Ventilation and Fresh Air

The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) Standard 62.2-2010 Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings states that the ventilation standard is 1 cubic feet per minute(cfm) of fresh air per 100 sq ft of area plus 7.5 cfm per person.

The only way to overcome negative pressure caused by, for example, duct leakage is to introduce fresh air into the building.

In hot and humid climates, like Florida, this fresh air cannot be introduced without removing water vapor that is present in the air. Therefore, the fresh air needs to be conditioned prior to entering the building. Small amounts of outdoor air can be ducted into the return network of the HVAC system. This is typically limited to 30 or 40 cfm.

Larger quantities of fresh air must be processed through a dehumidifier that is designed to process fresh air. This is usually limited to 90 cfm.

Even larger quantities of outdoor air are typically processed through Energy Recovery Ventilators (ERV). These devices precondition outdoor air through a heat exchanger that uses exhausting indoor air to remove the heat and moisture from the outdoor air. These devices are generally used to replace exhaust air rather than for adding positive pressure. However, some models have separate controls for the two sides that does allow for more incoming air than exiting air. The devices should be used in series with the HVAC systems.

Finally, the largest amount of outdoor air is processed through Dedicated Outdoor Air Systems (DOAS). These are in effect an HVAC system that is designed to process 100% outdoor air. These devices usually process 100 to 125 cfm per ton in Florida.

In all of these configurations, the fresh air duct should have a Normally Closed(NC) damper that is installed and there should also be a manual damper installed in the fresh air duct to permit balancing the fresh air flow. This damper is not necessary on the DOAS.

If there is no device to be connected to the fresh air duct, it should connect to the first return mixing box. If there is a dehumidifier, the ductwork should be independent of the HVAC system. This will permit the dehumidifier to operate independently. Furthermore, when dehumidifiers are connected to HVAC ductwork, the processed air can travel across the HVAC evaporator coil when the air handler is off and be re-humidified. Also, when the HVAC air handler is on, back draft can stall the dehumidifier. It should be noted that the air exiting non-desiccant type dehumidifiers is warmer than the air entering the unit.

When an ERV is employed its fresh air discharge can be ducted independently or connected to the return ductwork of the HVAC system and wired to run ONLY when the HVAC system is running. A DOAS system requires its own duct system.

All of these ventilation systems will provide a positive pressure to the building with the requisute conditioned fresh air.

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