HVAC Rooftop Units 030203d Flashcards
What is incorporated within the control unit?
System-on switch.
System-on indicating light.
Exhaust fan indicating light.
Plugged filter indicating light.
Page 4
What is a direct fired makeup air unit?
Does not have a heat exchanger. Burner heats the incoming air directly as it passes by.
Page 5
What are the two parts that the heating section is divided into?
Burner area and the heat exchanger area.
Page 7
What are the electrical components of a roof top unit?
Rollback switches.
Ignition transformer.
Flame rod.
Gas valve (operator)
High limit (temperature sensing)
Combustion blower (induced draft) motor.
Pressure switch.
Hot surface igniter.
Air prove switch.
Page 7
What are the mechanical components of a roof top unit?
Burners.
Pilot assembly if required.
Gas line connection.
Gas valve.
Page 7
What are the components in the heat exchanger area for roof top units?
Heat exchanger tubes.
Flue.
Sensing portion of high limit.
Sensing portion of fan limit when applicable.
Page 7
What are some ways to achieve two-stage heating?
Two separate gas valves feeding one burner.
A single burner fed from a gas valve that has two valve positions.
Two separate heating units, each with its own heat exchanger.
Page 8
What is an ignition transformer?
Located in or just outside the burner area. High-voltage transformer with a primary voltage of normally 120 V and a secondary voltage of approximately 5000 to 30000 V open circuit.
Can strike an arc of approximately 4 mm to 8 mm and ignite the gas.
Page 8
What is a rollback switch?
Located near the burners and the gas manifold.
N/C temperature switch that senses a higher than normal temperature if the burner flame rolls back from the burner.
If the switch opens the gas valve will close. Automatically resets when the temperature decreases.
Page 8
What is the flame sensing current value from a flame rod?
Approximately 1.5 uA to 4.0 uA.
Page 8
What are the components of the cooling section for roof top units?
Compressor.
Condenser fan motor.
Safety switches (low-pressure switch, high-pressure switch, and low-temperature switch)
Compressor.
Condenser fan motor.
Safety switches (low-pressure switch, high-pressure switch, and low-temperature switch)
Compressor heater.
Compressor lockout controller.
Page 9
What is the purpose of the compressor?
Takes low pressure vapour and compresses it, increasing both the pressure and temperature of that vapour.
This hot, high-pressure gas is then forced out into the condenser coil.
Page 10
What is the difference between a hermetic and semi-hermetic compressor?
Hermetic is a sealed unit and are replaced and disposed of when they fail.
Semi-hermetic is constructed so that all components are accessible and repairable.
Page 10
What could cause a low pressure switch to open?
Refrigerant leak in the rooftop unit.
If pressure drops to approximately 25 psi the switch will open.
Page 10
What psi level will cause a high pressure switch to open?
Approximately 410 psi.
It will automatically reset when pressure drops to approximately 210 psi.
Page 11
What is another name for a low-temperature switch and when does the switch open?
Freeze stat switch.
Will open at approximately 40 degrees F or 4 degrees Celsius.
Automatically resets if temperature rises to 58 degrees F or 14 degrees Celsius.
Page 11
What are the two types of compressor heaters also known as crankcase heater?
Insertion type
Band type (wraparound or belly band)
Both are a resistive-type electric element.
Page 11
What is the most common type of burner assembly for an HVAC rooftop unit?
In-shot burner.
What is the difference between Direct Spark Ignition (DSI) / Hot Surface Igniter (HSI) and an intermittent ignition?
DSI and HSI do not have a pilot flame.
Page 17
What is the free cooling / economizer function of the HVAC rooftop unit?
Provides comfort cooling without running the mechanical HVAC system. Provides 90% electrical energy savings.
Page 18
What are some names used to describe HVAC control systems?
Direct digital control (DDC) system.
Building automation system (BAS).
Energy management system (EMS).
Environmental control system (ECS).
Facilities management system (FMS).
Page 23
What are some typical inputs for basic rooftop controllers?
Room temperature sensor - located in the building space, typically a thermistor.
Discharge air temperature sensor - located in the supply air duct
Outside air temperature sensor - located outside (preferably on the north side) of a rooftop unit.
Return air temperature sensor - located in the return air duct
Fan status - located in the electrical control cabinet
Optional mixed air temperature sensor - located in the mixed air section near the return air duct and the outside air damper.
Optional manual override - overrides software programs and reverts to manual operation.
Page 24
What are some examples of typical outputs for a basic rooftop controller?
Fan on-off control.
Heating on-off control.
Cooling on-off control.
Page 24
