A Contractor’s Guide to Humidity Control

humidity control

Comfort Basics

Humidity is a key factor when it comes to interior comfort. Low or high RH can cause physical discomfort for the occupants and can damage products inside the building such as instruments or hardwood flooring. High humidity over time can become a health hazard, creating an environment for mold and bacteria growth, especially in levels above 50-70%. Interiors with high humidity are an ideal space for odors to develop, asthma to worsen, and condensation that can damage buildings.

Comfortable interior humidity levels vary based on location and climate. ASHRAE Standards state that the ideal ranges for comfort in the winter are 68 °F – 74 °F at 40-60% relative humidity. In the summer, this range is 73 °F – 79 °F at 40-60% relative humidity. These levels provide the best levels for comfort and for preventing unwanted microorganism growth and changes to building materials and fixtures.

Challenges with AC Sizing

As homes have become more energy efficient with better insulation and ventilation, contractors have been able to rethink AC sizing since cooling loads have shrunk over the past several years. However, although cooling loads have shrunk, latent loads (which involve moisture and humidity) are not as readily reduced. Because AC sizes have been optimized to better meet cooling loads, they may not be optimized for latent heat (humidity in the air) removal. More than ever, it is important that HVAC systems are correctly set up to remove moisture. If they are not properly installed to maximize humidity control, a possible result is cold and humid homes where the moisture in the air can develop into mold and cause structural damage.

Naturally, if the air conditioner is too small for the building, it will not be able to remove all of the humidity in the space or provide the proper cooling at times. On the other hand, when an AC unit is oversized, the system does not run as frequently due to its larger size. It quickly brings down the temperature, but the humidity itself is not removed in such a short time.

Now, it is more important than ever to perform a load calculation and determine the heat loss and gain of every building where an HVAC system is going to be replaced.

Fan Speed Adjustments

To ensure that the building’s HVAC system is working at maximum efficiency for controlling humidity levels, the fan speed needs to be checked and adjusted if needed. Any changes will be based on the size of the AC paired with the furnace and ductwork capabilities to make sure that the setups and sizes match and run together efficiently.

Lowering the fan speed is one way to control a large amount of interior humidity. A colder evaporator coil can remove more moisture from the air, but it is important that the evaporator coil doesn’t freeze. This would mean that it could no longer absorb sufficient heat and would block air from blowing across the evaporator coil; a condition like this can damage the compressor. Make sure to first understand what the thermostat, blower board, and motor are capable of and are already doing before making adjustments. Another precaution to keep in mind is that if the fan speed is overly low and is combined with an Electronic Air Cleaner, ozone may be produced, causing harm to the occupants’ health.

Unfortunately, when the fan remains running at a slower speed, this can lessen the overall efficiency of the system. However, when the AC works continuously at a low speed, interior humidity is reduced, and temperatures are more balanced.


In order for your HVAC system to run correctly and dehumidify the interior space efficiently, your equipment needs to be sized correctly before installation or replacement installation needs to be done properly with fan speeds set to the desired speed for a balance of efficiency, even temperatures, and moisture reduction. Equipment controls should also be set to a point that will best optimize the system.

Read Next: Top 5 Things a Homeowner Expects in Their HVAC System

humidity control


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4 thoughts on “A Contractor’s Guide to Humidity Control

  1. For AC, lowering fan speed can help help you achieve a higher temp differential across the coil. This is just one potential method to maximize existing equipment in an effort to remove moisture and lower the humidity. The assumption is that your evaporator coil is cold enough to condense the moisture in the air. Another way is to ensure your system runs long enough to have time to remove the moisture from the air.

    In your TD of 35 and high fan speed scenario, that points to a system that has a significant capacity to not only achieve a high Temp Diff but to also do that at high airflow across the coil.

    It is not unusual to reach the dewpoint at the coil which is an effective means to maximize moisture removal. In dehumidifiers that is the goal and then apply reheat to get the supply air back up above dewpoint. Dewpoint is the temperature to which air must be cooled at the evaporator coil to become saturated with water vapor and form dew (condense). If the Relative Humidity (RH) is 50% and the indoor air temperature is 80°F, the dew point would be 59°F. So if the coil is at or below 59 deg then moisture removal is being maximized.

  2. Getting the AC Units air down to dewpoint is key to removing moisture from the air passing across the evaporator coil.

    Dewpoint is not the same as Relative Humidty.

    55%RH is a target to achieve.

  3. I am confused kindly enlighten me.
    in a Fruit Refrigerator, the Fan speed is lowered + a lower TD in other to maintain high RH in the space according to Commercial Refrigeration Handbook.
    So I imagine that the TD of 35 in AC and high fan speed can remove more moisture from the space?

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