| [Image: Dehumidifying Ventilation System (FIgure 1).] |
| [Image: Fresh Air Supply Integrated with Central HVAC System (Figure 2).] |
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Energy-efficient homes in a humid climate, especially when ductwork is well-sealed or within the conditioned space, need little or no cooling during periods of mild temperatures. As a result, the air conditioner, which we often rely on to remove moisture from the air, doesn't run much, so there is not enough dehumidification and humidity levels in the house are higher than desired. This is not only uncomfortable but can cause increased levels of pollutants.
Here are three systems you can use to reduce humidity without sacrificing energy efficiency.
Dehumidifying ventilation systems. Spot exhaust fans and a separate ventilation system that supplies filtered and dehumidified fresh air to an air-tight home at a controlled rate is the optimal solution for air quality and comfort. It does it all — brings in clean, fresh, dry air through a duct instead of gaps, dehumidifies during mild temperature seasons when the A/C doesn’t run enough and maintains a slight positive air pressure. An energy-efficient home-ventilating dehumidifier may cost $1,500 - $2,000 but can be a good investment in health, comfort and moisture control, particularly for households with allergies or high moisture loads (30-50 liters per day). See Figure 1.
An enthalpy energy recovery ventilator (ERV). This is a fresh-air supply and exhaust system that exchanges heat and moisture between the incoming and outgoing air. The heat exchange has energy-saving benefits, but the level of dehumidification provided may not be sufficient and need supplementation. Thus, its benefits may not justify the cost in comparison to other dehumidification options.
Fresh air supply to AH + Dehumidifier. A dampered and filtered fresh-air duct can be added inexpensively to the return side of the central HVAC air handler with a specialized flow controller. The controller activates the AH blower for a few minutes periodically to draw in and mix fresh air. To prevent moisture problems, it’s important that the flow controller have a time-delay feature that does not run the blower for a set time after a cooling cycle to allow time for the coils to drain and ducts to warm. See Figure 2.
Since some of the fresh air drawn in is not conditioned, this system should be combined with supplemental dehumidification:
A separate whole-house, high-capacity dehumidifier unit may be installed near the air handler, yet with a humidistat in the living space. It can be installed to draw and dehumidify house air when it reaches your set relative humidity level, then distribute the drier air through the supply duct system. This option is comparable in effectiveness and energy efficiency; its system cost is similar or slightly lower than the dehumidifying ventilation system described above.
When dehumidification needs are modest (10-25 liters per day), a standard capacity dehumidifier placed in a closet with louvered doors or in the air return plenum (with a drain line) is an inexpensive ($200-$300), practical solution for many. Although low-cost, portable, standard dehumidifiers are less energy-efficient and produce noise and heat when operating. Choose an ENERGY STAR-labeled model.
Variable capacity air conditioner systems operate at different speeds and capacities to more closely match the home’s cooling load. As a result, they run longer and dehumidify more effectively than single-speed units. They are typically energy efficient with high SEER ratings but may cost up to 30% more than single-speed units.
The supplemental dehumidification described here is just one important method for managing indoor air to control biological pollutants.
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