Air Conditioners And Dehumidifiers

The truth is that a dehumidifier is nothing more than a modified air conditioner. Thus, before we discuss how a dehumidifier works, we’ll first talk about how an air conditioner works. Once you understand how AC systems work you’ll almost immediately be able to understand how dehumidifiers work. We’ll finish our guide by discussing the differences between dehumidifiers and AC systems and therein go over some of the unique features and functionality that make dehumidifiers able to dehumidify as efficiently as they do. By the end of this guide you should not only be well educated on how dehumidifiers work, but also have a very strong understanding of how AC systems work and how dehumidifiers and AC systems are similar but necessarily different in order for each to serve its own purpose most effectively.

Note that in the guide that follows, in describing how a dehumidifier works, we will be, more specifically, be describing how a compressor based dehumidifier works. While not all dehumidifiers are compressor based, most of the dehumidifiers we’ve tested and reviewed and the vast majority of dehumidifiers on the market are in fact compressor based. If you’re talking about a 50, 35, 22 pint or most commercial dehumidifiers you’re talking about a compressor based dehumidifier. Thus, for the rest of this guide, for the sake of convenience and because it represents what most people think of when they think of a dehumidifier anyway, we’ll refer to a “compressor based dehumidifier” simply as a “dehumidifier”. The two other types of dehumidifiers (thermo-electric and desiccant) are much less common. That being said, we have written comprehensive guides for these other types of dehumidifiers also. For more information on how thermo-electric dehumidifiers work see here. For more information on how desiccant dehumidifiers work see here.

How Air Conditioners Work

Air conditioners are found in any place that requires cooling – this includes homes, hotels, cars, and even boats. To simplify things we’re going to limit the scope of this discussion to central air conditioning systems, most of which are split systems – those air conditioning systems commonly found in homes and offices in which some of the parts that make up the air conditioning system are found inside of the home or office building and other parts are located outside of the home or office building. We’ll discuss the exact parts we’re referring to and their locations in greater detail later.

Before we begin, it’s important to know and understand that air conditioning systems are closed pressurized systems. Flowing through the system is a “magic fluid” that facilitates heat transfer. This “fluid” is called the refrigerant. The refrigerant is a chemical compound with the physical and chemical characteristics necessary to facilitate the proper operation of the system. This means that it needs to have the correct boiling point, freezing point, etc. to allow for the system to work correctly. If the chemical doesn’t meet the exact criteria required by the system then it cannot be used. This is why most residential and commercial air conditioning systems use one of only two different types of refrigerants: R-22 and R410A. Most dehumidifiers use R410A refrigerant.

Air Conditioner Parts

Both air conditioners and dehumidifiers share the same four critical components. They are:

1. Compressor – compresses refrigerant

2. Condenser – condenses refrigerant

3. Expansion Valve – facilitates abrupt reduction in pressure

4. Evaporator – evaporates refrigerant

Note that the name of each part tells you exactly what it does – the compressor compresses, the condenser condenses, the expansion valve “expands” (facilitates drop in pressure), and the evaporator evaporates. The design of the compressor and expansion valve, while interesting, is not fundamental to understanding how an air conditioner works. Do note that both the condenser and evaporator have a coiled and/or finned design (through which the refrigerant flows) to maximize exposed surface area (to the surroundings). The reason for this design choice is that it ensures maximum efficiency in transferring heat. As you’ll see below, most of the heat transfer within the system occurs at the evaporator and the condenser.

The Refrigerant Tells The Story

To better understand how central air conditioning systems work, let’s follow the path of the refrigerant as it travels through the system. Note the temperature, pressure, and phase (liquid/gas) of the refrigerant during each stage of the process as these qualities dictate the direction of heat as it moves into or out of the system.

We’ll start off with the refrigerant as it leaves the air conditioning system’s evaporator. Cool low pressure refrigerant vapor enters the compressor (located outside the home next to the condenser) where it is compressed into a hot high pressure vapor. The vapor is hot at this point because it has picked up heat from two different sources. It picked up heat from the warm air that was cooled at the evaporator and it also picked up the heat that was generated when the vapor was compressed from low pressure to high pressure inside the compressor.

The hot high pressure vapor then enters the condenser where the vapor is condensed into a much cooler but still hot high pressure liquid. The condenser, like the evaporator, has a multitude of metal fins with a large amount of surface area to facilitate the dissipation of heat. There is also a fan over the condenser that aids this process (the removal of heat from the system). The condenser is located outside the home (it’s the large metal box outside your home with a large fan over it).

