Air Distribution System Ducts, Plenums, and Fans
Air distribution system performance can have a big effect on overall HVAC system
efficiency. Therefore, air distribution systems face several mandatory measures and
prescriptive requirements, discussed below.
The 2016 Energy Standards specify mandatory requirements for air distribution ducts to be
sealed and tested in all climate zones. There are also several compliance credits available
related to duct system design.
Duct efficiency is affected by the following parameters:
1. Duct location (attic, crawlspace, basement, inside conditioned space, or other)
2. Specific conditions in the unconditioned space, for example, presence of a radiant
barrier
3. Duct insulation characteristics
4. Duct surface area, and
5. Air leakage of the duct system
In performance calculations, duct efficiency can be calculated in one of two ways:
1. Default input assumptions; or
2. Diagnostic measurement values.
Duct Sizing
The California Green Code and the California Mechanical Code both require that residential
duct systems be designed according to ACCA Manual D, or equivalent. If reasonable care
and judgment is used in designing the duct system (both return and supply ducts), and the
system is designed to reasonable parameters for airflow per ton, static pressure across the
fan, and friction rate, these systems should have no problem passing the diagnostic tests.
While undersized return ducts are very often the cause of poorairflow in many systems, they are only part of the overall system.
Following guidelines should be followed for Title 24 compliant duct sizing:
a. Make sure that the correct duct type is being used (vinyl flex, sheet metal, rigid
fiberglass, or other).
b. Make sure that all equivalent lengths and pressure drops are correctly accounted
for (bends, plenum start collars, t-wyes, filters, grilles, registers, and so forth.
c. Select a furnace that will provide at least 400 cfm/ton at the desired static
pressure of 125 to 150 Pa (0.5 to 0.6 inches water column).
d. Design the duct system to a static pressure across the fan of no more than 150
Pa (0.6 inches w.c.).
e. Consider upsizing the evaporator coil relative to the condenser to reduce the
static pressure drop. This results in better airflow and slightly better capacity and
efficiency. Manufacturers commonly provide performance data for such
condenser coil combinations.
f. Consider specifying an air handler with a better-quality fan motor.
Duct Insulation
Ducts that are entirely in conditioned space must comply with an installed R-value of R-4.2.
In all other cases, the minimum allowed duct insulation value is R-6. To determine whether ducts are entirely in conditioned space a Ratermust field verify by visual inspection.
(Excerpted from CEC Title-24 Part 6 Residential Compliance Manual)
