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.
- 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.