The Effect of Cooling on the Indoor Environment

The Principles of Cooling a Building

Cooling a building isn't about "making cold" but rather about removing heat. This is a fundamental concept in thermodynamics. There are two primary ways to achieve this: by transferring heat using a refrigerant or by using water and evaporation. The method chosen depends on the size of the building, the climate, and the required cooling capacity.

Residential Cooling Systems

Most residential cooling systems, like a home air conditioner, operate on a direct expansion (a refrigeration cycle where the refrigerant itself absorbs heat directly from the air) principle.

• The Refrigeration Cycle: This is the core process that makes cooling possible. It involves a chemical called a refrigerant (a substance that easily changes from a liquid to a gas and back again). The cycle has four main components:

  1. Compressor: This is the "pump" that pressurizes the refrigerant gas, making it hot.

  2. Condenser: Located in the outdoor unit, this coil releases the heat from the hot refrigerant into the outside air. As the refrigerant cools down, it turns back into a liquid.

  3. Expansion Valve: This device restricts the flow of the liquid refrigerant, causing its pressure to drop. This sudden drop in pressure makes the liquid rapidly cool down.

  4. Evaporator: Located in the indoor unit, this coil contains the now-cold refrigerant. A fan blows warm indoor air across the cold coil, and the refrigerant absorbs the heat, causing the air to cool. As the refrigerant absorbs heat, it evaporates and turns back into a low-pressure gas, returning to the compressor to start the cycle over.

• Evaporative Coolers (Swamp Coolers): These are a low-tech, energy-efficient option used primarily in dry climates (areas with low humidity). They work by drawing in hot, dry outdoor air and passing it over water-saturated pads. The water in the pads evaporates, and this process of evaporation naturally removes heat from the air, cooling it down before it's blown into the home. These systems add moisture to the air, which is a significant drawback in humid environments where it can make the indoor air feel clammy.

Commercial Cooling Systems

Commercial buildings, especially large ones, require a much greater cooling capacity and often use a different approach that separates the refrigeration process from the air delivery.

• Chillers: Instead of cooling air directly, these large systems cool water. A chiller uses the same refrigeration cycle as a home AC unit, but its job is to chill a large volume of water to around 42−45∘F (5.5−7.2∘C). This chilled water is then pumped through a network of pipes to different parts of the building.

• Cooling Towers: Water-cooled chillers (chillers that use water to dissipate heat instead of air) work with cooling towers. A cooling tower's job is to remove the heat that the chiller pulled from the building's water. It does this by spraying the hot condenser water over a porous surface while a large fan draws ambient air across it. A small amount of water evaporates, and this process effectively cools the rest of the water, which is then sent back to the chiller to start the cycle again.

• Air Handling Units (AHUs): This is where the cooling actually happens for the occupants. The chilled water from the chiller is pumped into the AHU, which is a large metal box containing fans and a coil. The AHU's fan blows indoor air across the cold water coil, cooling the air before it's distributed through the ductwork to different zones of the building.

• Variable Refrigerant Flow (VRF) Systems: This is a more modern, efficient technology that operates similarly to a residential mini-split system, but on a much larger scale. A single outdoor condensing unit connects to multiple indoor units, each with its own fan and coil. The key feature is the variable-speed compressor (a compressor that can adjust its operating speed to match the cooling demand). This allows the system to deliver precisely the amount of cooling needed to each individual room or zone, saving a significant amount of energy compared to traditional systems that are either fully on or fully off.

The purpose of a building's HVAC system is to do more than just lower the temperature; it is a critical tool for managing both temperature and humidity, which directly impacts occupant comfort and indoor air quality.

Sensible Heat vs. Latent Heat

To understand how an HVAC system handles temperature and humidity, it's important to differentiate between the two types of heat it removes from a space:

• Sensible Heat: This is the heat that you can feel and measure with a thermometer. It's the heat energy that changes the temperature of an object or substance. When you feel a room is hot, you're feeling sensible heat. An air conditioner's primary job is to remove this heat to make the air feel cooler.

• Latent Heat: This is the heat energy that is "hidden" in the phase change of a substance, such as when water evaporates into vapor. You cannot measure latent heat with a thermometer because it doesn't cause a temperature change; it's what's required to turn a liquid into a gas without changing its temperature. The moisture in the air is a form of latent heat. When you feel a room is "sticky" or "muggy," you are feeling the effect of high latent heat.

