Page 11

NORA Advanced Air Flow

National Oilheat Research Alliance 11 in size after three to four take-offs or every 200-400cfm. A reducing plenum system works well in larger buildings that require longer duct runs. When designed properly, the same pressure drop is maintained from one end of the duct to the other. This allows each branch duct to have about the same pressure pushing air into the takeoff. Recommendations and practices for laying out a reducing plenum duct system are: • The first main trunk system should be no longer than 20 feet. • The length of each reducing section should be no longer than 24 feet. • The first branch connection down from a single side taper should be at least 4 feet from the beginning of the transition fitting. The distance allows air turbulence caused by the fitting to die down before air is sent into the next branch. If the distance is less, the branch line could be hard to balance and noisy. • The trunk duct should extend at least 12 inches past the last takeoff. Loop Perimeter Duct System Loop perimeter duct systems (Figure 12) are common in structures built on concrete slabs. The perimeter loop is a continuous round duct of constant size imbedded in the slab. The duct runs close to the outer walls with outlets located next to the outer wall. The perimeter loop is fed by several branch lines from the plenum. When the furnace blower is running, there is warm air in the entire loop which helps to keep the slab at a more even temperature. Insulation around the slab will reduce heat loss to the outside. The loop has constant pressure throughout the system and provides equal pressure to all outlets. Duct System Components Building code requirements for installation of air distribution systems are not uniform and may vary from one location to another. Most localities have minimum standards or codes that determine the types of materials and methods that must be used. Some locations may have requirements for minimum insulation values for ducts in an unconditioned area, as well as for duct sealing and duct testing. Check with the local code authority before starting any work. The selection and size of trunk and branch lines is based on the air volume (CFM) needed to satisfy the heating and or cooling requirements for the building. A survey of the structure is needed, and then a load calculation must be done to find the heating and or cooling loads. Knowing the heating and or cooling loads, the minimum CFM requirements for a furnace can be calculated by using the following formula: CFM = Btu/HR 1.08 x TD Btu/HR is from your load calculation The 1.08 Btu/Hr is the amount of heat needed to raise or lower one cubic foot of water 1 degree at sea level. TD is the temperature difference or temperature rise. Assume a building with a heating requirement of 61,000Btu/Hr. By referring to the Figure 12


NORA Advanced Air Flow
To see the actual publication please follow the link above