HVAC Basics: Indoor Humidity & HVAC

Everyone knows that comfort is not only about temperature. Indoor humidity plays a big part in how warm or cool you feel. Humidity also affects much more than comfort: health and air quality are increasingly important concerns for your customers. 

As an HVAC service technician, you need to understand the impacts of indoor humidity so you can effectively talk with customers about their concerns and offer solutions. 

In this article, we will explore the effects of humidity on people, health, and air quality, as well as how the right HVAC solutions can help. But first, let’s define what indoor humidity levels should look like.

What exactly is indoor humidity and what is the optimum level?

Simply put, indoor humidity is the amount of water vapor (evaporated water) in indoor air. Not everyone agrees on the ideal indoor humidity level, but most of the adverse effects happen when relative humidity levels fall below about 30 percent or rise above about 60 percent. 

The usage of a space and the temperature can affect the ideal indoor humidity levels. Here’s one good benchmark for commercial indoor humidity levels: The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) recommends 50% relative humidity, at temperatures between 68°F and 78°F, for workplaces where people are doing sedentary work.

How indoor humidity affects comfort, health & indoor air quality

What happens when indoor humidity is too high?

High humidity slows the evaporation of sweat from the skin, which is how we regulate body temperature. When sweat does not evaporate as readily, we feel more hot and sticky than we do when the air is drier, even when the temperature is the same. 

Yet feeling too hot is only one effect of high humidity. Here’s what else can happen as relative humidity levels rise:

• Heat-related illnesses, including heat rash, muscle cramps, heat exhaustion and even heat stroke
• Growth of mold spores, dust mites, and other allergens 
• Bacteria and viruses that cause respiratory infections can more easily multiply and spread in high humidity
• Musty odors can linger in the space
• Increased levels of VOCs, ozone, and other pollutants 
• Damage to building structures and finishes from moist air and condensation

What happens when indoor humidity is too low?

When indoor humidity levels drop too low, we also experience physical symptoms. People may have sore eyes, dry throats, itchy or dry skin, headaches, or stuffy noses. But again, there are additional effects that range from annoying to serious concerns: 

• Bacteria and viruses (including the coronavirus that causes Covid-19) can also spread readily when indoor humidity is very low. 
• People with asthma and breathing difficulties may experience an increase in symptoms.
• Static electricity from walking on carpets in a dry room can cause annoying electric shocks. People also report itchy, red marks on ankles, which look like flea bites but are caused by static shocks. 
• Static electricity can also cause failure of computers and other technology.
• Dry air can damage fine wood furniture, paintings, floors, and woodwork.

How HVAC can help control humidity

Air conditioning and indoor humidity

Along with controlling temperature, AC systems are designed to control indoor humidity. Air conditioning systems remove excess water vapor from the air as part of the cooling process. When humidity levels get too high OR too low, that’s a sign that something is wrong. Here are some possible solutions when you find problems with indoor humidity levels:

Right-size the system capacity. In decades past, HVAC installers (and often customers) assumed that getting the highest-capacity unit the customer could afford was the best strategy. Today we know that oversized systems cause problems. They tend to cool quickly, reaching the set temperature after a short period of time, which often means they don’t run long enough to remove humidity effectively. Right-sizing the system can reduce excess humidity.

Offer a multistage cooling system. When it comes to controlling humidity, an air conditioner that runs at a single speed— basically, on or off—can have the same problems as one that’s oversized. Especially when the temperature isn’t too high but the air is very humid, the unit may turn on and off again frequently, not running consistently enough to remove humidity.

A multi-stage system, such as a VRF system, can be a great solution. They do a better job of humidity control because they can run longer at a lower capacity level.

Improve distribution and/or ventilation design. In areas that tend to have high indoor humidity, such as kitchens and bathrooms, proper ventilation is key to humidity control. Too little ventilation can cause humidity to build up. Also, too much ventilation (possibly due to a powerful kitchen exhaust fan) can cause negative air pressure that draws in more humid air from outside. 

AC distribution could also be an issue, especially if the space has been renovated since the air conditioning was installed. Adding vent fans, makeup units, and adding or moving duct runs can make a big difference in humidity levels.

Heating systems and indoor humidity

Heating is the main reason for indoor humidity levels that are too low in the winter. Heat from combustion systems produces very dry air. However, there are HVAC solutions that can help with that, too.

Add central humidification. For customers who complain about parched, dry air in the winter, you can recommend an add-on central humidification unit that works with HVAC systems to add more moisture to dry, heated air.

A central humidifier is attached to both the HVAC system and plumbing. The humidifier unit uses a humidistat to monitor the indoor humidity levels. When the air is too dry, the humidifier adds moisture in the form of water vapor to the air being circulated by the HVAC system.

Learn more about HVAC basics 

If you’re just getting started in the HVAC industry, or preparing yourself for a career as a service technician, we have lots of educational resources to help you move forward. Check out our helpful articles and videos.

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