The hot high pressure liquid leaves the condenser coils and is then routed back to the system’s evaporator (inside the home). Before it enters the evaporator the liquid is forced through an expansion valve. The expansion valve facilitates an abrupt reduction in pressure. This reduction in pressure causes the hot high pressure liquid to evaporate into a mixture of cool low pressure vapor and liquid. It is the partial evaporation of the hot liquid that causes a drop in the resulting mixture’s temperature (the process of evaporation requires energy and thus thermal energy is transferred from the system to facilitate evaporation – this is what cools the refrigerant).

The cool low pressure mixture is then routed to the system’s evaporator (usually located above/near your home’s furnace). Here, a fan blows warm air (from inside the home) onto the evaporator coils which causes the remaining liquid portion of the mixture to evaporate. This evaporation causes a further drop in the refrigerant’s temperature. We now have the cool low pressure vapor that eventually finds its way back to the system’s compressor.

Note, that in describing the process by which an air conditioning system works above, we have described two different ways in which air is cooled.

1 For the liquid part of the vapor/liquid refrigerant mixture within the evaporator coils to also turn into a vapor it needs energy. This energy is supplied in the form of thermal energy from the warm air that is being pulled onto the evaporator coils. Thus, an energy transfer takes place. The warm air is cooled and the liquid component of the refrigerant mixture inside the evaporator coils gets the energy it needs to change phase from a liquid to a vapor.

2 One of the basic principles of thermodynamics is that heat likes to flow from something warmer to something cooler. Thus, heat is also transferred from the warm air moving over the system’s evaporator coils, to the cooler refrigerant within the coils. The heat from the warm air is not enough to dramatically affect the refrigerant’s temperature. It remains a cool low pressure vapor as it enters the system’s compressor.

How A Dehumidifier Works

Differences Between AC Systems and Dehumidifiers (Compressor Type)

AC systems and dehumidifiers are nearly identical in both form and function. The one major difference between them in terms of functionality, is the fact that AC systems are optimized for cooling air while dehumidifiers are optimized for dehumidifying air. It’s important for us to note here that both systems do dehumidify air. The difference is that dehumidifiers are optimized for it – they remove moisture from the air at a much higher rate and much more efficiently than AC systems do. Note that both systems do not cool air. Dehumidifiers actually slightly warm processed air. AC systems exclusively cool processed air.

As far as design (form) is concerned, both AC systems and dehumidifiers consist of exactly the same four critical components – a compressor, condenser, expansion valve (or similar device), and evaporator. The major difference in design deals with the location of each system’s compressor and condenser. Central air conditioning systems have their compressor and condenser units located outside of the home and their evaporator units inside the home. Dehumidifiers have all of their components located within close proximity of each other within the same appliance. Thus, a central AC system has what is called a “split-system” design while a dehumidifier is an all-in-one appliance.

The location of each of the critical components we listed above is important as it relates to what each system is optimized for (AC systems for cooling and dehumidifiers for dehumidifying). Again, the evaporator and condenser coils are in two separate locations in a central AC system. One is located inside the home (evaporator) and one is located outside the home (condenser). In an AC system the air that is cooled over the system’s evaporator coils is circulated back into the home as the cool air coming out of the floor vents in your home. In a dehumidifier the air that is cooled over the system’s evaporator coils is instead immediately pulled through the dehumidifier’s condenser coils before it exhausts out of the top, back, or side of the dehumidifier. Why the difference in design?

Remember, as we discussed above, the refrigerant that leaves an AC system’s compressor is a hot high pressure vapor. This vapor enters the system’s condenser and is condensed to a hot high pressure liquid. This phase change (the refrigerant changing from a vapor to a liquid) requires an energy transfer between the refrigerant and the ambient air. It requires that the air around the condenser coils be colder than the coils (and the refrigerant within the coils), to allow for thermal exchange in the correct direction (thermal energy needs to move from refrigerant to the air to facilitate condensation of the refrigerant). If the ambient air isn’t colder than the condenser coils then the refrigerant within the coils won’t condense. This process is aided by a large fan (over the condenser) and the design of the condenser coils themselves (they are spread out over a large surface area to facilitate more efficient heat dissipation).

So, in a central AC system, the ambient air’s relative lower temperature (aided by the unit’s fan and condenser coil design) acts to condense the refrigerant within the condenser coils. This process takes place outside the home, and the air relevant to the process is the air outside the home. In a dehumidifier, on the other hand, the same cold air that was cooled over the unit’s evaporator coils is also used to “cool” the condenser coils and condense the refrigerant vapor leaving the dehumidifier’s compressor to a hot high pressure liquid. This means that the air that was cooled by the dehumidifier’s cold evaporator coils is almost immediately warmed by moving over the unit’s condenser coils.

