Interview Questions Air Handling Unit
  1. What is an Air Handling Unit (AHU)?

An Air Handling Unit (AHU) is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning system. An air handler is usually a large metal box containing a blower, heating or cooling elements, filter racks or chambers, sound attenuators, and dampers

2. What is the classification of AHUs?

According to Blower/Fan Arrangement

  • Draw Through
  • Blow Through

According to Mounting Location

  • Floor Mounted AHU
  • Ceiling Suspended AHU
  • Rooftop AHU

According to Structure

  • Horizontal Floor Mounted AHU
  • Vertical Floor Mounted AHU

According to Air Conditioning Characteristics

  • Outdoor AHU/ Once Through System/ Fresh Air Handling Unit
  • Recirculating Mixing AHU

According to Purpose

  • Single Zone
  • Multi Zone

According to Manufacturing

  • Custom built/ Factory Fabricated
  • Standard fabrication / Field Built-Up

According to Decker

  • Single Decker
  • Double Decker

According to Cooling Medium

  • Chilled Water AHU
  • DX Type AHU

Based on the frame-panel joints

  • Thermal break profile
  • Non thermal break

3. What are the types of the casing of AHU?

  • Single Skin Casing
  • Double Skin Casing

4. What are the types of skin?

  • Inner Skin
  • Outer Skin

5. What are the different types of material of skin?

  • Galvanized Iron (GI)
  • Stainless Steel (SS)
  • Aluminum (AL)

6. What are the different types of coating on the skin of the Air Handling Unit?

  • Plain
  • Pre-Coating
  • Powder-Coating

7. What IS the thickness of the panel skin of the Air Handling Unit? (Inner and outer)

Galvanized Iron (GI)

  • 0.6mm
  • 0.8mm
  • 1mm
  • 1.2mm

Stainless Steel (SS)

  • 0.63mm

Aluminum (AL)

  • 0.7mm
  • 0.9mm

8. Type of sandwich material used between the inner and outer skin of Double Skin Air Handling Unit?

  • Polyurethane Foam (PUF)
  • Rockwool or Mineral Wool
  • Polyisocyanurate (PIR)

9. What is the thickness of Sandwich material normally used in AHU?

  • 23mm ± 2mm
  • 43 mm ± 2mm
  • 50 mm ± 2mm 

10. What is the density of sandwich material used between inner and outer skin?

  • Polyurethane Foam (PUF) : 38-40 Kg/m3
  • Rockwool or Mineral Wool : 48 / 64 / 96 / 144 Kg/m3
  • Polyisocyanurate (PIR) : 40 Kg/m3

11. What is the effect of Puff / PIR Thickness on AHU cost?

If thickness increases then price also increase accordingly

12. What is the material of frame profile used in AHU?

Galvanized sheet steel

13. Type of Motor typically used in AHU?

  • 3 Phase TEFC (Typically enclosed Fan Cooled) Squirrel Cage induction motor with Class F Insulation.
  • Standard Motors are designed for 50 deg C ambient temperature.

14. What are the types of motor Poles?

  • 2 Pole (2880 RPM)
  • 4 Pole (1440 RPM)
  • 6 Pole (960 RPM)

15. What are the types of Motor EFFICIENCY?

  • IE1 or EFF2
  • IE2 or EFF1 (High Efficiency)
  • IE3 or EFF3 (Premium Efficiency)
  • IE4 (Super Premium Efficiency)

16. What is the application of Coil?

Air handlers may need to provide heating, cooling, or both to change the supply air temperature, and humidity level depending on the location and the application. Such conditioning is provided by a heat exchanger coil(s) within the air handling unit air stream, such coils may be direct or indirect in relation to the medium providing the heating or cooling effect.

