 85 thousand BTUs per hour. This range represents some of the highest sales volume of furnaces sold in North America.

How long do furnaces operate at High and Low firing rates? Of course, it depends upon many factors including your climate area, the outside temperature, the amount of insulation in your home, the thermostat setpoint, and how large of a furnace you have compared to the heating needs of your home, which is often referred to as "oversize."

The width of the sound level bar provides an approximation of the percentage of operating time that the furnace operates at this sound level during the heating season, again based upon a national average—the wider the bar, the more the furnace operates at that sound level. As stated before, the actual time spent at the various firing rates will vary based on a number of factors, throughout the heating season and from home to home.

Based on a national average, two-stage furnaces split their time of operation between their lowest and highest firing rates; with approximately 90% of their operating time at the lowest rate and approximately 10% at the highest rate. These furnaces will provide longer, more comfortable heating cycles than their single-stage counterparts.

Modulating furnaces will spend approximately 80% of the heating season operating at their lowest firing rate, based on a national average, and approximately 1% at their highest firing rate, with the remaining operation somewhere between low and high fire. Since these furnaces are able to modulate down to under 50% of their full firing rates, they will operate longer than other furnaces, provide more even heating, and usually provide the best comfort.

Startup Test Video

Whisper Comparison Video

Main Menu