313A system design and load calculation practice questions.

Five free practice questions on system design and load calculation: block C of the Red Seal 313A Refrigeration & AC Systems Mechanic exam, about 18 of its 125 questions. 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. Procedural · Selects HVAC/R equipment, components and accessories

   A single-stage R-404A freezer system is planned to run at 5 psig suction and 280 psig discharge on a hot day. Using psia = psig + 14.7, what compression ratio will the compressor see?

   1. 56.0 to 1
   2. 58.9 to 1
   3. 14.2 to 1
   4. 15.0 to 1
   Show the answer

   D. Correct. (280 + 14.7) ÷ (5 + 14.7) = 294.7 ÷ 19.7 = 14.96, about 15 to 1. That is well above the ratio most single-stage reciprocating compressors are applied at (roughly 10 to 1 or less per the manufacturer), so the plan should look at two-stage or compound compression, a scroll with vapour injection, or a lower design condensing temperature.

   Why not the others

   A. This divides gauge by gauge (280 ÷ 5). Compression ratio is always absolute discharge over absolute suction; gauge readings exaggerate it badly at low suction pressures.

   B. This adds 14.7 only to the discharge (294.7 ÷ 5). The suction pressure must be converted to absolute too, which is where the 14.7 matters most.

   C. This adds 14.7 to the suction pressure but not to the discharge (280 ÷ 19.7). Both pressures must be absolute before dividing.

2. Procedural · Selects control system components and accessories

   A mixed-air damper on a large air handler responds slowly, and the damper actuator supplied has a tri-state (drive open / drive closed / hold) input rather than an analog one. Which control strategy fits this actuator?

   1. Proportional control with a 0–10 V output
   2. Floating control
   3. Two-position control
   4. PID control through a 4–20 mA loop
   Show the answer

   B. Correct. Floating control pulses the actuator open or closed at its fixed travel rate when the mixed-air temperature leaves a deadband and holds position when it is inside. It suits slow processes and inexpensive actuators; because the controller does not know the damper position, it can drift after power loss and it hunts if the process responds faster than the actuator travels.

   Why not the others

   A. A proportional output needs an actuator that positions itself in proportion to an analog signal. This actuator has no analog input; its drive is open, closed or stopped.

   C. Two-position would slam the damper to full open or full closed. It ignores the intermediate positions a tri-state actuator can hold, and on a mixed-air damper it would swing the mixed-air temperature widely.

   D. PID is fine as a strategy, but it still needs an analog-positioned actuator to send 4–20 mA to. With a tri-state actuator the output form is the constraint.

3. Procedural · Determines placement of HVAC/R equipment, components and accessories

   A split-system heat pump outdoor unit is being located at a house in a region with a −30 °C (−22 °F) design temperature and heavy snowfall. Which placement is best?

   1. Directly on grade under the deck, where the structure shields it from snow and the noise is hidden.
   2. Against the wall beneath the dryer and bathroom exhaust vents to keep the line set short.
   3. In an enclosed shed to protect it from the cold and reduce noise for the neighbours.
   4. On a stand above the snow depth, with service clearances, clear of the roof drip line and prevailing wind.
   Show the answer

   D. Correct. In heating mode the outdoor coil collects frost and sheds defrost water, which freezes and builds up around the base; elevating it keeps the coil and drain clear. Roof drip and ice damage fans, wind through the coil disturbs defrost and low-ambient operation, and service clearance still has to be there in January.

   Why not the others

   A. Under a deck the unit is starved for airflow, recirculates its own discharge, and buries itself in shed defrost ice and drifting snow; it is also nearly impossible to service.

   B. Moist exhaust air freezes on the outdoor coil in winter and lint fouls it year-round. Exhaust and intake locations are an environmental placement factor the standard names specifically.

   C. An enclosure recirculates discharge air, raising head pressure in cooling and starving the coil in heating. Outdoor units are built for the ambient; they need airflow, not shelter.

4. Procedural · Determines placement of control system components and accessories

   An air handler mixes outdoor air at −25 °C (−13 °F) with return air in a large plenum. Which sensor type and placement should the plan specify for mixed-air temperature control?

