Conventional HVAC design focuses on indoor temperature, but keeping humidity under control is also very important. Excessive humidity and dryness have negative consequences, and the recommendation is to keep humidity within a moderate range. The U.S. Environmental Protection Agency (EPA) and the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) agree that the ideal range for indoor relative humidity is 30% to 60%.
In many cases, heating and cooling systems can provide adequate temperature and humidity simultaneously. However, when this is not possible, HVAC engineers specify humidification or dehumidification equipment as needed. As these systems consume additional energy, the recommendation is to purchase them only after professional evaluation.
Make sure your building has ideal air temperature and humidity.
Problems caused by excessive humidity
High humidity causes many problems that affect both buildings and their occupants. Moisture can damage some building materials while causing discomfort and health problems.
Depending on where it accumulates, moisture can damage many materials. This often leads to costly repairs, consuming funds that could be used for energy efficiency measures or other building improvements. The following are some examples of moisture damage:
- Condensation on metal surfaces causes rust. Old plumbing systems made from cast iron are especially vulnerable to rust.
- Moisture can also accumulate in spaces within walls. Wood and paint are especially vulnerable to damage from accumulated moisture.
Air-conditioned spaces with excessive humidity often feel like a refrigerator, causing discomfort and health problems. This is a common problem with oversized air conditioning systems: they cool the air too quickly and moisture is not removed effectively.
Moisture also encourages the growth of mold and dust mites, both of which cause respiratory irritation and allergies. In the case of asthma patients, exposure to mold spores or dust mites can also cause attacks. Unfortunately, mold and dust mites are very difficult to eliminate directly:
- Mold often grows hidden from view, constantly spreading spores inside buildings.
- Dust mites are microscopic and survive by feeding on organic matter, such as dead skin shed by humans and pets.
Removing all mold and dust mites from the interior of a building is impractical, and a much better approach is to lower the humidity in the air. Both organisms die when air humidity is reduced to levels recommended by the US EPA and ASHRAE.
Problems caused by excessive air dryness
Excessive dryness also causes property damage, discomfort and health problems. Moisture from the air should be removed when necessary, but not to the point that the relative humidity drops below the recommended range.
Wood tends to warp and crack when the air humidity is too low and paint can start to fall off surfaces. Low humidity also increases the chance of electrostatic discharge, which can damage electronic equipment. Data centers are especially vulnerable to electrostatic discharge, as it can damage expensive equipment and compromise important data.
Dry air tends to irritate the skin, eyes and respiratory system. Small particles also stay in the air longer with low humidity, and this includes dust and viruses. As a result, allergies and viral illnesses are more common when relative humidity is low.
Maintaining relative humidity within the ideal range
As mentioned previously, humidification and dehumidification systems can maintain air humidity at ideal levels. However, these devices have a price and operating cost. The best approach is to size HVAC equipment for optimal temperature and humidity control, using additional measures only when necessary.
For example, if your building has humidity issues due to large air conditioners, adding dehumidification is not the most effective solution. Instead, you should consider upgrading your air conditioners as you can get new units with adequate capacity and greater energy efficiency. On the other hand, a separate dehumidification system increases energy costs.
Some HVAC configurations have built-in dehumidification capabilities without the need for additional components. For example, dehumidification is possible when you have air handlers with heating and cooling coils:
- The cooling coil reduces air temperature and humidity. If additional dehumidification is required, the cooling power is increased to remove more moisture from the air.
- If this process cools the air excessively, the heating coil can be used to increase its temperature again.
Some steam heating systems allow for simultaneous heating and humidification, which is very useful on cold, dry winter days. Although hot water is preferred over steam in space heating applications, your property may benefit from steam-based humidification if a process that uses it already exists – healthcare institutions and industrial facilities are two examples.
Energy recovery ventilation systems typically exchange heat between the outside air supply and the exhaust. However, some configurations also allow for moisture exchange, a concept called enthalpy recovery ventilation:
- When the outside air is dry, the ERV system can recover moisture from the exhaust air.
- Conversely, when the outside air is very humid, exhaust air can be used to remove some of the moisture.
- In other words, ERV can complement both humidification and dehumidification.
Conclusion
A well-designed HVAC system not only controls indoor temperature, but also relative humidity. A moderate level of air humidity is recommended, as extreme humidity and dryness cause property damage and health problems.
For best results, get the professional opinion of HVAC engineers. They can provide high-performance mechanical design for new construction or suggest upgrades to improve performance in existing buildings. Adding humidification or dehumidification may seem like the simplest solution, but better results may be possible by improving your existing HVAC installation.
