Frequently Asked Questions


1. What is a Small Duct High Velocity (SDHV) system?

The product is so unique in its application ability and principles of operation that the air conditioning industry identifies the Unico System with a separate classification called "SDHV" Small Duct High Velocity. There are only 2 companies in this classification, Unico and SpacePak. There are other companies that call themselves SDHV but cannot fall within the definition listed below:

  • 220 - 350 cfm per ton
  • 1.2 in.wc. or greater supply plenum pressure
  • Supply Outlet velocity of 1000 fpm or greater
  • Less than 6 of supply outlet opening

2. Are both sides of the system considered high pressure high velocity?

Only the supply side of the system has relatively high pressure high velocity. The return side is the same as a conventional system. This means you can apply accessories to the return side of the system the same as a conventional system. Humidifiers, UV lights and Electronic Air Cleaners. Do not use a bypass humidifier.

3. What pressure and velocity are we talking about?

A conventional duct system is designed for .1 or .2 in.wc. (inches of water column) of duct pressure. The Unico system operates at approximately 1.5 in.wc. duct pressure. A conventional system will have supply duct velocity of 500 to 700 fpm, the Unico system is 1500 to 2000 fpm and the outlet supply velocity will be 1000 to 2000 fpm.

4. How many cfm per ton is the Unico system designed for?

The Unico system is designed and tested at 250 cfm per ton. 200 cfm per ton could be applied in very moist climates if necessary but 250 cfm per ton is a must in dry climates.

5. What types of cooling or heating methods can Unico provide?


  • Refrigerant using outdoor condensing units
  • Chilled water supplied by a chiller or water to water Geo thermal


  • Standard air to air heat pump
  • Hot water from a boiler or hot water from a water to water heat pump
  • Electric heater

6. What types of refrigerant are Unico coils rated for?

R-22 / R-410A / R-407C

7. How many air handler capacity sizes does Unico offer?

There are 5 modular and 3 one-piece vertical up flow models:

  • M1218 - 1 to 1.5 Ton Modular Unit
  • M2430 - 2 to 2.5 Ton Modular Unit
  • M3036 - 2.5 to 3 Ton Modular Unit
  • M3642 - 3 to 3.5 Ton Modular Unit
  • M4860 - 4 to 5    Ton Modular Unit
  • V2430 - 2 to 2.5 Ton Modular Unit
  • V3036 - 2.5 to 3 Ton Modular Unit
  • V3642 - 3 to 3.5 Ton Modular Unit

8. What configuration and orientation?

The 2 thru 5 ton units can be mounted Horizontal, Up flow, and down flow. The 1218 can only be mounted up flow and horizontal.

9. What voltages are available?

(208v / 230v single phase) and some models are available in 120v

10. How do you set airflow?

For the PSC motor (standard single speed) CFM is initially set using the blower restrictor plate on the outlet of the blower. You move it up or down to set motor amperage listed on the side of the blower module. CFM and mass flow (pounds of air per minute) are related. As the cfm goes up so does the mass flow and amperage. With the ECM motor (variable speed) the motor control board has 2 factory settings which can be custom set with a laptop. There are 6 different speeds available.

11. How is cfm measured?

CFM is measured with a Turbo Meter by Davis instrument. The meter is positioned over the outlet opening and is measured in knots. A knot to cfm conversion chart is provided to determine cfm. The Turbo Meter is the only method we tested to measure air flow. It is available thru Unico.

12. How do you charge a Unico system?

Unico uses a TXV for the refrigerant metering device. TXVs are normally charged by measuring sub cooling at the outdoor unit. Lift and line set length will change this value. UNDERSTAND SUBCOOLING.

13. Will 250 cfm per ton affect my suction pressure?

Measured suction pressure will approximately be 10% less than a conventional. As an example with R410A, instead of 128 psi you will read about 115 psi.

14. Will 250 cfm per ton in heating on a heat pump effect head pressure?

Measured head pressure can be 30 psi higher than listed on your charging chart.

15. Will 250 cfm per ton change my discharge air temperature in cooling?

The outlet air temperature in cooling will be colder than a conventional system. Normally on a conventional you will have 15 to 18° drop across the coil, on a Unico you will have 22 to 28° temperature drop.

16. Will 250 cfm per ton affect my discharge air temp in the heating mode with a heat pump?

Because of a lower air flow the Unico heat pump outlet air temperature will be approx. 25° warmer than a conventional.

17. How many 2” supply outlets per ton do I design for?

Six to 8 outlets per ton at Sea Level and a moist climate, 7 to 10 as you go up in altitude or a dry climate. Starting with a minimum of 7 per ton is recommended at 2,000 ft. elevation.

18. How do I size my refrigerant piping?

You must follow your outdoor unit manufactures recommendation for refrigerant piping size for the outdoor tonnage. Basically the pipe sizing is the same as a conventional air handler.

