IAQ Battle: Duct Board Vs. Metal
The battle of mold in residential
duct systems presents an interesting question: Given the exact same condition
variables (size, distance from plenum, identical relative humidity conditions,
etc.) would a length of duct board attract more or less mold than its metal
At this point there's not enough
research to answer the question, but generally if a choice exists between duct
board and metal for ultimate IAQ, many experts believe metal duct is the better
choice. Duct board manufacturers have produced studies that claim duct board
attracts no more mold than metal. While the jury is still out on mold, it's
scientific fact that metal duct has less pressure drop, has a smoother surface
that's less likely to accumulate dust, and is easily cleaned with a hard
surface biocide if mold occurs.
Another disadvantage to duct board
is the seams sometime leak over time, which could be due to aging tapes or
installation error. Duct tape is not approved for any ductwork and foil tape
tends not to hold as long as mastic/cloth tape combination.
These openings created by the
connecting tape's failure to adhere over the life of the duct, creates voids
that can expose the cold air supply to warm humid condensation-creating
conditions outside the duct. Where there's moisture, there's a good chance mold
will develop. That's why the moisture source—whether it's a leaking pipe, roof
or condensation-collecting void in the ductwork connection—must be rectified.
**The duct board spoken of above; it
gets wet from condensation and moisture like a sponge and air borne spores
adhere to it as this unit was only two years old. It is a natural home for
fungal growth and accelerates the reproduction and amount of spores that are
expelled. This is what your airways look
like inside of the unit creating the “conditioned air you breathe in your home.
It should be bright yellow on the
front of the board that was cut out of the supply plenum.
This is the same plenum with sheet
metal, and the insulation on the outside of the airway. This supply plenum is
7-8 years old and still clean and sanitized.
A homeowner complaining of
respiratory problems should have the home's RH determined, because mold/mildew
can't exist without moisture. This is accomplished with either:
- determining wet bulb and dry bulb temperatures and
calculating RH with a psychrometric chart;
- using a humidistat;
- documenting an extended period of RH readings with a
humidity recording instrument to determine house's tendencies;
To lower the RH, the air
conditioning system should be turned on. Much of this boils down to a
humidistat and educating the homeowner on using the air conditioning unit to
control RH regardless of whether the indoor temperature is cool enough or not.
This is particularly important in summer months if the home is unoccupied for
Setting the thermostat lower will
reduce RH, but it also tends to chill the home's occupants. There are
thermostat/humidistat combinations that when set for 55-percent, will run
continuously until the RH set point is achieved, regardless of whether the
space temperature is too cold in the house. If the occupants complain about the
low temperature in the conditioned space, an air comfort solution to the
chilling effect is an installed humidity control board. While not efficient
this reheat control board activates the furnace to reheat the air when the
space if the temperature is too chilling while achieving the targeted RH. A
humidity control board is easily wired into the thermostat wiring anywhere
between the thermostat and the fan coil unit and works with essentially any a/c
or heat pump models.
An oversized air conditioner might
be the cause of a high RH because it's not moving enough air across the
evaporator coil. Its oversized cooling capacity achieves the set point room
temperature too quickly (short cycling) without running long enough to
circulate sufficient air across the evaporator coils to remove the humidity at
Another remedy for oversized a/c
units might be adjusting the blower motor speed (for fixed speed units). For
example, increasing blower speed will move more air across the evaporator coil,
which in turn circulates more air. However if the blower speed is increased too
high, the evaporator air exit temperature might be above the dew point
temperature and there will be no condensation or moisture from the air. The key
is to look at the evaporator air exit temperature. Once the evaporator coil is
at steady state temperature, determine if the evaporator air exit temperature
is below the dew point. If it is well below the dew point, increase the blower
speed. If it's above the dew point, decrease the blower speed to dehumidify.
Fan speed can be adjusted by moving a fan lead to a different position or using
a different fan-power lead (refer to equipment instructions). The downside of
increasing motor speed is the fact that fan noise increases.
There are two reasons why the HVAC
contractor should provide IAQ services to their clients, the contractor's realm
of the HVAC system can both create mold and also prevent it. Thus it should
only be the certified IAQ/HVAC contractor that should remedy the homeowner's