Inspecting Whole-House Dehumidification Systems

According to Section 3.5 of InterNACHI’s Home Inspection Standards of Practice, "The inspector shall inspect the cooling system, using normal operating controls." Although inspecting whole-house dehumidification systems is not required by the SOP, home inspectors can learn about their different components and functions in order to broaden their knowledge of cooling systems.

Air conditioning provides two key benefits that enhance indoor comfort: cooling and dehumidification.  Dehumidification involves removing sensible heat and latent heat. 

Sensible heat and latent heat are two types of energy absorbed or released into the atmosphere. Latent heat is related to changes in phases between liquids, gases and solids. Sensible heat is related to changes in the temperature of a gas or object that experiences no change in phase.

Removing sensible heat creates a cooler air temperature, while removing latent heat removes humidity from the air. 

If the HVAC system’s coil temperature is below the dew point of the air (which is the temperature at which water vapor will condense into liquid water), the air-conditioning system will simultaneously lower the air temperature and remove moisture. Home inspectors should note that, together, sensible heat and latent heat constitute the cooling load on the HVAC system.

For maximum effectiveness, a well-designed HVAC system depends on an accurate room-by-room calculation of the heating and cooling loads. The system's cooling load is calculated by adding the amount of sensible heat and latent heat that needs to be removed from the air. Home inspectors should keep in mind that the important factors that contribute to sensible and latent heat calculations include outdoor climate, building orientation, and building design.

The optimal cooling system should be sized to meet its projected loads, resulting in better temperature and humidity control.  Oversized cooling systems can cause short-cycling of equipment, or short periods of “on” time when cooling is provided, followed by long periods of “off” time when no cooling is provided.  In homes with oversized air conditioners, the thermostat quickly reaches the desired set point before moisture can condense on the coil and be removed from the home. In this situation, the temperature in the home drops but the relative humidity or RH climbs.

Comfort and Health

Comfort in the home means more than just having adequate relief from the heat.  In hot-humid climates, human sweat doesn't evaporate, so it's difficult for the body's circulatory system to regulate its internal temperature and release excess heat.  An underperforming cooling and dehumidification system in the home could lead to heat-related health problems.  This is especially important to address as the world's climate continues to get hotter.

Home inspectors should take in to account that, in addition to creating an uncomfortable space, indoor relative humidity consistently greater than 60% can promote mold growth and create an environment conducive to dust mites and other pests. In hot-humid climates, additional dehumidification may be necessary. Accurate load calculations based on the home’s envelope characteristics, climate zone, and specific orientation can help address these issues by aiding in the proper sizing and design of cooling and dehumidification systems.

Figure 1. IECC Climate Zone Map (image courtesy of the DOE)

In order to ensure the proper sizing of heating and cooling systems, the HVAC industry uses Manual J, a calculation worksheet developed by the Air Conditioning Contractors of America (ACCA). The Manual J worksheet accounts for factors such as the local climate (see Figure 1.), the orientation of the home, and the thermal performance of the building enclosure, including windows, roofs, walls and floors. After completing the load calculation based on Manual J, the HVAC designer then uses a procedure from Manual S to properly select equipment capable of meeting both the sensible and the latent loads in the home. Home inspectors should note that in hot and humid climates, the air conditioner alone may not be able to remove enough latent heat to keep relative humidity below 60%. In such cases, HVAC design must also take into account any additional dehumidification capacity, such as additional system controls, or a standalone dehumidifier.

 How to Improve Dehumidification in Warm-Humid Climates

The types of equipment used for providing dehumidification to homes include standalone dehumidifiers, central fan-integrated dehumidifiers, and additional dehumidification controls for existing HVAC systems.

Standalone Dehumidifiers

Standalone dehumidifiers can be ducted or completely separate appliances. The major benefit of a standalone dehumidifier is its ability to control the indoor relative humidity in the home without having a major effect on temperature. This is very valuable in areas that have a low sensible load but a higher latent load (humidity), such as coastal climates.

