Design & Installation Guide

This instruction is a summary of the basic rules and applies to most applications. For help with designs, remember that we offer a free Duct Design Service as well. For applications outside the scope of this guide, contact our customer service team, or your local representative.

Duct Layout


• Minimum 6 - 2" outlet, Minimum 5 - 2.5" Per Ton (3.5 kW). For refrigerant cooling applications the airflow must be between 200 and 250 CFM per nominal ton [27 to 33 L/s per nominal cooling kW]. For hot water or chilled water systems, refer to performance charts to determine the required airflow. The allowable airflow range per outlet is 20 to 40 CFM [9.4 and 19 L/s], where the typical outlet will deliver 35 CFM [17 L/s] if the plenum static pressure is 1.5 inches [0.37 kPa] and the branch duct length is 10-foot (3 meter) without any balancing orifices. Therefore, the average project will require about six 2" outlets per nominal ton [6 outlets per 3.5 kW] although more will be needed if the branch ducts are longer, balancing orifices are used, the plenum static pressure is less than 1.5 inches of water [0.37 kPa], or it is desirable to make the system as quiet as possible. For example, two runs with 50% balancing orifices are equal to one branch run without any orifices.

• 10% Rule. For supply ducts longer than 10 feet (3 meter), the air is reduced in that run by 10% for every 5 feet over 10 (every 1.5 meter over 3 meters). For example, a 30 foot [9 meter] run is 60% of an outlet that is 10 foot [3 m] yielding a reduction of 40% (30-10=20, 20÷5=4, 4×10=40%).

• Consider Traffic Pattern. Place outlets out of traffic pattern. A corner, 5-inch [127 mm] from each wall, is a good location, or along walls, or in soffits blowing horizontally. Consider floor outlets (with screens) for units located in basement. Slotted outlets can be used for high wall locations or in ceilings where there is insufficient room for bending tubing.

• Allow for Aspiration. Locate outlets so the air stream does not impinge on any objects or people – at least 3 feet [1 m] away. Use outlet deflectors and outlet balancing orifices sparingly as they disrupt the aspiration.

• Minimize Length, Minimize Restriction. Keep the supply duct length as close to 10 feet [3 m] as possible and never less than 6 feet [1.8 meters]. Use the fewest number of bends as possible. Maximize the radius of any bends making sure the bend in the sound attenuator tubing near the outlet is at least 6-inch [152 mm].


• Maximize Length, Minimize Restriction. Run main trunk (plenum) as long as possible; it is better to lengthen the plenum if you can shorten even two outlet runs. Use full flow tees with turning vanes (when applicable) and full flow elbows. The maximum total plenum length is 150 ft [45 m]; consider the first tee equal to 30 ft [9 m] and elbows equal to 15 ft [4.6 m].

• 60/40 Rule. When using a tee split the flow as close to 50/50 as possible – no more than 60/40. Always use a turning vane.

• 70/30 Rule. Turn the tee 90° to make a side branch with no more than 30 percent of the air. Do not use a turning vane

• Horseshoe Patterns. (Best Method). Use a tee at least 24 inches [610 mm] off the unit. For the 4860 unit, use 10-inch [254 mm] metal up to and including tee; then use 9-inch [229 mm] in both directions. For the 3642 unit, use 9-inch [229 mm] insulated metal up to and including tee; then use 7-inch [178 mm] in both directions. If possible, close the horseshoe into a perimeter loop.

• Shotgun Pattern. For the 4860 unit, use 10-inch [254 mm] insulated metal or fiberglass duct for the first 30 percent; then reduce to 9-inch [229 mm] if desired. For the 3642 unit, use 9-inch [229 mm] insulated metal or fiberglass duct for the first 40 percent; then reduce to 7-inch [178 mm] if desired. For the 2430 unit, 7-inch [178 mm] may be run the entire length.

• 24-inch (610 mm) Rule. Use at least 24-inch [610 mm] of straight plenum before any fitting, such as an elbow, tee, or takeoff. Electric duct heaters require 48 inches [1.2 m]. Avoid elbows directly off units.

• Space Takeoffs Evenly. Maintain distance between takeoffs as evenly as possible. Space the takeoffs at least 6-inch [152 mm] apart and 12- inch [305 mm] from end cap.


Sound Attenuators. Always use at least 3 feet [1 m] of the UNICO supplied sound attenuator supply tubing (UPC-26C or 226C) at the end of each run. For runs up to 12 feet [3658 mm], you may use the sound attenuator for the entire run. For greater lengths, use the aluminum core supply tubing (UPC-25 or 225) with a 3 foot [1 m] sound attenuator at the end.

• Return Air Duct Attenuation. Use the UNICO Return Air Duct (UPC-04), duct-board, or sheet metal with acoustical duct liner. Never use flex duct with a solid plastic liner in place of UPC-04.

• Isolation. Isolate the air handler with foam rubber strips under the unit. Either hang the unit from the structure using angle iron framework under unit (do not hang directly with hooks in the cabinet) or set on a platform.


• Secondary Drain Pan. Always use a secondary drain pan wherever overflow of condensate can cause water damage. Do not trap secondary drain line or connect to primary drainpipe. Place secondary drain line exit so that it is apparent when being used. For example, pipe the drain line so it drips on an outdoor windowsill and causes splashing to be noticed.

• Primary Drain. Always trap primary drain line and run drain line per local plumbing codes.

• Refrigerant Lines. Follow the outdoor section of manufacturer’s instructions for running refrigerant lines. Size and trap per the instructions.

• Check Total System Airflow. Check the airflow at each outlet with a Turbometer centered over the outlet. Add up the cfm for all outlets – it should not differ by +/- 5% from the design airflow.


• Check Static Pressure (Optional). Measure the external static pressure in plenum 2 feet [610 mm] from unit and before any fitting. Set as close as possible to 1.5 inches of water [0.37 kPa] or less, but no less than 1.0 IWC [0.25 kPa] or greater than 1.8 IWC [0.48 kPa], by providing adequate number of full outlets and minimum plenum fittings (full flow).

• Check Amperage. Measure the amperage and voltage of the motor. Verify that it matches within +/- 5% of the values for the desired (design) cfm in the installation manual or the motor amperage table shipped with each air handler. Also, use a Turbometer® to measure airflow from each outlet. If the total airflow measured by the Turbometer at the outlets is significantly less than airflow determined by motor amperage, check the system for leaks.

• Check for Full flow. Inspect each outlet for full flow (except where balancing orifices are used or for long branch ducts); it should be approximately 35 CFM [17L/s] per outlet. Investigate for blockage or kinks if flow is insufficient.

• Check Refrigerant Charge. Charge unit per outdoor manufacturer's instructions and Unico's Installation instructions. For best results use the subcooling method during the cooling cycle. Heat Pumps should be checked in both heating and cooling.