Mammoth 1/2 to 6 Tons: High Efficiency Single Circuit Horizontal and Vertical (A-Vintage) - Manual (Page 7)

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MAMM-WSHP-IOM-1MA
Ductwork and Attenuation
Discharge ductwork is normally used with A-vintage
horizontal and vertical single circuit units. Return air
ductwork may also be required.
Horizontal, or ceiling-mounted heat pumps virtually
always have discharge ductwork attached to the unit.
A flange is provided on the discharge to facilitate
attachment of the ductwork. Good design practice
requires a flexible connector between the flange and
supply ducting. This flexible collar isolates unit
vibration and noise from the supply duct, and simplifies
unit removal. The filter section is also supplied with a
flange to accommodate return ductwork. A flexible
connector should also be used to connect the unit to
the return ducting to attenuate unit sound levels and
allow ease of unit removal. Ducted returns are typically
used on acoustically sensitive applications. Return
ducting should run straight to the unit a minimum of 12”
to better distribute return air across the filters and coils.
For applications that are especially acoustically
sensitive, consider an acoustic kit that includes
additional unit lining and dampening material beneath
the entire unit.
Ductwork should be lined with an acoustic/ thermal
insulation a minimum of ½”. For sensitive installations,
use 1” insulation a minimum of five to 10 feet prior to
each diffuser. Directional changes in ductwork, such
as tees, elbows and dampers can cause airflow
turbulence and noise. Minimize the use of these items
where possible. Placing a straight run of duct about
the width between fittings will decrease turbulence and
associated noise. Diffusers located in the bottom of a
trunk duct pose acoustical problems. Volume control
dampers should be placed well upstream of any air
outlets.
Applications that have a single duct discharge, such as
hotels, should limit the velocity to a maximum of 600
fpm. Use a short run of duct with an elbow, if
necessary, without turning vanes. Return air grilles
should be located low on the sidewall and route up to
the ceiling plenum. An attenuator placed at the return
opening will provide added acoustical protection. ASHRAE and SMACNA Suggested Supply Air Ducting
ASHRAE and SMACNA Suggested Return Air Ducting
For vertical units, discharge ductwork should include a
non-insulated transition from the unit connection to a
flexible connector at the full duct size, a short run of
duct and an elbow internally lined with insulation but
without turning vanes. The main duct tees into branch
circuits with discharge diffusers.
Return air is typically brought in through a grille in a
mechanical closet door. The unit should be located
ninety degrees from the door return to eliminate any
“line of sight”. Return air can be brought in through the
grille and ducted to the unit.
Return ducting is facilitated through use of a filter rack
that is designed to accept return air ducting. Return
ducting will likely increase the required width of the
closet.
Outside air may be required for ventilation. The
temperature of the ventilation air must be controlled so
that the mixture of outside air and return air entering
the unit is within application limits. It is typical to close
off the ventilation air system during unoccupied periods
(i.e. night setback).
The ventilation air system is typically a separate
building subsystem with distribution ductwork. Simple
introduction of the outside air into each return air
plenum chamber reasonably close to the unit air inlet is
recommended. Do not duct outside air directly to the
unit inlet. Provide sufficient distance for the thorough
mixing of outside and return air.
Ventilation Air