17. What are the types of coil used in the Air Handling Unit?

  • Chilled water Coil
  • Hot Water Coil
  • DX Coil

18. What are the various types of coil material?

  • Copper (Plain)
  • Stainless Steel

19. What are the different parameters for Coil Selection?

  • Flow (CFM)
  • Velocity of water
  • Type of system DX or water
  • Coil leaving and entering temp
  • Application

20. What is the standard thickness and material of the Fins of the Cooling coil?

  • 0.13mm Plain / Blue coated Al fins

21. What should be the standard number of fins per inch (FPI) & Fins per meter?

  • 11-12 FPI & 472 Fins per meter

22. What is a type of coil header used to connect the Chilled water pipe?

  • Copper header & MS end connection

23. What are the types of coating on the coil?

  • Hydrophilic coating

24. What are the different types of filters based on efficiency?

  • Pre -Filter: – Prefilter is used for initial filtration of the air from the mixing chamber. Generally we use G-4 Filter which has a filtration of 10 Micron.
  • Bag- Filter: – It is the second stage of air cleaning after pre-filter, the air pass on to bag filter.
  • Fine Filter-Fine Filter is located after the blower. It is used only when the air to be supplied should have less particle sizes than 10 microns (Critical Application).

EU 4-10 Microns

EU 5-5 Microns

EU 7-3 Microns

  • HEPA (High efficiency particulate air)

EU 13-Hepa 0.099 %

EU 14- Hepa 0.0099 %

25. What should be the velocity across the filter?

  • 500 FPM

26. What is an eliminator & its application?

  • To avoid water carry-over in humid areas or when the velocity of air across the cooling coils is high, drip eliminators are provided.

27. What is the material of the drain tray?

  • SS- The condensed water tray is made of stainless steel or galvanization steel, free from rust and erosion

28. What are the accessories used for Air Handling Unit ?

  • Differential pressure switch
  • Airflow switch
  • Volume control damper
  • Hinged door panel
  • Filters
  • Cooling coil
  • VFD
  • Magnehelic Gauge
  • Marine Lamp
  • UV Lamp
  • Measuring Nipple and Sight Glass

29. What is a fresh air damper & why it is used?

  • Damper Place in fresh air duct to modulate the fresh air entering the AHU is fresh Air Damper. It is used to suffuse the fresh air requirement inside the area to be catered.

30. What are the different types of Blowers?

  • DIDW Forward Curve
  • Backward Curve
  • Aero foil
  • Plug fans
  • EC Plug Fans

31. What is the internal pressure drop for various types of equipment of AHU?


4- Row Deep – 20mm

6 Row Deep – 20mm

8 Row Deep -25 mm


2- Row Deep – 10 mm

4- Row Deep – 15 mm

6 Row Deep – 15 mm

8 Row Deep -20 mm


2- Row Deep – 10 mm

4- Row Deep – 15 mm


EU 4: 10 mm

EU 5/EU 6/ EU-7: 20 mm

EU-8 / EU-9: 30 mm

EU-13: 50 mm

  • HUMIDIFER- 3 mm
  • AHU CASING – 10 mm

32. What are the different types of Certification of Air Handling Unit?

  • Eurovent AHU (Eurovent Certification certifies the performance ratings of air-conditioning and refrigeration products according to European and international standards.
  • UL Certification- For Fire Rating

33. What are the different parameters controlled by Air Handling Unit?

AHU Control the Following parameters of the Space

  • Temperature
  • Humidity
  • Air Movement
  • Air Cleanliness

34. What are the different dampers & Louvers in Air Handling Unit?

  • Fresh air damper
  • Return air damper
  • Supply air damper
  • Relief/Purge/Bleed air damper
  • By Pass air damper
  • Sand Trap Louver

35. What are the Essential Requirement for Air Handling Unit Selection?

  • Ambient Temperature- Deg. C or Deg. F
  • Total Capacity – MBH, BTU, KW, TR.
  • Sensible Capacity- MBH, BTU, KW, TR.
  • Supply Air- CFM, L/S
  • Fresh Air- CFM, L/S
  • Entering Coil Temperature- Deg. C or Deg. F
  • Leaving Coil Temperature- Deg. C or Deg. F
  • External Static Pressure- Pa, In.wg
  • Power Supply- V-Ph-Hz
  • Chilled Water Entering Coil Temperature- Deg. C or Deg. F
  • Chilled Water Leaving Coil Temperature- Deg. C or Deg. F
  • Chilled Water Flow- GPM or L/S
  • The filtration requirement
  • Location – above ceiling or in separate room
  • Some other requirements like body type, casing type, fan type.
  • Fan selection from highest pressure drops in ducts and air inlet outlets.
  • Control selection like it is coming with which types of controls, is it standalone type or can be controlled by BMS only.
  • Discharge  type  –– upward/downward/side  flow

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