Dehumidifier Effectiveness in Winter
How effective is your dehumidifier in winter? The answer probably depends on when and where you are using it.
Dehumidifiers used in a crawl space are an effective tool in combatting moisture. They are often an integral part of your crawl space moisture control system. However, they have limitations that may affect their ability to be effective in certain climates and/or seasons. As temperatures decrease, the amount of moisture that can be removed by the dehumidifier decreases too.
Let’s look at a commonly-used crawl space dehumidifier rated for 70 pints per day at 80˚F. If you look at the first dehumidifier in the graphic, you can see that at 80˚F, it will extract 70 pints per day and has the ability to dry the air to the range of 25-28% relative humidity. All looks great, right? Now compare to the lower one. This is the same dehumidifier at 55˚F. At this lower temperature, it can only extract 10-15 pints per day, and it can only achieve a 57-66% relative humidity. Keep this in mind as you keep reading.
A good measure of moisture in your crawl space is relative humidity. Relative humidity is the amount of moisture in the air at a given temperature. It is important to control the humidity inside your crawl space to prevent moisture buildup. A dehumidifier is one tool to combat this moisture that can lead to issues for your home. A dehumidifier is generally set to run based on a target humidity level to control the moisture level within your crawl space. If you are using a crawl space dehumidifier, where do you have it set to operate?
How a Dehumidifier Works in the Winter
The dehumidifier works to dry your space by pulling warm moist air into the unit and cooling the air to extract the moisture. It cools the air so the moisture in the air condenses and becomes water. The condensed water drains to the outside. The resulting drier air is heated and expelled back into your crawl space. A sensor located in the dehumidifier is set to a specific humidity target. The unit continues to cycle the air until it reaches the set target humidity level.
The amount of moisture removed from the air is typically measured in pints of water removed in a 24-hour period. Many dehumidifiers tout their pints/day assuming conditions of 80˚F and 60% humidity. However, it is important to understand that the cooler the temperature the less effective the dehumidifier will be. Therefore, the pints of water extracted per day decreases as the temperature decreases. Many dehumidifiers are in a basement or in a crawl space where the air temperature tends to be cooler. In other words, your dehumidifier may not be as effective in winter or in cooler spaces as you think!
Dehumidifiers are most effective when the ambient air is above 60°F and less effective in winter when temperatures are much cooler. In spaces with temperatures below 60˚F, it becomes more and more difficult for the dehumidifier to further cool the air and results in small amounts of moisture removed from the air. The dehumidifier’s coils must be cooler than the air to condense the air to remove the moisture.
This graph shows the decreased effectiveness of a high capacity energy efficient dehumidifier at lower temperatures. This study performed by Acker & Associates is representative of how dehumidifiers work across the board.
Standards for testing portable residential dehumidifiers have recently been updated. Specifically, the new standard is to test the portable units at 65˚F assuming a 60% relative humidity or 73˚ for whole house dehumidifiers. These testing standards were mostly likely updated to better represent the temperatures of a crawl space or basement where these portable units are often placed. The conditions of your crawl space or basement rarely reach temperatures as high as 80˚F where testing standards were previously set. Accordingly, with a 15˚ drop in the testing standard, you will notice the rating of the pint per day capacity will decrease as well even though nothing about the dehumidifier has actually changed.
Target Humidity Level
For a variety of reasons, a crawl space dehumidifier’s target humidity level is often set low – in the 50-55% range. This may be to compensate for far reaching corners away from the dehumidifier or simply because of the “more must be better” mindset. A dehumidifier runs until it receives a signal to stop – typically when its sensor reads a humidity level below its target. However, what happens if the target humidity level is unattainable?
Let’s revisit the example at 55˚F keeping in mind that there are crawl spaces that sometimes get colder than this. At this temperature, the unit will continue to cycle the air but may never reach its set humidity level under 55%. This is extremely inefficient from an energy standpoint as the dehumidifier is working non-stop with little benefit. This can lead to an overdrying in areas closer to the dehumidifier. Furthermore, this can even lead to a freeze up or burning out of the unit if it does not shut off.
It is important to consider the effectiveness of your dehumidifier in winter and cooler spaces. Temperature does play a role in the effectiveness of your dehumidifier. Therefore, read your instruction manual for your specific dehumidifier. Various brands and models vary in capacity, operating temperatures, and other important considerations. Additionally, consider the temperatures of your space and other drying methods to complement your dehumidifier.
When a dehumidifier is connected to an ATMOX controller, temperature is evaluated to avoid operation in extreme cold conditions to protect the dehumidifier. Additionally, the ATMOX controller allows for a combination of drying from both the dehumidifier and outside air when beneficial.
When temperatures are colder outside, the air tends to be very dry. Therefore, using outside air in these conditions to dry the crawl space is much more effective and efficient. Both the dehumidifier and outside air are effective drying tools when used and controlled properly. Each has strengths during different temperatures due to seasonality. The ATMOX controller will direct the operation to use the best tool for moisture reduction at any given time.