TP-M11
12
2) Select a water heater with an output
that is at least 120% of the heat loss
Duct Layout
Make sure a proper duct design has
been completed for the dwelling using
HRAI, ASHRAE or other approved
design method.
Supply air Plenum
Provide a supply air plenum that is the
same dimensions as the outlet flanges of
the air handler and at least 36 inches
long. Acoustical lining is recommended.
A smooth, square-to-round transition
may be used in place of the supply air
plenum for horizontal installations where
there is only one supply main.
For vertical applications where the air
conditioning coil is installed in the supply
air plenum, the cooling coil must be
supported at least 4 inches above the
heating coil face on brackets or channel
to ensure unimpeded airflow through the
heating coil.
Plenum takeoffs may be mounted on the
end of the plenum or the sides of the
plenum, but not both. Spin in collars or
transition takeoffs may be used, but not
both.
Supply mains
Plan duct layout to avoid branch runs in
outside walls or attics and to minimize
the length of the main duct
Where practical, provide parallel main
ducts to various floors or zones rather
than running a single larger duct with
tees. For applications with 3 or more
floors or any application where a large
seasonal adjustment in airflow is
anticipated, parallel main supply ducts
must be used. Volume dampers in each
of the main supply ducts must be
installed and must be accessible for
seasonal adjustments. For example; a 4-
storey town house will probably require
vastly different airflow rates for the upper
floors between cooling and heating
seasons. If there are two main supply
trunks, the supply trunk serving the upper
2 floors can be damped down in the
heating season to better balance the air
flows.
Supply mains may be round duct or
equivalent rectangular duct.
Round duct can be spiral duct, welded or
snap lock seams.
Rectangular duct must be at least 26
gauge for all dimensions. The aspect
ratio for square ducts (wide dimension
over short dimension) shall not exceed
2.5 to1.
Return air duct
The return air duct should be sized for a
total pressure drop of not more than
0.15”w.c. Using conventional sizing
methods and installed in accordance with
HRAI guidelines or equal. It is
recommended that the return air duct or
drop be acoustically insulated for at least
6 feet nearest the air handler.
Refer to the
small-D™
design guide for
additional requirements for small
diameter/higher static duct applications.