Figure 2. Example of a standalone dehumidifier in a one-story home's closet (image courtesy of the Building Science Corp.)

Integrated Ducted Dehumidifiers

A ducted dehumidifier can be integrated with the central air-conditioning distribution system (see Figures 3 and 4 below). The fan inside the dehumidifier unit draws air from the living space or the air-handler plenum and returns it to the supply plenum. From there, the dehumidified air is distributed throughout the home through the HVAC ducts. A remote dehumidistat located in the living space can trigger the dehumidifier to turn on when indoor humidity levels exceed a set point. Controls connecting the air handler to the dehumidifier can activate the central air-handler fan when the dehumidifier is activated. Controls could also be configured to operate a damper on a duct that brings outside air to the return side of the air handler to prevent over-ventilation with humid outside air during excessively humid periods.

 Figure 3. A supplemental dehumidifier can be integrated with the central air handler to use the HVAC ducts for air distribution, with controls that include a dehumidistat located in the living space to switch on the dehumidifier and the central fan when extra dehumidification is needed (image courtesy of the Building Science Corp.).

Figure 4. A supplemental dehumidifier is integrated with the home’s HVAC air handler to provide extra dehumidification when needed (image courtesy of the Building Science Corp.).

Additional Dehumidification Controls

With additional dehumidification controls in the HVAC system, the cooling system is operated with a slower fan speed to allow the air to spend more time on the coil's cold surface. This results in additional dehumidification. Because the system is still operating as an air conditioner, home inspectors should keep in mind that dropping the temperature of the home may result in potential comfort issues.

Adding controls to the HVAC system is generally less expensive than installing a central or standalone dehumidifier. However, they are also more suited for climates with shorter shoulder seasons because of potential comfort issues while running the air conditioner in cooler weather. The standalone dehumidifier may be a costlier upfront investment, but this option generally costs less to operate and does so without the same comfort issues.    

How Professionals Conduct a Manual J Calculation

A contractor can utilize one of the HVAC industry-adopted software programs based upon Manual J to assist with calculating HVAC system design loads.

A Manual J report includes:

• climatic data, such as the “Outdoor Design Conditions.” This information includes the local elevation, dry-bulb and wet bulb temperatures, and the daily temperature range;
• indoor design temperature and relative humidity for heating and cooling;
• heat loss and gain for heating and cooling for each of the following elements of building loads:
  • windows and glass doors;
  • skylights;
  • wood and metal doors;
  • above- and below-grade walls;
  • ceilings and roofs;
  • floors and foundations;
  • air infiltration;
  • internal gains for both appliances and occupants;
  • duct location and tightness; and
  • ventilation loads.
    
    

Note that the U.S. Environmental Protection Agency (EPA) Indoor airPLUS Specification 7.3 requires the builder to provide the homeowner with a signed copy of the HVAC, duct and ventilation system design documentation, including Manual J and Manual D analysis reports.  These reports should include room loads, duct system criteria by room, and duct layouts, e.g., drawings illustrating return sizes, supply trunks, run-out duct sizes, and the cubic feet per minute (cfm) of conditioned air delivered to each room.
 

Summary

Air conditioning provides two key benefits that help enhance indoor comfort:  cooling and dehumidification.  In addition to contributing to discomfort for the home's occupants, indoor relative humidity consistently higher than 60% can promote mold growth and create an environment conducive to dust mites and other pests.

The types of equipment that can provide dehumidification include standalone dehumidifiers, central fan-integrated dehumidifiers, and additional dehumidification controls for existing HVAC systems. Adding controls to the HVAC system is generally a less expensive option than installing a central or standalone dehumidifier.  Home inspectors can offer this information to their clients who complain of comfort or pest problems related to indoor humidity or cooling issues.

This article was sourced from the U.S. Department of Energy and InterNACHI®.