All installations and services must be performed by qualified service personnel.
6.The following method can be used to size ductwork when air velocities are low to moderate.
a.Using a floor view of the residence, determine, or layout, the locations of the supply registers and the return air grills. (Generally, supply registers should be located close to sources of heat loss, i.e. windows and doors, around the perimeter of the building. Return grills should be located in central positions as far away from the supply registers as practical.)
b.Find a location for the appliance outside the building that minimizes the amount of ducting required to connect the appliance to the supply and return air duct systems. Consider issues of access to the oil supply and electrical service, required service and venting clearances, exposure to sunlight, and operating noise when selecting this location.
c.Plan an efficient layout for the ductwork connecting each of the supply air registers in the supply system to the unit. Plan and layout ductwork connecting each of the return air grills in the return system to the unit. Measure or estimate the length of duct between each register and grill.
d.Select values for the airflow through each register and grill.
e.Select values for the pressure drops of both the supply and return air systems. Each branch of the supply (or the return) air system will have this pressure drop. The total pressure drop of the supply and return air systems added together cannot exceed the maximum external static pressure that can be supplied by the appliance blower.
f.Determine the required flowrate for each branch of the supply and return air systems. The total air flowrate, by adding the air flowrate of each branch of the supply system, must equal the minimum required air flowrate (refer to part 3, above). Likewise, the air flowrate of each of the branches of the return air system must sum to the required minimum flowrate.
g.Using the selected air flowrates for each component of the duct system and manufacturer’s literature, or published literature on duct system pressure drops, the pressure drop for each component in the duct system can be estimated.
(Chapter 32 of the ASHRAE Handbook – Fundamentals is an excellent source of duct system design principles and pressure drop data.) Conversely, for a specified type of fitting, it is also possible to determine the required size or diameter of the component for a specified pressure drop and flowrate.
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