Lesson 12. Air Distribution Components

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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Lesson 11. HVAC Air Distribution Basics

HVAC Air Distribution Basics

What do we mean by Air Side?  We have focused so far on the equipment that produces cooling and heating.  The air side is the distribution of the conditioned air to produce a comfortable environment for the building occupants.

Air Distribution in its simplest form consists of a fan or blower that forces air over an evaporator and/or heating heat exchanger.  The air is then forced through a system of air pipes called air ducts.  Finally, the air is directed into the room by a grill or register at the end of each air duct.  This air duct system must be designed and installed using sound engineering and installation practices.  The air distribution system is usually the cause of a poor performing or an inefficient system.

ACCA Manual D for duct design is the best resource for small residential and commercial HVAC duct systems design.

The Basics

  • Cubic Feet per Minute, CFM:  Air flow quantity is measured in cubic feet per minute.  A cubic foot is a cube 1’ x1’ x1’.  If this cubic foot of air moves at a velocity of 1 foot per minute, this = 1 CFM.  The blower for air conditioning is usually sized to produce 400 CFM per ton of cooling.
  •  Feet per Minute, FPM:  This is the speed of the air or wind.  You could measure the speed in miles per hour or other units, but the standard in HVAC is FPM.
  •  Static Pressure:  Air has weight and acts in a manner similar to water.  The pressure produced by the fan and the air duct resistance to moving air produced a static pressure against the walls of the air duct.
    Velocity Pressure:  This is the force exerted by the moving air along the axis of movement.
  • Total Pressure:  Total pressure is Static pressure + Velocity pressure.
  •  TESP, Total External Static Pressure:  This is the pressure you will measure and compare to the manufacturers data or the unit nameplate.  To determine Total External Static Pressure of a unit you first measure the supply (+) pressure near the discharge of the unit, between the furnace and the coil. Then you measure the return (-) pressure near the inlet of the unit, between the filter and the furnace.   Add the two pressures together to determine the system’s TESP. When adding the positive and negative pressures together, disregard the + and – signs.

System Pressure Budget:  Most small residential and commercial systems are designed to produce a maximum TESP or .5 inches. Check the unit label or literature to find out.  When designing the air distribution system, make sure you establish a TESP budget and dont exceed it.  A typical budget for a split system with a furnace might be:

  • Evaporator coil:  .2
  • Supply duct:  .1
  • Return air .1
  • Filter .1
  • Total .5

Air Distribution Tools

  • Velometer:  The 4” vane anemometer is a great tool for measuring velocity and CFM.  A typical return air grill can be traversed and CFM calculated in a couple of minutes.  Taking this measurement is like measuring your blood pressure when you go to the doctor, measures it every time and so should you.  A good tool will cost $500.00
  •  Manometer:  Pressure can be measured with a Dwyer Magnahelic, about $100.00 or with the newer digital tools.  You will also need static pressure tips and flexible tubing.

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