Fine-Tuning Forced-Air Heat - Continued
When designing a zoned system, the first step is to know the
Btu and airflow demands of the zone. Once the actual air
requirements are verified, the ductwork should be designed and
installed at a slightly larger size (10% to 15% larger) than
standard ductwork. Oversizing each zone's ducts helps to
dissipate any extra airflow when only one zone calls for
heating or cooling.
3. Motorized zone dampers, like these examples
from Jackson Systems, are available for both round
ducts (top) and rectangular ducts (bottom).
Zoning is accomplished by installing motorized zone dampers
(Figure 3). Since the premise for zoning is to reduce the air
going to the area where the temperature is satisfied and
deliver air to the area that needs the heating/cooling, each
zone will need dampers. Manufacturers of zone dampers include
Carrier, Jackson Systems, and Robertshaw Controls (see ""
If there is room for a dedicated trunk line to serve the
zone, it is usually easier and cheaper to install a zone damper
in the trunk line. In that case, the individual branch lines
that are tapped off the dedicated trunks will not require zone
dampers. When there is no room for a dedicated trunk line, the
area can be zoned by installing a series of dampers in the
branch lines serving the area, and then controlling the dampers
together with a multi-damper enabler. Usually, the enabler is
purchased from the zone damper supplier.
A multi-zone system requires individual thermostats to
regulate the temperatures of each zone. The low-voltage
thermostat wire is fed to a main zone panel. Wires are then run
to the equipment and each zone damper. High and low temperature
sensors are usually placed in the supply-air plenum to serve as
unit safeties in the unlikely event of a zone damper
Supply ductwork is the only part of the system with dampers.
When one zone calls and gets supply air, the returns are still
being drawn from the entire home. Therefore it's important to
locate adequate returns in each zone. Two-story homes should
have a combination of high and low return grilles.
Extra air. When only one zone in a multi-zone
system calls for heat, there needs to be some way to dissipate
the extra cfm output of the furnace. Some brands of zone
control ignore this problem, and let the high airflow howl
through the small duct. Other brands will allow for the other
zone(s) to open slightly and allow for the air to "leak" into
areas that do not actually require conditioning. A third option
is to install a bypass damper that allows the excess air to be
recirculated back to the return. How the "extra" air is handled
is a matter of contractor preference. The surplus air issue is
much less of a problem with a two-stage gas furnace or a
two-stage heat pump, especially one with a variable-speed fan
-- one more reason for installing two-stage equipment.
Doing Ductwork Right
In unconditioned spaces like crawlspaces and attics, use
insulated duct for both supply and returns. For ducts in
conditioned spaces, insulation is highly recommended, but not
required. During the cooling season, uninsulated metal ducts
can become cold enough to sweat.
Minimizing leaks. Many
studies have shown that the typical forced-air system has leaky
ductwork. Leaky ductwork wastes energy dollars and can lead to
pressure imbalances in a house. An excellent resource for
information on duct sealing can be found at .
4. Sheet-metal duct joints must be screwed
together before mastic is applied.
Joints in sheet-metal duct should be screwed together and
sealed with mastic (Figure 4). Using mastic is always good
practice, although some contractors omit mastic on ducts in
conditioned spaces. Four water-based duct mastics are Glenkote
181, Hardcast Versi-Grip 181, RCD, and Uni-Mastic 181 Duct
Sealer. Duct mastic has the consistency of mud and is spread
with gloved hands or a paint brush (Figure 5). Wide gaps in
ductwork can be bridged with fiberglass tape before applying
5. Joints in sheet-metal duct should be sealed
with mastic, which can be applied with a paint brush
(left). When mastic is used on duct board, the joint
should first be bridged with fiberglass tape or scrim
Joints in rigid fiberglass duct (duct board) should be
sealed with a UL-181 heat-activated tape, like Ideal Tape #490
(Figure 6). Heat-activated tape works better and lasts longer
than the aluminum pressure-sensitive tape (See "").
6. The female end of a duct board joint is
pulled taut and stapled through to the male end (left).
The joint is completed by applying heat-activated tape,
which is warmed with an Amcraft duct board iron
Keep flex duct short and fat. Insulated flexible duct is
usually much faster to install than rigid duct. However, flex
duct must be sized right and installed properly. Flex duct
should be supported every 4 to 6 feet. Flex duct has high
friction losses because of the coiled interior spring liner, so
sharp bends should be avoided. The diameter of flex duct must
be adequately sized for the airflow required, especially for
runs longer than 12 feet.
Avoid pressurized rooms. If a room has a
supply grille but no return grille, the room can become
pressurized. To avoid this problem, such rooms need a
low-resistance path for the return air. Verify that the door is
undercut by 11/2 to 2 inches or that transfer grilles are
installed in a partition between the room and the hallway.
How much will the suggested upgrades cost? The cost of
upgrading a 100,000 Btu/h gas furnace from a single-stage unit
to a two-stage unit with a variable speed fan is between $750
and $900. A 3-ton air-source heat pump with a 10 SEER
efficiency rating can be upgraded to a two-stage unit with a
14.9 SEER rating for an added cost of about $1,900.
Customers who choose the upgrades will reap returns on their
investment: not only increased comfort, but energy savings from
the improved efficiency of the equipment.
is vice president
of H.B. McClure, a heating and cooling contractor in
Duct Tape, Mastic, and Zone Damper
5144 Enterprise Blvd.
Toledo, OH 43612
Amcraft 7150 heat seal iron for sealing
heat-activated duct tape
P.O. Box 4808
Syracuse, NY 13221
903 W. Kirby
Wylie, TX 75098
Versi-Grip 181 duct mastic
1400 Middlesex St.
Lowell, MA 01851
UL 181 heat-activated duct tape
100 E. Thompson Rd.
Indianapolis, IN 46227
2400 Fairwood Ave.
Columbus, OH 43207
Uni-Mastic 181 Duct Sealer mastic
2850 Dillard Rd.
Eustis, FL 32726
100 W. Victoria St.
Long Beach, CA 90805
Air Station Industrial Park
Rockland, MA 02370
Glenkote 181 duct mastic