   1. A single-point probe in the return duct upstream of the mixing box, where the air is uniform.
   2. A single-point probe at the supply fan discharge after the coils, where the air is fully mixed.
   3. A single-point probe mounted just inside the outdoor-air damper, where the air is coldest.
   4. An averaging (capillary) element serpentined across the mixed-air plenum downstream of the mixing point.
   Show the answer

   D. Correct. Cold outdoor air and warm return air do not blend evenly; a plenum can be stratified by 10 K or more from one side to the other. A single probe reads whichever stream happens to pass it and the loop hunts. An averaging element samples the whole section and returns the true mixed temperature, which also makes the freeze stat and economizer behave.

   Why not the others

   A. Return-air temperature is a useful reading, but it is not the mixed-air temperature the sequence controls. The sensor must be after the two streams meet.

   B. After the coils the sensor reads discharge temperature, which is what the heating and cooling loops control. Mixed-air control needs a reading before the coils, and a single point is still the wrong sensor for a stratified plenum.

   C. That reads outdoor air, not mixed air, and at −25 °C it would drive the heating valve wide open regardless of the true mix.

5. Procedural · Verifies HVAC/R system parameters and requirements

   A small server room is to be cooled by a unit delivering 2000 CFM at 55 °F (12.8 °C) with return air at 75 °F (23.9 °C). Using the sensible heat formula from the sheet, what sensible cooling does that airflow provide?

   1. 180000 Btuh
   2. 21600 Btuh
   3. 43200 Btuh
   4. 118800 Btuh
   Show the answer

   C. Correct. Q = CFM × 1.08 × ΔT = 2000 × 1.08 × 20 = 43 200 Btuh, or 3.6 tons at 12 000 Btuh per ton. The 1.08 constant is the specific heat of standard air per CFM; it applies to sensible (dry-bulb) change only.

   Why not the others

   A. This uses the 4.5 constant, which belongs with an enthalpy change (BTU/lb) for total heat. With a dry-bulb difference the constant is 1.08.

   B. This is half the correct value, the result of using a 10 °F difference. The temperature difference across the coil is 75 − 55 = 20 °F.

   D. This multiplies by the supply temperature (55 °F) instead of the temperature difference. The formula needs ΔT, the return minus supply dry bulb.

02Answer key

03What block C covers

This block covers planning an installation before anything is ordered. It includes cooling loads and heat transmission, airflow and psychrometrics, sensible heat ratio, selecting compressors, TXVs and coils, sizing suction risers and liquid lines, equipment placement and clearances, machinery rooms, material take-offs, and planning the controls: differentials and set points, PI and floating control, BACnet and MODbus, and sensor placement.

The Red Seal Occupational Standard names this block “Plans installation”. It carries 14.4% of the exam, about 18 of the 125 questions, split across two tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Formula sheet air calculations

Sensible heat is CFM × 1.08 × ΔT and counts dry-bulb change only. Total cooling uses 4.5 × CFM × the enthalpy change, because enthalpy carries both sensible and latent heat. Heat through a wall is U × A × ΔT, with U = 1/R.

Compare sensible to sensible

Sensible capacity is total capacity times the SHR. A 66 000 Btuh unit at an SHR of 0.72 gives only about 47 500 Btuh sensible, short of a 60 000 Btuh sensible data-room load. A high-SHR unit is needed there.

Risers and liquid lift

A suction riser must keep vapour velocity high enough to carry oil at minimum load, so it is sized smaller or doubled. On a liquid riser, lift and friction lower the saturation temperature; if that drop exceeds the subcooling, flash gas reaches the TXV.

Differentials and offset

On a thermostat, cut-in equals cut-out plus the differential, so -22 °C with a 4 K differential cuts in at -18 °C. A proportional-only loop settles with a standing offset from set point; adding integral action removes it.

05The other blocks

The 313A exam has six blocks. Each has its own page of free questions:

The 313A Refrigeration & AC Systems Mechanic practice exam page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.