Are you Depending on your HVAC System to Remove Moisture?
With today’s upgrades and improvements to heating and cooling systems the secondary benefit, moisture removal, is now non-existent. Thankfully, HVAC systems are much more energy-efficient and blast colder air in short increments. There is nothing wrong with the improved upgrades. In fact, it is doing exactly what it is created to do, cool the air in less time. ASHRAE’s article, Choosing the Right Size System for your Home, states “the insufficient moisture removal resulting from over-sized cooling equipment can lead to mold growth and other types of moisture related damage.”1
Money is a huge factor in the construction world, typically the most cost-effective option wins the bid. For the past few years, multifamily housing developments are displacing standard HVAC systems with alternate cooling & heating techniques including mini-splits, Packaged Thermal Air Conditioners (PTAC’s), and Variable Refrigerant Flow (VRF’s) for their convenience, availability, and most importantly cost.
Since many assume that a bigger HVAC system means better heating and cooling capabilities, using price as a determining factor often leads to purchasing and installing an oversized unit. Unfortunately, an HVAC system that is “too large” will only waste energy and money, on a product that doesn’t remove moisture. HVAC units were created to remove heat from the air to cool the space, not to remove moisture from the air. ASHRAE explains further that “a cooling system that is too big cools down the temperature very quickly, but it does not run long enough to remove sufficient moisture or “humidity” from the air. The result is you feel cool but clammy.”1
According to Building Performance “oversized equipment operates less efficiently and costs more than properly sized equipment.”2 An oversized HVAC system releases quick bursts of cool air to bring room temperatures to thermostat set point, but do not dot condense moisture. In fact, any moisture collected on the coils, will evaporate back into the air once the system is no longer running.
Problem with choosing an oversized HVAC System
• Higher indoor humidity– these systems are only able to condense and dry the air while running. They cannot independently control both temperature & humidity.
• Decreased efficiency- the system builds up pressure to start, larger systems require more pressure but only need to run briefly to bring the temperature to set point, which reduces efficiency.
• Shorter lifespan – HVAC systems disperse hot and cold air, to bring room temperatures to the set point. Thermostats measure air temperature, airflow, and moisture content of air, to get an accurate indoor air temperature. If moisture is not removed, air temperature fluctuates, causing short-cycling, which shortens the unit’s lifespan.
• Inconsistent temperature – installing a unit designed for a larger floorplan can lead to inconsistent heating and cooling performance.
• More expensive– Initial costs of unit will cost more, for not only the product but installation, as well as long-term maintenance, and operation of an inefficient unit.
HVAC systems cannot independently control temperature and humidity indoors. Supplementing a dehumidification system into your property will not only remove excess moisture in the air but will help reduce energy costs by lowering the HVAC systems runtimes. Reducing runtimes not only helps save money on electric bills, but also protects simple wear and tear of costly HVAC parts and maintenance. According to John F. Straube, Ph.D., “controlling or managing moisture and reducing the risk of moisture-related problems by judicious design, assembly, and material choices is the most practical approach for designing buildings and conditioning systems.”4 Integrating an independent dehumidifier will compliment your system by providing an escape route for moisture while maintaining the longevity of your HVAC system.
Oversized HVAC System = More Money + Higher Energy Usage
When it comes to HVAC Systems for Multi-Family Communities…
Bigger isn’t always Better!
The Science of Condensation
Moisture accumulates indoors from basic everyday tasks including cooking, showering, cleaning, or even a cold drink condensing on the counter, they all increase indoor relative humidity. Strube, Ph.D. states in his ASHRAE Journal article Moisture in Buildings, “a significant amount of moisture can be released or generated by occupants, their activities, and processes. Table 1 lists different internal sources and their significance.” 4
Water moves from areas of high concentration to areas of low concentration (evaporation) and moves from warm temperatures to cooler temperatures (condensation).3 This is the basic science behind how a dehumidifier works; dehumidification reduces the relative humidity (high to low) of an area while the cold coils (warm to cold) condense moisture. The dry air and cold coils work together to convert water vapor (gas) to water (liquid).
Multifamily housing developments are being built tighter and tighter to provide energy-efficient housing, and drastically reduce sensible cooling loads. Supplemental dehumidification is not only beneficial but a necessity to reduce moisture trapped indoors, especially in warm and humid climates. Conventional cooling systems are designed to be as efficient as possible, controlling temperature to setpoint and cooling systems.
Moisture Removal Solutions
- ASHRAE, Choosing the Right System for your Home, accessed October 2021,
- Building Performance, HVAC Systems: “Right Size” HVAC systems to ensure efficient operation, 2019, accessed October 2021, <https://buildingperformancenc.org/performance/hvac/>
- 5.2E: Osmosis, Mechanism of Osmosis 2020, LibreTexts, accessed October 2021
- ASHRAE Journal; John F. Straube, Ph.D. 2002, ResearchGate, accessed October 2021, <https://www.researchgate.net/profile/John-Straube/publication/271706272_Moisture_in_buildings/links/57fd01f108aeea8c97c8635f/Moisture-in-buildings.pdf>