Brief Summary
This video explains how dehumidifiers work, focusing on their primary function of protecting against moisture damage rather than enhancing comfort. It details the science behind dehumidification, including relative humidity, latent heat, and the refrigeration cycle, to clarify why dehumidifiers generate heat and how they interact with air conditioning systems. The video also offers practical advice on choosing effective dehumidifiers and discusses alternative technologies for managing humidity.
- Dehumidifiers are designed to protect against moisture damage, not to enhance comfort.
- The process of dehumidifying air generates heat due to the conversion of latent heat into sensible heat.
- Dehumidifiers are most effective in unconditioned spaces like basements and garages.
Intro - they do not make you comfy
The video introduces dehumidifiers and their function of removing water from the air to reduce humidity. The purpose of dehumidifiers is not to make people comfortable or to aid air conditioning systems. Dehumidifying air generates heat, potentially as much as a space heater, which can be counterproductive during hot weather. The video aims to explain how dehumidifiers work, revealing that they are more complex than they appear and understanding their operation can change how and where they are used. If there isn't a significant humidity problem in a home, a dehumidifier is unnecessary and will only waste energy and produce unwanted heat.
Basics of humidity problems
Indoor relative humidity persistently above 65% is considered problematic, leading to mold, mildew, accelerated corrosion, material degradation, pest attraction, and even structural issues. Dehumidifiers are commonly used in damp environments like Midwestern basements and crawl spaces, especially those prone to stormwater seepage. Basements and crawl spaces are often unconditioned or poorly conditioned, and their cooler temperatures lead to persistently elevated humidity. Relative humidity indicates air's ability to absorb more moisture, with warmer air capable of holding more moisture than cooler air. Cooling air increases relative humidity, making it harder for liquids to evaporate and increasing the risk of damage.
Dehumidifiers protect your stuff
Dehumidifiers are a form of air conditioning that extracts moisture from the air without providing cooling, making them less energy-intensive. They are suitable for spaces that are cool but damp, maintaining a set humidity level by switching on and off as needed. The primary function of a dehumidifier is to protect belongings from humidity damage by keeping the air dry.
Recommendations and advice
To be effective, a dehumidifier needs to cost more than $100 and be a heavy, substantial machine. Small, cheap dehumidifiers are generally ineffective. Dehumidifiers are rated by pints of moisture removal per day. To avoid manually emptying the water bucket, a condensate pump can be used to automatically drain the dehumidifier. These pumps use a float switch to activate and pump the collected water to a more convenient drain location.
What's inside the machine
A dehumidifier contains the same components as a small window air conditioner, including a compressor, blower fan, evaporator coil, and condenser coil, simply rearranged. Unlike an air conditioner, a dehumidifier is controlled by a humidistat rather than a thermostat and collects condensate in a bucket or drain. It operates like an air conditioner but does not cool the air; instead, it heats the air when running in a humid environment.
A demo of surprise heat
A dehumidifier was placed in an enclosed bathroom with a temperature data logger to measure the exhaust temperature. Initially, the dehumidifier consumed 300W of power, and the exhaust temperature slowly rose to about 90°F (32°C). When humidifiers were added to increase humidity levels, the dehumidifier worked to counteract this, and its exhaust temperature climbed significantly, reaching over 100°F (38°C) in 15 minutes and nearly 105°F (40°C) by the end of the hour. This increase in temperature demonstrates the large amount of heat energy being expelled, despite the dehumidifier only using 300W.
Heat pumps and the refrigeration cycle
An air conditioner moves heat energy from one location to another by exploiting the physical properties of a refrigerant. Refrigerants have specific relationships between their boiling points and pressure. The refrigerant, such as difluoromethane (R32), is compressed, which elevates its boiling point. As the refrigerant condenses, it releases heat, and when it passes through a metering device into a low-pressure area, its boiling point plummets, causing it to vaporize and absorb heat. The refrigerant cannot boil away until it absorbs its latent heat of vaporization, which is the heat energy required to change phases from a liquid to a gas.
Water condenses on cold surfaces
The goal is to create a cold surface for water in the air to condense on, removing water vapor from the air. Air can only hold a certain amount of water vapor, and cooler air holds less. Cooling air below its dew point (the temperature at which it becomes fully saturated) causes excess moisture to condense on cold surfaces. A dehumidifier forces air through a large, cold surface to cause condensation on a large scale, reducing humidity in a space.
Water's latent heat weirdness
Refrigerants aren't the only substances with a latent heat of vaporization; water also has this property. Water vapor releases latent heat energy when it condenses into a liquid. This latent heat is difficult to notice because it cannot be felt. When condensation forms on a cold beverage, the water imparts its latent heat into the can, causing the contents to warm up more quickly.
Why dehumidifiers convert latent heat to sensible heat
A dehumidifier converts the latent heat from condensing water vapor into sensible heat that can be felt. In a dehumidifier, the evaporator absorbs heat, and the condenser releases heat. When humid air passes through the cold evaporator, water vapor condenses, releasing latent heat that the refrigerant absorbs. The evaporator can only absorb a certain amount of heat, so if it absorbs latent heat from condensing water, it loses its ability to cool the air. The condenser releases all the heat the evaporator absorbs, latent or not, converting the latent heat into sensible heat.
Dehumidifiers don't help your air conditioner
Air conditioners also deal with latent heat from water vapor condensing on their evaporators, but they move that heat outside. An air conditioner's cooling capacity is reduced by the energy it spends condensing water out of the air. Adding a dehumidifier does not reduce the total heat load; it simply converts the water's latent heat into sensible heat, which the air conditioner still has to handle. Using a dehumidifier in an already air-conditioned space generally does not help.
Can they save money?
Whether dehumidifiers save money depends on the situation. They are beneficial if they prevent moisture damage. If a dehumidifier removes more moisture per kilowatt-hour than an air conditioner and the air conditioner can handle the extra heat, it might lower energy costs. However, dehumidifiers are not very energy-efficient, and modern air conditioning systems are highly efficient. A dehumidifier might save costs when used while away from home and not using air conditioning, ensuring humidity doesn't get too high.
Cold temperature performance problems
Dehumidifiers do not work well in colder temperatures because their simple refrigeration circuits cause the refrigerant pressure in the evaporator to drop too low, potentially freezing the condensed water. This can plug up the evaporator with ice, limiting its effectiveness. Decent dehumidifiers can detect this and shut off the compressor to allow the ice to melt. This issue may not matter in climates where dehumidifiers are not needed in cold weather.
Alternative technologies
Rotary desiccant dehumidifiers do not have the cold ambient performance problems of vapor-compression dehumidifiers but have other drawbacks. Small, cheap dehumidifiers that use a Peltier element instead of a refrigeration circuit are ineffective. Proper dehumidifiers have large catch buckets and remove significantly more water from the air while being energy-efficient due to the power of a heat pump.
