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How to Size a Humidifier


W hile more simplistic in their operation and with a limited range of functions and features to look into, humidifiers can actually be surprisingly confusing to acquire. Tank size and moisture output are the two elements everyone tends to look into first, but without knowing how to size the humidifier to the room, space, or entirety of the house, chances that you make a wrong pick and not match the system’s capacity to your requirements are high.

Basically, to get rid of dry air indoors, it doesn’t suffice to add a humidifier, but one that is on par with the space size demands. Otherwise, your nasal and throat membranes will still dry when winter comes, and susceptibility to colds and viral infections will remain high. In the following, we will provide you with guidance on this topic so that the humidification enhancing system you acquire meets your needs and performs as expected to relieve dry air symptoms and prevent unwanted repercussions.

Single Room Humidification

Usually, people seek humidification increase for specific rooms or spaces in their homes, generally the nursery or bedroom. If you intend to use the system for a single space or for different rooms that are somewhat similar in size, you will evidently look into a portable machine that you can relocate as needed and that won’t take up much space.

While manufacturers provide listings of room size compatibility, the data isn’t always a trustworthy indicator of how well the device raises humidity in an area of that specific size. The info provided are usually based on ideal operating conditions and with the system running at its highest output capacity, so the GPD output and specific area coverage are not being ideal guiding aspects as you are unlikely to have perfect temperature indoors or run the machine on the top speed at all times.

How to choose: If you intend to run the humidifier in a room rather than cover a whole floor, for example, you should first calculate the square footage of the space you want to use it in and opt for a machine that is rated to cover an area of 100-200 sq. ft. larger than that to make sure that no matter the moisture output level you pick, humidity will spread evenly throughout the whole space.

Whole House Humidification

To increase moisture levels throughout the entirety of the house, you can use either a stand-alone whole house humidifier that features a large-size reservoir and generally attaches to a sink to serve as water supply, or you can opt for a furnace humidifier that is evidently incorporated as part of a furnace and have direct supply for humidification at all times. Regardless of the type you opt for, there are several factors to take into account and make sure you are properly sizing the system to provide whole house humidification, more precisely building tightness, GPD output, area volume, and load determination.

Building Tightness

All buildings are classifiable in one of the following categories – tight, average, and loose. By definition, tight buildings are insulated properly with vapor barriers, and windows and doors with weather stripping. In tight buildings, 1/2 air exchanges per hour occur, whereas, in average and loose buildings that aren’t insulated properly or are old, there is an average of 1 to 1-1/2 air exchanges per hour.

The tighter the building construction is, the better as humidity isn’t needed in absurd amounts to uphold proper RH levels, air exchanges per hour not occurring at a rapid pace.

Output Rating

The GPD output of the humidifier is tightly linked to the square footage it can handle, the higher the gallons per day the system promises to produce under specific conditions, the bigger the area it can cover is. As with single room humidification, you should only vaguely base your decision on this element rather than blindly guide yourself after it as manufacturers only provide test results under specific, ideal environmental and operational conditions.

Area Volume

Knowing the square footage of the building doesn’t suffice for proper sizing as you need to take into account the ceiling height as well. Usually, humidifiers are rated to provide their stated GPD in spaces with 8’ high ceilings. However, if the height to the ceiling is shorter or higher than that rated in the calculation of the machine’s GPD, you must factor in this aspect and make proper adjustments. If the ceiling height is bigger than 8’, then you need a system with a higher GPD than normal for your square footage, while if the ceiling height is shorter than 8’, you can acquire a machine with a slightly smaller GPD rating.

In case the ceiling is vaulted, make an approximate estimation to figure out whether you should go higher or lower with the humidifier’s moisture output. Take a few measurements, one at the highest point to the ceiling, and a few along the road.

Load Determination

The load refers to the humidity that is normally produced in a building through everyday living. For example, a couple (household composed of two members) generates an estimated 1 gallon of water per day that is released into the air as a result of casual activities like cooking, showering, and so on. Take this aspect into consideration by simply multiplying or dividing the aforementioned guiding measurement according to the number of people who reside in your home and deduct the result from the total amount of moisture you reckon is needed for the humidifier to output per day so that normal RH levels are maintained.

Final Words

There is no perfect method to use and make sure 100% beforehand that what you opt for is going to provide the ideal humidification you seek as the manufacturing companies generally provide data regarding the operation of their products under specific circumstances that aren’t met in real life. However, using the guidelines provided here and by taking a few estimations, you are more likely to properly size the moisture-enhancing machine to the room or space you intend to place it in so that low RH won’t be an issue after you install it.

Tobey Hunter
Tobey Hunter
Tobey is the editor-in-chief at Optima Institute, his prior experience as a niche product reviewer in air quality improvement systems helping shape his path to conduct thorough research toward selecting and analyzing products so that customers are satisfied with the provided recommendations. During his carrier, Tobey also gathered technical input that makes him well versed in understanding advancements in modern systems, helping readers in turn to better understand how modern devices help improve life quality.