CA C-20 HVAC common mistakes
CA C-20 HVAC · Understand the traps and how to avoid them.
Sizing supply and return ducts from a cfm-per-ton shortcut (e.g., 400 cfm/ton) without running a room-by-room load or a friction-rate/equivalent-length calculation, then discovering the trunk cannot carry the blower's rated airflow.
The shortcut is fast and 'always worked' on similar jobs, and the exam's layout questions look like simple arithmetic, so candidates skip the Manual D-style pressure budget and the equivalent-length of fittings.
What to do instead
Before picking a size, compute the run's total equivalent length including fittings, pick a friction rate (IWC per 100 ft), and use the duct slide rule/chart. Verify total external static pressure at design cfm against blower data. Practice one full trunk-and-branch layout end to end, showing the pressure budget on paper.
Turning in a takeoff that lists only the net square footage of duct, omitting seam/lap allowance, fitting cutouts, and a waste factor, so the ordered galvanized coil comes up short on the job.
Candidates measure the finished duct area on the plan and stop there, forgetting that each joint and Pittsburgh seam consumes metal and that field cuts generate scrap.
What to do instead
For every item, convert net area to ordered area by adding lap/seam allowance (commonly ~1 in. per joint on the development) plus a standard waste percentage, and list length of coil vs. sheet stock separately. Build a checklist: net area, seam allowance, fitting allowances, waste, fasteners, sealant.
Diagnosing low suction pressure as 'undercharged' and adding refrigerant when the real cause is restricted airflow, a dirty evaporator coil, or a liquid-line restriction.
Low suction looks identical on the gauge whether the problem is charge, airflow, or a restriction, and adding refrigerant produces an immediate pressure rise that feels like a fix.
What to do instead
Always measure superheat, subcooling, and evaporator delta-T together before touching the charge. High superheat plus high subcooling points to a restriction; high superheat with normal subcooling and low delta-T points to airflow; then confirm with temperature readings across the coil and the drier. Only add charge when subcooling is genuinely low.
Testing a heat pump in heating mode the same way as cooling - checking superheat and expecting low-side pressures to behave like a cooling call - and misjudging charge as a result.
Candidates learn one charging method (superheat in cooling) and apply it year-round, not realizing that in heating the indoor coil is the condenser and the correct target is subcooling (or a weigh-in per data plate).
What to do instead
Confirm the mode of operation first (reversing valve state, which coil is the condenser), then use the manufacturer's heating-mode charging chart: subcooling at the indoor coil or weigh-in charge. Memorize which mode the reversing valve is energized in and verify with the solenoid rather than assuming.
Using the nameplate Minimum Circuit Ampacity as the breaker/fuse size, or sizing the disconnect at MCA instead of the marked Maximum Overcurrent Protective Device.
MCA and MOP appear side by side on the data plate and both are amps, so candidates grab the larger-looking or first number.
What to do instead
Size conductors to at least MCA (with ampacity adjustment as required), but size the branch-circuit overcurrent device at the MOP/HACR maximum, never above it. Write both numbers on the work sheet next to the unit before selecting wire and breaker.
Brazing joints without a continuous nitrogen purge through the line set.
Purging takes setup time and a regulator, and the joint 'looks fine' afterward - the black oxide scale forms on the inside where it cannot be seen until it plugs a screen, TXV, or compressor.
What to do instead
Flow a low-pressure dry-nitrogen purge (a few cfm, vented to atmosphere) for the entire time the joint is above brazing temperature, and use a heat-sink or wet rag near valves and driers. Treat purge setup as a required step on every braze, not an optional one.
Judging evacuation by the manifold gauge's compound needle reaching '30 in. Hg' instead of using a micron gauge on the system with valve cores removed.
The manifold gauge bottoms out at roughly 30 in. Hg, which corresponds to thousands of microns - it looks like a deep vacuum when the system is still wet, and the gauge is on the wrong side of the hoses.
What to do instead
Use a dedicated electronic micron gauge connected at the system (core tools or a dedicated port), pull through large-bore hoses with cores removed, and require a standing vacuum decay test (isolate and hold, typically under 500 microns with minimal rise).
Replacing components by guesswork ('parts cannon') - swapping the capacitor, then the contactor, then the compressor - before taking electrical and refrigerant measurements.
A no-cool call creates time pressure, and swapping an inexpensive part feels faster than tracing a circuit; candidates also fear the meter if they are not fluent with it.
What to do instead
Follow a fixed sequence every call: verify power and control voltage, check capacitor microfarads, winding resistance and insulation, then refrigerant pressures and temperatures (superheat, subcool, delta-T) and airflow. Write each reading down and let the data pick the component; change one part at a time and re-test.
Installing a condensate drain with no trap (or a trap of the wrong depth), and omitting the secondary drain pan/switch or the code-required safety device on an attic or ceiling installation.
Drain details are treated as finish work and get done last, after inspection attention has moved on, and the trap depth calculation (based on the unit's static pressure) is not intuitive.
What to do instead
Before rough-in, calculate trap depth from the fan's total static pressure, install a trapped primary drain with a cleanout and slope per code, and add the secondary pan with a float switch or secondary drain piped to a conspicuous location. Make drain and safety-device details a line item on the layout drawing so they cannot be skipped.
Quoting a job from equipment and material cost alone, leaving out permits, labor burden, refrigerant and brazing consumables, crane/lift, disposal, or sales tax.
Candidates price what they can see on the takeoff and forget the soft and indirect costs that determine whether the job makes money; exam estimating items are designed to catch exactly this omission.
What to do instead
Use a standard bid template with fixed lines for labor hours x loaded rate (wage plus burden), permits and inspection fees, consumables, equipment rental, disposal, tax, and overhead/profit. Run the template on a sample job and compare the total to the material-only figure to see the gap.