The air that exhausts out of a dehumidifier is therefore warm dry air – air that is slightly warmer (and much dryer) than the air that entered it – the ambient air. Remember that a dehumidifier is used to dehumidify air, not cool it. By this design it achieves the goal of dehumidifying air, without cooling it.

How Does Condensate Form?

Thus far we’ve seen how AC systems work and also how dehumidifiers work. We’ve seen all of the parts that make up each system and how they work together to cool air that enters each system. But how does the cooling of air facilitate moisture removal? We answer this question next.

As we discuss here, temperature and humidity are inherently related. As temperature increases, assuming a constant amount of moisture in the air, relative humidity will decrease. Conversely, if temperature decreases, relative humidity increases. This relationship between temperature and humidity is what enables the heating/cooling technology of AC systems and dehumidifiers to dehumidify air. A drop in temperature causing an increase in relative humidity is the reason why condensate forms on either system’s evaporator coils. As the air immediately surrounding the coils is cooled its relative humidity can shoot up past 100% at which point condensate forms.

This is also what meteorologists refer to as the dew point – it’s simply the temperature (a low temperature) at which dew (condensate) forms. AC systems and dehumidifiers are essentially bringing temperatures to a localized dew point next to their evaporator coils. In a central AC system the condensate drips down into a set of collection trays (which are either attached to a condensate pump or connected to a floor drain) while in a dehumidifier the condensate drips down into a large removable condensate collection bucket (it can of course also be drained).

Other Ways In Which A Dehumidifier Is Optimized For Dehumidification

We saw earlier that a dehumidifier’s design allows it to dehumidify air without cooling it permanently. Thus, dehumidification is achieved without greatly affecting the temperature of the air that is dehumidified. Central air conditioners dehumidify air, but at the cost of permanently cooling the air in the process.

Dehumidifiers have several other advantages over AC systems when it comes to dehumidification. First and foremost, most dehumidifiers are equipped with a built-in humidistat while AC systems are not. AC systems are instead controlled by a thermostat. Just like you set a desired temperature on a thermostat, you can set a desired humidity level on a dehumidifier’s hydrostat. The dehumidifier will cycle on (compressor and fan turns on) when it senses that the actual room humidity is above the desired humidity level and it will automatically cycle off when it senses that the actual room humidity has fallen below the desired humidity level. This is one of the most important ways in which a dehumidifier is optimized for humidity removal. It can be set to a desired humidity level and it will stay on for as long as it takes to achieve that desired humidity level. There’s no way to control humidity in this way with a central air conditioning system.

Other advantages that dehumidifiers have over AC systems include:

1. Portability – they can be moved to different locations within the home where they can be used to dehumidify only specific areas. Dehumidifiers can also be used to dehumidify areas that cannot be reached by your home’s central AC system – a garage, a shed, etc.

2. Additional features such as timers and defrost modes – dehumidifiers can be set to cycle on or cycle off at specific times. Many also offer additional modes to enable more efficient moisture removal.

3. Much lower power consumption – dehumidifiers draw much less power than central air conditioning systems.

And to summarize those advantages we already discussed above:

4. Unchanged air temperature – dehumidification is achieved without greatly affecting air temperature.

5. Built-in hydrostat – allows the consumer to set an exact desired humidity level.

Final Thoughts

In reading about the differences between AC systems and dehumidifiers above, you may not be convinced that you need a dehumidifier. Especially during the summer, can’t you just run your AC to reduce humidity in your home?

The truth is that while AC systems do remove some humidity, moisture removal within these systems is nothing more than a side effect. Moisture removal is a product of the system’s evaporator coils cooling the air. The primary aim of the system, however, is to cool the air. That’s what it’s designed to do.

A dehumidifier, on the other hand, while borrowing much of an AC system’s design, is first and foremost built to dehumidify air. It does so much more efficiently. Air in the room is pulled directly onto the unit’s evaporator coils where it is immediately dehumidified at a very high rate. The air is exhausted out of the unit much drier and at close to the same temperature at which it entered the dehumidifier.

The bottom line here is that while both air conditioners and dehumidifiers share many of the same components and much of the same design, they are two very different appliances with two very different functions. If your aim is to cool the air in your home and thereby slightly reduce the moisture in that air then run your home’s air conditioner. If your aim is to dehumidify air that is at a high uncomfortable level of humidity then you definitely want to run a dehumidifier instead.