HVAC Interview questions

HVAC: heating, ventilation and air conditioning
BTU: British Thermal Units
1BTU: It is the amount of energy required to raise 1 Degree Fahrenheit temperature of 1 Pound of water
Sensible Heat: It can be sensed or measured by an ordinary dry bulb thermometer. E.g.: Human body temperature can be measured using a thermometer
Latent Heat: It can’t be sensed or measured by an ordinary dry bulb thermometer. It is a hidden heat. Latent heat is absorbed or released whenever there is a change in phase. E.g.: When the water becomes vapor and vice versa
TON or Ton of refrigeration: A ton is the cooling capacity of an air conditioning system. One ton is equal to the amount of heat required (288,000 Btu) to melt one ton of ice in a 24-hour period. A one-ton air conditioner is rated at 12,000 Btu per hour (288,000/24).
1 ton = 12000 BTU/hr = 3.516 KW
The volume of air or the unit of air flow is measured in terms of:
CFM = Cubic Feet per Minute, 1 CFM = 0.417 L/s
CMH = Cubic Meter per Hour, CMH = 1.7 x CFM, CFM = 2118 x m3/sec
M3/Sec = Cubic Meter per Second/Sec = Liters per second 
ASHRAE: American Society of Heating Refrigeration and Air-conditioning Engineers
What is the Room Temperature that you maintain in any room? 24 Degree Celsius or 76 Degree Fahrenheit
Pumps supply fittings: Thermometer, Shutoff valve, pressure gauge, strainer, test point, drain cock, union flexible connector, and Eccentric reducer
Pumps return fittings: Eccentric reducer, flexible connector, union test point, check valve, strainer, gate valve, check valve, temp sensors
Relative Humidity: Relative humidity compares the actual amount of water vapor in the air at a given temperature with the total amount of water vapor the air can hold at that temperature. Warm air has more capacity to hold water vapor than cold air. Relative humidity is stated as a percentage, with a 40% to 60% range common for indoor environments.
Type of Dampers: Fire dampers = Butterfly damper, Non Return Damper, Motorized Damper, Fire damper, Smoke damper, Gravity damper
Motorized damper: Motorized volume damper, Motorized fire and smoke damper
VFD: Variable frequency drive. It is used to control the speed of a motor by controlling the frequency applied to it.
How do you control or balance the chilled water? Double regulating valve in main line return (usually controlled from return using dry)
How do you control the noise or vibration in the duct? Lining (nc-35) and Reinforcement.
primary pumps and secondary pumps: The pumps from chiller to building is Primary and in building transfer is Secondary.
What are duct heaters and why do we use them if we already have heating option in AHU? Just to provide heating in some specific areas.
VAV: To control the flow of air and temp. 
VAV & CAV: Variable Air Volume & Constant Air Volume.
Pressure dependent & Pressure independent:
Pressure dependent: The volume of air supplied by the terminal unit varies depending upon the static pressure (SP) in the primary air duct.
Pressure independent terminal units: have flow-sensing devices that limit the flow rate through the box. They can control the maximum and minimum cfm that can be supplied and are therefore independent of the SP in the primary air duct.
* Almost all HVAC systems installed or retrofitted at present have pressure independent VAV terminals. Pressure independent systems can be balanced and will allow the correct airflow from each terminal.
2 way valve: it will regulate the flow as per demand and it will be fitted in the return line.
3 way valve: if the AHU is closed and is running on min load then it will by-pass (Connected to BMS).
How do you measure & control the noise level in A/C system? Noise level can be measured by dB meter.
Fire dampers (FD) are used to stop the fire wherever duct passes through a firewall.
Splitter dampers (SD) are used to split the main duct for proportional air balancing
Non Return Dampers (NRD) are used to stop the backflow of air in the duct. NRD's used in exhaust and ventilation system only.
What are the different types of exhaust fans used to extract the air? Axial type propeller fan, forward curved, twin fan, direct driven centrifugal fan.
What are the different duct shapes normally used? Rectangular, Round, Spiral, Oval
How do you control the vibration of AHU & Chillers? Spring vibration isolators, Rubbers pads, Kinetic Chain, flexible bellows
Type of Louvers: Fresh air louvers, sand trap louvers, door louvers, acoustic louvers, supply air and return air louvers, back draft louvers
Securities checking in chillers: Flow switch, Chiller and Pump interlocking, High-Pressure cut-out, Oil Pressure switch, High Suction superheat, Anti-Freeze.
temperature & RH required for human comfort: 22 ±2 degree Celsius & 50%RH.
At what temperature degree Ferhenite & degree Celsius are same? -40°C.
Which gas is used to purge the refrigerant line during the brazing? Nitrogen gas (inert(non-combustible), nonmetallic, colorless, odorless and tasteless)
Fan Coil Unit (FCU) is a simple device consisting of a heating and/or cooling heat exchanger or 'coil' and fan.
Air Handling Unit (AHU) is used to re-condition and circulate air as part of a heating, ventilating and air-conditioning system. The basic function of the AHU is to take in outside air, re-condition it and supply it as fresh air to a building.
Name the four components that make a refrigeration system complete? Compressor, Condenser, Metering device(Expansion valve), and Evaporator
What is the purpose of “Lock out tag out”: This procedure ensures your safety and others when working on equipment.
Give us an example of a “Ternary(composed of three items)” refrigerant? R-410a, R-134a as alternative refrigerants to R-22
What does “Ternary” mean in the HVAC trade? This is a refrigerant blend, which means this refrigerant is a three part blend.
When looking for leaks on a refrigeration system what “signs” should you be looking for? Oil stains on the refrigeration system. 

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70 Question & Answers for HVAC Engineer Interview - PDF