Air Distribution Components
Basic Air Distribution Components
Blowers and Fans: There are many types of fans and blowers that move air. The majority of the furnace and air handlers use a centrifugal squirrel cage blower mounted inside sheet metal housing. Smaller blower wheels use a direct drive motor and larger blowers are driven by motors connected with pulleys and belts. This type fan is quiet, efficient and low cost. Always be aware of the TESP (total external static pressure) the fan assembly or blower will produce.
Blower Motor: Most systems use a constant speed electric motor. The specific motor for residential systems is a Permanent Split Capacitor, PSC, Motor. These motors typically can be wired for three different speeds. The speed is field selected based on the pressure and air flow needed. A more expensive and more energy efficient option is a variable speed motor. The motor speed can be changed electronically to produce the exact pressure and CFM required. The most common type variable speed motor in small systems is called an electronically commutated motor, ECM.
Return Air Grill: Air enters the system through the return air grill or multiple grills. These grills are unintentionally undersized in many installations. If enough air cannot get into the fan because the return is undersized or has obstructions, the fan cannot deliver enough air to the rooms. You can’t get more out of a fan than you put in. Return grills are often undersized, but you can’t oversize a return. Make a mental note and look for air flow problems if you find grills sized less than 200 square inches per ton. A 3 ton system would need 600 square inches or a 25” x 25” grill providing 625 square inches. Smaller returns can work, but they will reduce the fan capacity available for the supply components and ductwork. Many small systems have the air filter installed in the return grill. It is good practice to size the filter/grill not to exceed 300 FPM velocity. High velocity will also generate more air noise. Remember that you need 400 CFM per ton. You should always measure the air flow as part of your diagnostic procedure.
Air Filter: The primary purpose of the air filter is to keep your evaporator coil clean and efficient. Common filters have flat media or pleated media. Pleated media has more filtering surface than flat media and will last longer. Pleated filter media will have less resistance to air flow. In recent years it has become popular to use the AC system as an air cleaner. Since most returns are undersized it would be careless to recommend an air filter with a rating higher than MERV 4 without a full airflow analysis. MERV is an efficiency rating for filters. Make sure the filter will allow 400 CFM per ton after 30 days of use. The filter has a much higher resistance to airflow when dirty than when new. The higher the MERV rating the smaller the particle that the filter stops. A pleated MERV 8 rated filter is a good choice for a properly installed residential or small commercial system. Be aware of your static pressure budget for the system and how much pressure drop the filter will produce clean and dirty.
Plenum: The plenum is an air distribution box at each end of the furnace/coil or AHU. The system will have a supply air plenum and a return air plenum. Air duct is attached to the plenums in such a manner as to minimize pressure loss.
Evaporator Coil: The evaporator coil is in the air stream. It is listed here because it can have a significant effect on the flow of air. Inspect the coil or measure pressure drop to determine if it’s clean and has adequate air flow.
Ductwork: The purpose of the ductwork is to efficiently move the air to the room or location where it is needed. The duct system is sized for the amount of air in CFM needed. Turns in the duct creates resistance to airflow so turns are kept to a minimum. Ducts are typically constructed from sheet metal or fiberglass and flexible duct is constructed of plastics and spiral wire. Ductwork in non-air conditioned spaces must be insulated to prevent sweating and heat gain in cooling applications or heat loss for heating systems.
Supply Grills and registers: The ducts are terminated with a ceiling grill, wall grill or register. The purpose of the grill is to throw the air into the room to the locations needed. Grills and registers are designed to distribute the air one-way, two-way, three-way or all directions, four-way. The grill is also designed to “throw” the air a specified distance.
The amount of cooling and heating required is determined by performing a heat gain/heat loss calculation. The industry standard for residential and small commercial is described in Manual J available from ACCA.
Ducts are designed as described in Manual D, also from ACCA.
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