Multifamily buildings present complex design and engineering challenges in every area, and HVAC design is no exception. In the past, less stringent construction standards allowed for less efficient heating, cooling, and ventilation mechanisms. But today’s high-efficiency building techniques and requirements have changed the field dramatically. One of the most concerning challenges is humidity control in multifamily buildings due to its impact on many different quality-of-living markers.
Why is Humidity Control in Multifamily Buildings a Significant Concern?
Single-family homes can experience humidity issues if proper systems aren’t in place. And in multifamily buildings, tenant density, small spaces, and tighter envelopes in newer apartment buildings can all cause humidity control issues to multiply exponentially. Mold is also an ever-present threat so long as there is adequate heat, moisture, and food for spores to grow.
Through mere breathing, a single apartment occupant can generate 0.25 pints of moisture each hour and an additional 0.25 pints per hour through regular movement and activity. That’s not even counting the moisture generation from everyday cooking and bathing.
If large multifamily buildings lack adequate dehumidification and ventilation systems, mold can become a problem. And if HVAC systems aren’t built correctly, facilities struggle to balance tenant health and comfort with energy efficiency.
Oversized Cooling Units and Humidity Control
The balance of health, comfort, and efficiency starts with properly sized HVAC units. Conventional sizing codes require cooling and condensation units designed to meet peak load capacities during the hottest temperatures of the year. The result is that for the majority of the time a cooling system is in use, it’s oversized for the space it cools.
The benefit? Rapid cooling for the space, which is great for comfort. But this faster cooling means that the coils for the unit don’t remain cold long enough to build up condensation and naturally pull humidity from the space. In most systems, this condensation provides the necessary dehumidification to keep an area healthy—but only if the cooling unit can run for at least 10 to 15 minutes. Oversized units, though, may only run for a couple of minutes to meet the demand in smaller and more tenant-dense spaces. Again, this is great for quickly cooling a room or two. But it eliminates the utility of the system’s inherent humidity control mechanism.
Challenges with Energy-Efficient Buildings
In the past, building techniques left more than enough gaps for natural ventilation within a system, allowing airflow and heat exchange to occur naturally and mechanically. As such, exhaust-only ventilation was adequate for humidity control in multifamily buildings and single homes alike.
But with tighter building envelopes, energy-efficient buildings now require better systems for ventilation. Both supply and exhaust systems should be part of the design in multifamily buildings to maintain stable pressure from unit to unit and provide fresh air.
Seasonal Variations and Humidity Control in Multifamily Buildings
Now, with the need for a better sizing standard for cooling units in multifamily buildings, it’s important not only to consider peak temperature for load calculation but also seasonal changes in temperature, cooling needs, and humidity control. If cooling units are sized for peak load capacities in hot weather, their innate ability to dehumidify is compromised even more during shoulder seasons. Fall and spring typically require very little forced air ventilation to maintain a comfortable interior temperature. But this lack of airflow and increased moisture leads to a less healthy interior environment.
In response to this need, SEER can provide a better rating for overall energy efficiency by looking at a more comprehensive picture of a building’s needs, specifically taking into account seasonal humidity and moisture changes outside of the system. SEER ratings in a building can be improved by combining a larger air handler with a smaller condensing unit, but a multi-stage unit and handler can also help during these shoulder seasons. By switching to the first stage, when load capacity drops below about 2/3 of the second stage capacity, the smaller condensing unit can stay cool long enough to create the dehumidification effects desired.
However, this doesn’t completely compensate for the occupant-generated moisture levels in a large multifamily building. Instead, HVAC engineers must design a more complex system to accommodate increased tenant density, building envelope efficiency, and seasonal demand fluctuation. The beginning of the design and engineering process is the best place to guarantee the balance between health, comfort, and efficiency in multifamily buildings.
Designing Solutions that Focus on Health First
All multifamily units should include heating and cooling functions. But to guarantee health, comfort, and efficiency, tenant-dense buildings need a more comprehensive design for HVAC. In fact, this is the best possible way to create an efficient system that also provides the comfort desired by occupants, regardless of the season.
Systems should include:
Here’s how these five components work together to create the ideal HVAC system:
Heating and Cooling Systems
For occupants, this will be the most familiar component. Controlled by a thermostat in each apartment, the heating and cooling systems adjust the temperature of the rooms. For most people, this is enough to maintain comfort during the majority of the year. However, when heat increases, relative humidity often increases with it. Likewise, airflow is necessary to clear excess moisture from the apartment. If heating and cooling are the only components, stagnant air and excess moisture can cause both discomfort and a breeding ground for mold.
As mentioned above, exhaust-only ventilation systems cannot provide the proper airflow to clear an apartment without negative pressure concerns. A supply-only system is also unsatisfactory since it may result in improperly conditioned air entering the apartment. Both supply and exhaust ventilation should be a part of the design in order to create the proper airflow in modern buildings with tight envelopes.
In order to eliminate microbial contamination and scrub air, filtration is a necessary component of any forced-air system. Furnace filters, at the end of cold air returns, are the most common solution, although more technologically advanced options can be used. Still, this does not eliminate moisture in the air.
Humidity control in multifamily units is best addressed through unit-level dehumidifying devices since they can be installed without tenant controls in order to maintain appropriate humidity levels, regardless of heating and cooling settings. Whether they are installed in-line with the heating and cooling systems or placed as an independent component, a dehumidifier will help maintain an appropriate moisture level within buildings, so long as it isn’t turned off by occupants.
Why Humidity Control in Multifamily Buildings Works Best
Heat flows, air flows, and moisture flows all play a role in comfort and health, and unbalanced flows can compromise all three goals. Efficiency is also lost when these three flows are not properly addressed:
- Increased heat = increased humidity (no condensation to remove moisture)
- Decreased air flow = increased humidity (no exhaust to push moisture out)
- Decreased moisture flow = increased cooling load (apparent temperature effect)
When heat increases, water vapor has less opportunity to condense properly, leaving enough moisture in the air to grow mold. Senior living apartments are prone to this more than other apartments since most seniors prefer warmer temperatures to counteract the effects of poor circulation.
Decreased airflow creates a stagnant environment, which allows humidity to build. Just like a breeze can cool you on hot summer days, a steady airflow can improve comfort. However, if the fan motor is linked to an oversized cooling unit’s operation, it will run only for short times and only occasionally.
The result of increased humidity within an apartment is a feeling of a warmer environment (apparent temperature effect), even if the actual ambient temperature remains the same. For example, a temperature of 75 degrees with 50 percent humidity will feel two degrees cooler than the same ambient temperature with 70 percent humidity. Decreasing humidity actually increases the setpoint for air, which improves energy conservation.
Humidity Control in Multifamily Buildings: The Magic Bullet
In essence, decreasing humidity will also decrease cooling costs. In addition, decreasing humidity, especially in winter months, will decrease the risk of mold development. Both comfort and efficiency are met by addressing health concerns related to mold development. And when the system is designed from the start to address humidity control in multifamily buildings, the balance between health, comfort, and efficiency is near perfect.
If your next HVAC engineering and commissioning plan does not include humidity control for multifamily buildings, we can help you plan a full-featured HVAC system that puts health, comfort, and efficiency at the center of its design. With decades of combined experience and a focus on multifamily living projects, Innovative Engineering Solutions can lead your project from start to finish with leading technology, better-than-compliant design, and incredible efficiency that promises an investment that will last for decades. Contact us today to see what we can bring to your next building project.