High Interior Humidity
2011-12-02
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends an indoor relative humidity range of 30% to 60%. High (rather than low) humidity is the predominant humidity problem in Florida homes, and can lead to discomfort, odors and mold growth. Water intrusion, cooking, laundry, and other sources of moisture may contribute to or cause a humidity problem within a home. However, in our experience, an air conditioner (AC) problem is the most common reason for high humidity in a Florida home. Most homes in the sunshine state have AC to reduce temperature and humidity. This is accomplished by drawing or pushing interior air across evaporator coils. These coils are cooled by refrigerant as it changes state from liquid to vapor. This same refrigerant is compressed and condensed at the exterior, where heat is given off. As air is drawn across the evaporator coils and cooled, moisture is removed and the interior humidity is reduced. A sufficient air flow must occur in order to properly reduce interior humidity. 'Short cycling' is a term used to describe an AC system that is cycling too quickly, or the compressor and evaporator are being called back into service after being stopped for a relatively short period of time. Short cycling reduces an AC system's ability to remove humidity. This is because a short cycling AC system will not run long enough to draw water from a sufficient quantity of air. Short cycling causes additional wear and tear on the AC system’s components, and may lead to premature failure. This is because under normal conditions the compressor discharge pressure is allowed time to dissipate through the expansion valves at the evaporator. If the compressor starts before this pressure dissipates, it must work against the higher pressure, resulting in increased power usage, operating temperatures, and mechanical stress. Higher electric bills are sometimes noticed in conjunction with high interior humidity. Causes of short cycling include an over-sized AC unit, too much refrigerant in the system, inadequate movement of air through the home, or an improperly placed thermostat. Any of these conditions, acting alone or in combination, can disrupt the proper operation of the AC system and contribute to higher interior humidity. Clogged filters, the lack of return vents, and inadequate undercut at bedroom doorways can also contribute to inadequate air movement through a home. These factors result in a deficiency of the home’s relative warm, moist air being drawn across the evaporator coils. The ideal is a continuous flow of relatively warm, moist air from all areas of the home to the evaporator coil, and the relatively cool, dry air returning back to the entire home. If this air movement through the home and across the evaporator coil is insufficient, the evaporator coil may not be given the volume or circulation of air flow required to properly remove moisture and evenly cool the home. CODE DISCUSSION – RETURN AIR The 2010 FBC, Residential contains the following pertinent requirements with regards to heating, cooling, and ventilation equipment return air. These paragraphs require that unbalanced air flows and pressure differentials be avoided:M1602.1 Return air. Return air shall be taken from inside the dwelling. Dilution of return air with outdoor air shall be permitted.
M1602.2. Prohibited sources. Outside or return air for a forced-air heating or cooling system shall not be taken from the following locations:
1. Closer than 10 feet to an appliance vent outlet, a vent opening from a plumbing drainage system or the discharge outlet of an exhaust fan, unless the outlet is 3 feet above the outside air inlet.
2. Where flammable vapors are present; or where located less than 10 feet above the surface of any abutting public way or driveway; or where located at grade level by a sidewalk, street, alley or driveway.
3. A room or space, the volume of which is less than 25 percent of the entire volume served by such system. Where connected by a permanent opening having an area sized in accordance with ACCA Manual D, adjoining rooms or spaces shall be considered as a single room or space for the purpose of determining the volume of such rooms or spaces.
Exception: The minimum volume requirement shall not apply where the amount of return air taken from a room or space is less than or equal to the amount of supply air delivered to such room or space.
M1602.4 Balanced return air. Restricted return air occurs in buildings when returns are located in central zones and closed interior doors impede air flow to the return grill or when ceiling spaces are used as return plenums and fire walls restrict air movement from one portion of the return plenum to another. Provisions shall be made in residential buildings to avoid unbalanced air flows and pressure differentials caused by restricted return air. Pressure differentials across closed doors where returns are centrally located shall be limited to 0.01 inch WC (2.5 pascals) or less. Pressure differentials across fire walls in ceiling space plenums shall be limited to 0.01 inch WC (2.5 pascals) by providing air duct pathways or air transfer pathways from the high pressure zone to the low zone.
Exceptions:
1. Transfer ducts may achieve this by increasing the return transfer one and one half times the cross sectional area (square inches) of the supply duct entering the room or space it’s serving and the door having at least an unrestricted 1 inch undercut to achieve proper return air balance.
2. Transfer grilles shall use 50 square inches (of grille area) to 100 cfm (of supply air) for sizing through-the-wall transfer grilles and using an unrestricted 1-inch undercutting of doors to achieve proper return air balance.
3. Habitable rooms only shall be required to meet these requirements for proper balanced return air excluding bathrooms, closets, storage rooms and laundry rooms, except that all supply air into the master suite shall be included.