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Updated: Aug 18, 2022

This article was edited from an article, “HVAC Systems and Humidity Fan Speeds and Sizing” written by Priority Energy, providers of education and information to HVAC contractors.

"Have you ever been in a building that feels cold and clammy at the same time? Or to be more specific, have you experienced what cold and humid conditions feel like? Chances are you have, hopefully not in your current home or work place, however it seems it is more likely than not.

As our buildings have become more energy efficient through increased insulation levels, air tight envelopes and higher SHGC windows (better windows), our homes are facing new challenges when it comes to cooling. These improved buildings have caused our cooling loads to shrink over the last decade+, and our HVAC contractors have finally been forced to rethink their air conditioner sizing policies and methodologies. The adoption of our IECC energy codes (which mandate mechanical sizing) has had a lot to do with this. This is great, because correctly sized AC’s will run longer, which helps them remove humidity. So problem solved, right? Ah, if only things were that simple.

Air conditioners remove two types of heat from the building:· Sensible heat - think pure temperature rise like we measure on our thermostats· Latent heat - think heat gain related to moisture in the air… Humidity.

Air conditioners remove both types of heat, but not equally. Central air conditioners do a good job at the sensible removal; 70-80%+, but only 20-30% of latent removal. We call this the sensible heat ratio (SHR). SHRs are typically 70-80% sensible heat removal to 20-30% latent removal.

Let’s review the 3 main components of an air conditioning system and how they impact heat removal:

1. The Outdoor Unit -- This is the part of the system that moves the refrigerant, releases heat picked up from the house to the outside, and compresses the refrigerant to a liquid.2. The Evaporator Coil is connected to your furnace/air handler, it is where the refrigerant picks up heat from the house when it changes to gas, and ideally causes the air from the house to condensate as it moves over it.3. The Fan blows the air over the evaporator coil from the house and pushes the cooled air into the home. When air conditioner systems are working correctly; the units are charged with the perfect amount of refrigerant, causing the refrigerant to change state at exactly the right time in the evaporator coil. This allows the evaporator coil to maximize the removal of water from the air in your home because it is operating at a temperature below dewpoint… meaning the coil is sufficiently cold to cause the air from the house to condense allowing the water to drain out of the house. The critical components of this process are the amount of refrigerant in the system (which must be matched to the outdoor unit's ability to move/compress the refrigerant) and the velocity of the air over the evaporator coil.

Latent Heat is Indoor Moisture and Humidity

So, here’s the problem, as mentioned, cooling loads have shrunk – it’s true, but only the sensible side of the equation has gotten smaller. When it comes to latent loads, unfortunately we haven’t found a way to reduce the amount of moisture we put into the house (breathing, showers, cooking, toilettes, plants, etc.). The result is we are reducing the size of our air conditioners, so they can better meet the loads, but in doing so we’re reducing the amount of latent heat/moisture they can remove, which means it’s critical the AC’s are setup correctly to maximize moisture removal. If we do not, we can end up with the type of buildings we started this article with – cold and humid. Worse, we can end up with homes with high moisture levels which turns into mold and ultimately does catastrophic damage to our homes.

In SummaryFor your furnace and AC to work properly, and keep humidity in check, your system first needs to be sized correctly. Second, it needs to be installed correctly, and fan speeds need to be set to deliver CFMs between 350 and 400 CFM/ton of cooling.Finally, controls should be in place to optimize the equipment used. This article was edited from an article, “HVAC Systems and Humidity Fan Speeds and Sizing” written by Priority Energy, providers of education and information to HVAC contractors. "

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