19. What is and why use sound attenuator tubing?

The sound attenuator tubing core is made of spun nylon, covered with insulation and an outer jacket of Mylar. The Mylar is a vapor barrier and maintains internal duct pressure. The sound attenuator must be used if the outlet sound level is important. You must use at least 3 feet of sound attenuator tubing before the outlet to prevent air noise. Attenuator tubing comes in 12ft lengths; you can use the full length if you choose.

20. When should I use the aluminum supply tubing?

Aluminum tubing is more rugged. If the outer jacket is torn air will not escape. You can use it on long runs where you are going up walls and across attics with sharp corners. Aluminum comes in a 25 ft length and is a little less expensive than sound attenuator. You can run aluminum and sound attenuator if you choose but the last 3 ft. or more must be sound attenuator. There is a coupling for joining or extending lengths in the install kits.

21. How far can you run the supply tubing?

You can run the supply tubing up to 30 ft if required. If airflow is reduced from length so will the delivered BTU’s. To compensate you may have to add more outlets to that area to heat and cool properly.

22. How many styles of finished outlets does Unico have?

There are two styles. The round outlets you are familiar with and slotted outlets. Both styles can come in plastic, metal and a variety of woods.

23. How many types of outlet boots are available?

There are round outlets and angled round outlets in case you are installing in a vaulted ceiling. There are straight slotted boots and a slotted boot with a 90° bend. The 90° boots can be used in a 2" x 4" wall with side wall air discharge.

24. Can any condenser or heat pump work with a Unico system?

Almost all makes of condensing units will work quiet well. You will lose about 2 points on a 13 seer and 5 to 15% Btu capacity at certain conditions. This is because we are operating at 250 cfm per ton instead of 400 which lowers Btu capacity. Unico operates at a higher duct static which lowers cfm per watt. SEER = BTUH / WATTS. The Unico duct system is much more efficient at delivering BTU’s to the space that we regain the efficiency and capacity because of our more efficient duct system.

25. How high a SEER rating can I install with a Unico System?

If the highest rated unit efficiency on paper is what is required by the homeowner then Unico will not equal a conventional systems matched combination. Even though the Unico’s delivered efficiency can be higher. A 13 SEER to a two stage 19 SEER will work very well with Unico air handlers. Be sure the control scheme is compatible. Some units have communicating indoor and outdoor sections. We use standard 24 volt control.

26. What is aspiration or induced flow?

At 1000 fpm a low pressure develops around the air stream. The low pressure promotes room circulation. A return air in the same room is generally not needed to create circulation but a path must be provided to get back to the return air intake.

27. Are there install kits for rough in?

There are rough in kits available for the round and slotted outlets. The rough in kits look like a light receptacle kit. A metal bracket will expand to fit between the joist spaces.

28. How do I handle changes in altitude?

You must adjust your air flow design as you change altitude. Increasing altitude reduces air density and increases cubic feet per pound. Reducing air density reduces poundage of air crossing the coil. The higher you go the more cfm you will have to move to equal the same mass flow as at sea level. For example at sea level 1 lb of air occupies 13 cubic feet, but at 5000 ft it takes approximately 16 cubic feet. At sea level you would move 250 cfm per ton and require 8 outlets per ton and at 5000 ft you would move 300 cfm per ton and require 10 outlets per ton. Refer to our technote #103 or Call Unico technical support about high altitude installations, you may exceed the capabilities of the blower because of air density.

29. Where did 33 cfm per outlet come from?

Unico used to recommend 5 outlets per ton, (200 cfm / 5 = 40 cfm per outlet. An outlet was noisy at 40 cfm. The cfm was reduced until the outlet became quiet which is approximately 33 cfm. 200 cfm / 6 = 33 cfm per outlet. The Unico system is designed for 250 cfm per ton. 200 cfm per ton is possible only in moist climates where the condensing moisture yields heat into the coil to keep suction pressure above freezing.

30. What is a FRO (Full Rated Outlet)?

When we talk about the number of outlets per ton we really mean the number of FRO outlets per ton. A FRO means 10 ft long and 33 cfm

31. What is a high ambient heat pump kit?

If a heat pump is operated in mild weather meaning 60° plus you may have a head pressure problem. In mild weather the heat pumps capacity can exceed the cooling capacity. The mild weather kit cycles the outdoor fan off if head pressure gets too high. Stopping the outdoor fan reduces outdoor coil load which drops suction pressure and in turn lowers the head pressure. The outdoor fan will cycle on and off on head pressure to prevent high pressure faults.

32. Can the Unico system be zoned?

The Unico system can be zoned. A number of design scenarios can be applied. Please refer to technote 110 or call Unico technical support with your design application and they will select the proper system configuration.


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