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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.


Figure 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 failure. 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 .


Figure 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 mastic.


Figure 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 (right).

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 "").


Figure 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 (right).

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.

Jeri Donadee

is vice president of H.B. McClure, a heating and cooling contractor in Harrisburg, Pa.

Duct Tape, Mastic, and Zone Damper Suppliers


5144 Enterprise Blvd.

Toledo, OH 43612


Amcraft 7150 heat seal iron for sealing heat-activated duct tape


P.O. Box 4808

Syracuse, NY 13221


Zone dampers

Hardcast Carlisle

903 W. Kirby

Wylie, TX 75098


Versi-Grip 181 duct mastic

Ideal Tape

1400 Middlesex St.

Lowell, MA 01851


UL 181 heat-activated duct tape

Jackson Systems

100 E. Thompson Rd.

Indianapolis, IN 46227


Zone dampers

McGill AirSeal

2400 Fairwood Ave.

Columbus, OH 43207


Uni-Mastic 181 Duct Sealer mastic


2850 Dillard Rd.

Eustis, FL 32726


Duct mastic

Robertshaw Controls

100 W. Victoria St.

Long Beach, CA 90805


Zone dampers


Air Station Industrial Park

Rockland, MA 02370


Glenkote 181 duct mastic