Duct Sizing: For total duct lengths up to
25', use a minimum 10" diameter round or
equivalent rectangular. For longer lengths,
use a minimum 12" diameter or equivalent.
Grills or diffusers on the duct ends must
not excessively restrict airflow.
Isolated Areas: Effective dehumidification
may require that ducting be branched to
isolated, stagnant air flow areas.
Use 8" or larger diameter branch ducting to each
of two or three areas, use 6" or larger to each of
four or more areas. Provisions must be made to
provide airflow from supply locations to central return
location. Proper air distribution is important to ensure
even humidity control throughout the structure.
4.4B Ducting for Fresh Air — Option
Fresh air may be brought into the structure by
connecting an insulated duct from outside the
structure to the 6" inlet of the DH90. Advantages
of this form of ventilation include:
1. Outside air is filtered before entering the
2. Outside air will be dehumidified before
entering if the DH90 is running in
3. Drawing air from outside and blowing
inside aids in slightly pressurizing the
Figure 3: Basement or crawlspace installation.
For HVAC Installer Only • Installation Guide
An insulated 6" diameter duct is generally
sufficient to provide up to 75 CFM of outside air.
Large quantities of outside air will impact DH90
performance positively or negatively, depending
upon the inside and outside air conditions.
The outside air duct should be connected to the 6"
round collar on the side of the unit. The amount of
outside air can be restricted by the blade damper
in the 6" collar. The actual amount of ventilation
air being delivered through the ventilation ducts
must be measured in each installation. Refer to the
instructions provided with the W8150 Ventilation
Control for proper setup instructions.
4.4C Installation in a Basement or Crawl Space
with an Existing Forced Air HVAC System
Install a separate 10" return for the DH90 in a central
area of the structure. Optional: Duct the supply of
the DH90 to a 10" x 10" x 10" tee/damper, adjusted
to 20% open to the basement. Duct the other side
of the tee to the air supply of the existing HVAC
system with a backdraft damper. Connect a duct
from outside to the 6" collar of the DH90 if you
wish to provide ventilation air. See Figure 3.
structure. This helps prevent dirty and
humid air from entering elsewhere.
Exhaust fans are recommended in the
bathrooms and kitchen.
DRY AIR TO