4th Class Power Engineering heating and cooling systems practice questions (4B).

Five free practice questions on heating and cooling systems: unit B11 of paper B (4B) of the 4th Class Power Engineer certificate exam, written to the SOPEEC syllabus. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Recall · Describe the auxiliary equipment used in a steam heating system, including air vents, radiator valves and traps, and condensate return equipment

   In a two-pipe vacuum steam heating system at a university, the vacuum pump unit connected to the return lines

   1. separates flash steam from the condensate for reuse in the mains
   2. pumps fresh make-up water into the boiler as its level falls
   3. draws air and condensate out of the returns and vents the air
   4. raises the pressure in the returns to push condensate back
   Show the answer

   C. Correct. The pump keeps the returns below atmospheric pressure, pulling air and condensate through the radiator traps into its receiver. It discharges the air to atmosphere and sends the condensate back to the boiler, so steam fills the radiators quickly even at a low supply pressure.

   Why not the others

   A. Flash steam is separated in a flash tank on higher-pressure condensate lines. The vacuum unit's job is to draw air and condensate from the returns and vent the air.

   B. Make-up water for losses comes from a separate make-up or feed arrangement. The vacuum unit handles the system's own condensate and air, which it draws out of the returns.

   D. The unit does the opposite: it lowers the return pressure below atmospheric. That vacuum draws air and condensate through the traps; the unit's water pump then returns the condensate to the boiler.

2. Procedural · Describe the purpose and function of standard hot water heating system accessories

   An operator closed a branch's calibrated balancing valve to isolate a fan-coil unit for a coil repair. When the repair is done, the operator should

   1. reopen the valve fully to give the coil full flow
   2. reopen the valve to its balanced setting
   3. reopen the valve until the room is at set point
   4. reopen the valve to match the branch beside it
   Show the answer

   B. Correct. The balancer set each valve to pass its branch's design flow, and that setting is recorded in the balancing report; many valves also have a memory stop that holds it. Returning the valve to it restores the balance; any other opening changes this branch's flow and, with it, the flow left for the others.

   Why not the others

   A. Fully open, this branch would take more than its design flow and rob the units farther out on the system. The valve was balanced partly closed on purpose.

   C. Room temperature is the job of the unit's control valve. Using the balancing valve to chase the set point upsets the flow to the rest of the system.

   D. Each branch was balanced to its own design flow, and the neighbouring branch's setting may differ. This valve goes back to its own recorded setting.

3. Procedural · Discuss the concept, application, construction, and operation of infrared heaters

   An aircraft hangar has large doors that open several times a shift. Gas-fired infrared tube heaters hung high in the roof are chosen over unit heaters mainly because they

   1. warm people, floors and aircraft directly instead of the air
   2. heat the incoming outdoor air before it reaches the work area
   3. need no flue to carry their combustion products outdoors
   4. move more warm air than unit heaters of the same input
   Show the answer

   A. Correct. Infrared energy passes through the air and is absorbed by the people, floor and aircraft it falls on. The warm floor and objects hold the heat, so when the doors open and the air is swept out little of the heat goes with it, and comfort returns quickly once they close.

   Why not the others

   B. Infrared heaters warm surfaces, not the air coming in through the doors. Their advantage is that the heat is held in the floor and objects, so less is lost when cold air rushes in.

   C. Gas tube heaters burn inside a steel tube and commonly discharge their products outdoors through a vent. The reason for choosing them here is that they heat the floor and objects directly.

   D. Infrared heaters have no fan; unit heaters use fans to move warm air. Infrared works by radiation, warming the surfaces it strikes.

4. Recall · Describe general layout and operation of central air conditioning systems

   In a VAV system, each office's terminal box has a hot-water reheat coil, and the air handler supplies air at 13 °C. As an office cools on a winter morning, its box controller first

   1. cuts the airflow to its minimum, then opens the reheat valve
   2. opens the reheat valve while holding the airflow at maximum
   3. raises the airflow to bring more supply air into the office
   4. shuts the airflow off and heats the office with its coil alone
   Show the answer

   A. Correct. Air at 13 °C cools the room, so the box first reduces it to the minimum flow set for ventilation. Only if the room is still too cool does it open the reheat valve, heating that minimum airflow. Reheating full airflow would waste both cooling and heating.

