Dehumidifier selection is one of the most important elements of the drying process, yet significant pressures often interfere with making the best decisions about dehumidifier use.

No other piece of drying equipment receives as much attention from insurance adjusters, with questions asked frequently about the quantity, size and type of dehumidifiers.

This should be no surprise, as humidity control is the single most important drying factor to prevent collateral damages to otherwise unaffected materials. Specific formulas help with basic dehumidifier decisions, but not all same category dehumidifier models perform the same. And the restorer also has to weigh other factors.

This mix of pressures requires the restorer to take extra steps to ensure that each dehumidifier used is providing maximum benefit to the project. There are several components to ensure this happens. These include maintenance, updating technology, proper installation, proper selection and accurate project data Dri-Eaz/Legends Brandused when sizing.


There are three critical areas in dehumidifier maintenance that should receive regular attention. Each directly affects the ability of the machine to control humidity, and if left unattended, will either dramatically reduce efficiency or cause the unit to fail. All three relate to cleaning the machine regularly, and removing collected dust and debris.


Filters are intended to prevent dust from entering and collecting on sensitive interior surfaces of the dehumidifier while still allowing ample airflow.  It is a difficult balance to both filter effectively yet allow high volumes of air into the machine. Each dehumidifier has been designed for use with a specific filter type in order to maintain the best balance between airflow and filtration. It is therefore critical that filters are replaced with the manufacturer’s recommended filter.

As filters load, the dust and debris in the filter will restrict airflow and reduce efficiency. It will also potentially lead to dust passing through the filter and collecting on coils and in pumps. To prevent this, filters must be cleaned and/or replaced regularly, per the dehumidifier owner’s manual. Mismanaging filter maintenance can easily reduce the performance of a dehumidifier by more than 50 percent.


The primary component in a refrigerant dehumidifier that removes moisture is the evaporator coil. The name is a little confusing, as it refers to what happens inside the coil (the refrigerant evaporates into a gas and becomes colder). What we’re concerned about is what happens on the outside surface of the coil.

Because of the cold temperatures inside, water condenses on the surface of the coil, collecting into drops and eventually draining down into a tray and pump. If dust builds on this surface, two problems develop. First, the dust tends to insulate the coil surface and reduces the energy exchange required to produce condensation. Second, the dust fills the spaces where air is supposed to flow, and the overall airflow through the dehumidifier suffers.

Both of these issues dramatically reduce water removal — as much as 90 percent. In fact, these conditions can cause the unit to fail entirely. Regular filter replacement will prevent much of this from occurring.

It is important to inspect the coils and interior surfaces regularly, and if you note excessive dust accumulation, it should be removed. The best method for cleaning this coil surface should be described in your owner’s manual. Coil cleaning products make it easy to clean coils on a regular basis.


The last area for regular maintenance is the pump reservoir and internal pump mechanisms. If you’ve discovered dust and debris on the coil assembly, check your pump. It will likely be filled with the same material.

Over time, this debris can begin to prevent mechanisms like float valves and flow check valves to fail. If they do, the dehumidifier will likely spill water out onto the floor instead of pumping it away, resulting in a very unfortunate “catch and release” program!

Updating technology

One major misconception in the restoration industry is that all dehumidifiers are created equal, that one ”100-pint” dehumidifier will provide the same benefit as another “100-pint” dehumidifier.

In reality, it is much more complex. Newer technologies have allowed low grain refrigerant (LGR) dehumidifiers to overcome a major performance limitation: Continuing to perform well at lower levels of humidity.

A decade ago, the best refrigerant dehumidifiers commonly had a five percent ratio of high-to-low humidity performance. In other words, a 100-pint dehumidifier (when tested at the 80°F/60% RH Association of Home Appliance Manufacturers’ test condition) would produce approximately five pints in low humidity conditions, usually 80°F/20% RH.

Today, newer LGR dehumidifier models are capable of well more than twice this ratio. It may not seem like much of a difference, but consider the following simple math.

Old Technology

Consider a job that requires 250 pints of dehumidification per day. The restorer installs three 100-pint machines (300 pints) to match the job. Just 24 hours later, the conditions have fallen to 20 percent RH. The same three machines are now only removing five pints each, for a total of 15 pints per day.

New Technology

The same project receives three 100 pint dehumidifiers, but these newer machines each pull 13 pints at the 20 percent RH level. With three of the newer LGR units in place, the job is seeing 39 pints of dehumidification, more than twice as much as the older units are able to remove. What impact will that have on drying rates?

Proper installation

Placement of dehumidifiers on a project is important, but not as important as the placement of air movers. To get the most out of each dehumidifier, air movers need to be properly installed to create even and full circulation throughout the affected area. The air leaving the dehumidifier is relatively warm and dry. This air must be promptly distributed throughout the space. In turn, the humid air near materials must be constantly directed toward the dehumidifier.

When air movers are installed properly, the placement of the dehumidifier becomes much less important. A good installation of drying equipment will yield a consistent temperature and humidity throughout the space. Evenly distributed drying is best verified by taking measurements with a hygrometer.

There are also a few other simple, yet important, factors to consider when placing dehumidifiers. Ensure the dry air leaving the machine does not damage sensitive surfaces by blowing directly on them, avoid blowing air directly into the dehumidifier inlet or outlet and try to avoid placing the dehumidifier over sensitive flooring (like hardwoods) in case there is a fault that causes the machine to leak water.

Selection and sizing

For many years, the industry has used formulas from the ANSI/IICRC S500 3rd Edition to size and select dehumidifiers for projects. The calculation in the S500 is a good starting place.

However, several additional factors should be considered when selecting and sizing to a specific job. These include outdoor humidity, anything affected by occupants, the preference of the building owner for speed of drying, the number and complexity of building assemblies and other water sources that may be present. Any one of these factors may create a need for additional dehumidification.

The most important tool for evaluating if more dehumidification is necessary, though, is a hygrometer. If hygrometer readings indicate that the humidity continues to rise after dehumidifiers have been installed, more dehumidification is needed. When humidity begins to level off or decline, enough have been installed.

Generally, it is important to wait at least a half an hour after starting the equipment before making this analysis, as most projects will initially show a “spike” in humidity regardless of how many machines are installed.

Maximize productivity

Dehumidification is a critical part of the restoration process. Several factors have placed pressure on the restorer to limit the number of dehumidifiers used. For these reasons, contractors must make sure they are getting the most production possible from each unit installed.

Proper maintenance, staying current with technology and apply sound principles for sizing and placement of related equipment will help ensure success.

Brandon Burton is the technical education manager for the Restoration Sciences Academy (RSA), a part of Legend Brands. He teaches IICRC-approved classes in the categories of Applied Structural Drying (ASD) and Water Damage Restoration (WRT). Burton has served the restoration community for more than 15 years as an IICRC-approved instructor, ANSI/IICRC S500 chair, RIA restoration council member, and many other industry roles. You can contact him at