The Dehumidifying Function of an HVAC System

The HVAC system's ability to remove humidity is a direct result of the refrigeration cycle we previously discussed. The process of removing latent heat happens at the same time as the removal of sensible heat.

When warm, humid air from the building is pulled through the return ducts and passes over the cold evaporator coil (the part of the indoor unit where refrigerant absorbs heat), two things happen simultaneously:

1. Sensible Heat Removal: The air's temperature drops as the coil absorbs its sensible heat.

2. Latent Heat Removal: As the air cools, it drops below its dew point (the temperature at which the air can no longer hold all of its water vapor, and the vapor turns into liquid water). When this occurs, the water vapor in the air condenses on the surface of the cold evaporator coil, just like condensation forms on the outside of a cold glass of water on a hot day. This condensed water is collected in a pan and routed away through a drain line.

This process of condensation releases the latent heat that was stored in the water vapor, effectively removing the moisture from the air. By removing both sensible and latent heat, the HVAC system provides not only a cooler but also a more comfortable and less humid environment.

How an Air Conditioner's Cooling Capacity Works

It's a common misconception that turning a thermostat to its lowest setting will cool a building faster. In reality, an air conditioner's ability to cool is based on a fixed temperature differential—the amount of temperature reduction it can achieve across the unit. Cranking the thermostat down doesn't make the air colder; it only causes the system to run continuously, wasting energy and not cooling the space any faster.

The Fixed Rate of Cooling

An air conditioners' cooling capacity is basically fixed. The system's main job is to remove heat and moisture from the air and push it back into the room. It does this at a set rate. The air passing through a properly functioning AC unit is typically cooled by a specific amount, known as its Delta T (a scientific term for the change in temperature).

For most residential and light commercial units, this temperature drop is between 18−22∘F. This means if the air entering the unit from the room is 80∘F, the air exiting the unit will be around 58∘F to 62∘F.

Whether you set your thermostat to 75∘F or 60∘F, the air coming out of the vents will have that same 18−22∘F temperature drop. The system's cooling ability doesn't change based on your thermostat setting.

The Inefficiency of "Cranking Down" the Thermostat

When you leave a building and turn the air conditioning off, the indoor temperature will rise, often to 85∘F or higher in the summer. When you return and "crank down" the thermostat to 60∘F, you're telling the system to cool the space to a temperature it cannot realistically achieve.

Instead of cooling faster, the unit simply runs nonstop in a cycle that will never reach the desired temperature. The air coming out of the vents will still be cooled by the same 18−22∘F, but the system will consume a huge amount of energy trying to reach a setting that is well below its effective cooling range. This not only wastes money but also puts unnecessary wear and tear on the unit.

The most effective way to cool a home is to set the thermostat to your desired temperature—such as 75∘F—and let the system run until it reaches that temperature and then cycles off. For even greater energy savings, a programmable thermostat can be used to raise the temperature when the building is unoccupied and then begin cooling a little while before you return.

Condensation is a very common source of moisture in buildings, especially during the cooling season. When cold air from an air conditioning system cools a surface below a certain temperature, the water vapor in the warmer, more humid air around that surface will condense into liquid water.

Condensation Around Air Registers

During the summer, an air conditioner's cold air passes through metal registers and chills their surface. When this cold metal is exposed to the warm, humid air inside a room or from an unconditioned space, the metal's temperature often drops below the dew point (the temperature at which air becomes saturated and water vapor begins to condense).

This causes the water vapor from the room's air to condense on the surface of the register. This is similar to what happens on the outside of a cold glass of water on a hot day. Condensation on a ceiling register, for example, can drip onto the drywall below, leading to water stains and, over time, a high risk of mold growth.

Leaky Ductwork in Unconditioned Spaces

A similar problem occurs with air ducts that run through unconditioned spaces like an attic or a crawlspace. These ducts are not air-sealed from the outside and can be exposed to the hot, humid air in those spaces.

As the air conditioning system sends cold air through the ducts, the exterior of the ductwork also becomes very cold. When the hot, humid air surrounding the ducts hits this cold surface, the water vapor in that air condenses. This causes the ducts to "sweat." The moisture from this condensation can then drip down, soaking insulation, drywall, and wood framing, creating a perfect environment for mold growth and structural damage.