   Why not the others

   B. Heating a full flow of cooled air is the costly constant-volume reheat approach. A VAV box reduces the cold air to its minimum before it reheats.

   C. More 13 °C air would cool the office further. As the room temperature falls, the box reduces the cold air it admits.

   D. The box keeps a minimum airflow for ventilation, and its reheat coil can only heat the air passing through it. With no airflow it could not deliver heat to the room.

5. Procedural · Describe heat gain and heat loss analysis in a building or plant

   In a 30-storey residence tower in January, the ground-floor lobby is drafty and hard to heat, cold air pours in whenever the entrance doors open, and warm air leaks out around the top-floor windows. The main cause is

   1. the supply fans pressurizing the lower floors
   2. wind pressure on the tower's windward face
   3. stack effect from the warm air in the tower
   4. solar gain warming the tower's upper floors
   Show the answer

   C. Correct. Warm indoor air is lighter than the cold outdoor air, so the tall column of it presses outward near the top and draws outdoor air in near the bottom. The colder the day and the taller the building, the stronger the effect; vestibules and revolving doors at the entrance reduce it.

   Why not the others

   A. Fans pressurizing the lower floors would push air out through the lobby doors, not draw cold air in. The inflow at the bottom comes from the stack effect.

   B. Wind pushes air in on the side it blows against and out on the other, at every floor, and shifts with the wind direction. Air entering at the bottom and leaving at the top on a cold day is the stack effect.

   D. Solar gain varies with the sun and the face it strikes; it does not draw cold air in at the lobby. The pattern of inflow low and outflow high is the stack effect.

02Answer key

03What unit B11 covers

The heating and cooling systems a power engineer looks after: steam heating devices, traps, vents and piping layouts, hot-water heating layouts, accessories, pumps and expansion tanks, warm-air, infrared and electric heating, unitary, central and combined HVAC systems, heat gains, heat losses and heat recovery, and HVAC control strategies.

Part B of the SOPEEC 4th Class syllabus names this unit “B11 Heating and Cooling Systems” and lists six objectives under it (objectives 46 to 51 of 52). SOPEEC does not publish how many exam questions come from each unit, and neither do the ABSA, Technical Safety BC, TSASK or TSSA pages we checked. The right-hand column is how many questions our bank holds on each objective, Standard and Advanced together, not a count from the exam.

04What the questions turn on

Steam heating needs air out and condensate away

A thermostatic trap closes when steam reaches its element and opens for cooler condensate and air. A one-pipe radiator heats only as fast as its air vent lets air out, and its supply valve is kept fully open or fully shut, because condensate drains back through it against the steam.

Hot-water flow from Q = m c ΔT

The flow a hot-water loop needs is the load divided by c times the supply-to-return drop; the room temperature does not enter it. A glycol fill carries less heat per kilogram per degree and is more viscous, so the same pump speed delivers less heat.

Heat loss through a wall is U A ΔT

Thermal resistances in series add, and U is one over their total. Heat loss is U times area times the inside-outside temperature difference. Heat-recovery effectiveness is the outdoor air's actual temperature rise over the largest rise possible; in winter, stack effect pulls cold air in at the bottom of a tall building.

Control strategies save energy

Outdoor reset lowers the hot-water supply temperature in a straight line as the outdoor temperature rises. A VAV box cuts its airflow to minimum before it opens its reheat valve. To dry the air without overcooling, the coil cools below the dew point and the air is reheated.

05The other units

Paper 4B has twelve units, B1 to B12. Each has its own page of free questions:

The 4th Class Power Engineering practice exam (4B) page describes the full question bank; paper A has its own 4A practice exam. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.