SCO Plumbing & Gas common mistakes
SCO Plumbing & Gas · Understand the traps and how to avoid them.
On venting questions, sizing or routing a vent using drain-pipe rules (e.g., assuming a 1.5-inch fixture drain may carry a 1.5-inch vent for its full developed length), rather than applying the separate vent-sizing table and its length/developed-length limits.
Candidates study drainage tables heavily and treat venting as an afterthought; the two systems share pipe sizes on drawings, so the drain-size answer feels correct even when the vent table gives a different permitted length or size.
What to do instead
For every venting item, first identify whether it is a stack vent, individual vent, common vent, or wet vent, then look up that vent type's own table. Practise converting developed length (horizontal + vertical run) before selecting size, and re-check that the selected size satisfies the length limit, not just the fixture count.
Treating a wet vent as an ordinary dry vent and ignoring the extra restrictions: the wet-vented fixture must connect downstream of the water closet, the wet vent must be sized for the total fixtures it serves, and it must be no smaller than the drain it vents.
Wet venting is one of the few places where a drain also performs vent duty, so candidates apply the dry-vent logic they practised and miss the ordering and sizing conditions.
What to do instead
Memorise the wet-vent checklist as a sequence: fixture type permitted, connection order relative to the WC, minimum size based on combined load, and horizontal-to-vertical limits. Run through at least ten worked wet-vent layouts, checking each condition explicitly.
Calculating water-supply pipe sizing from fixture count alone, without applying the demand-load (fixture-unit to gpm) conversion, the pressure-loss allowance for the highest fixture, or the developed-length and elevation penalty.
Fixture units are the visible number on the plan, so candidates stop there; the pressure-availability and friction-loss steps feel like extra work and get skipped under time pressure.
What to do instead
Work supply sizing in fixed steps: total fixture units, convert to probable demand, subtract pressure losses for meter, elevation, and developed length, then confirm residual pressure at the most remote fixture meets the minimum. Do timed drills where you must show each intermediate number.
Answering trap-seal and fixture-clearance questions from memory of a 'standard' number (e.g., assuming one universal trap-arm length or one universal seal depth) instead of the code's specific trap-arm distance, seal depth, and fixture-to-wall clearances.
Several similar numbers circulate (trap arm length, seal depth, vent distance, water-closet clearance), and candidates conflate them because they are all small distances expressed in inches.
What to do instead
Build a one-page number sheet grouping these values by category (trap, vent, clearance, slope) and test yourself by category, not by number. When a question gives inches, ask which category the dimension belongs to before choosing.
On slope and drainage questions, applying a single slope (commonly 1/4 inch per foot) to every pipe size, instead of using the larger slope required for smaller-diameter drains and the minimum slope permitted for larger mains.
The 1/4-inch-per-foot figure is the most rehearsed number and gets applied universally; candidates forget that small drains need steeper fall to remain self-cleansing.
What to do instead
Learn the slope table by pipe size with the reason attached (self-cleansing velocity), and practise questions that state pipe size and ask for minimum fall, and vice versa. Include the maximum-slope caution for fittings that could lose seal.
Selecting gas pipe size from a single capacity table without correcting for the specific gas, its heating value and specific gravity, inlet pressure, and the allowable pressure drop to the appliance.
Candidates memorise one capacity column (often natural gas at low pressure) and reuse it for propane or for higher-pressure systems, because the table looks like a simple size-to-load lookup.
What to do instead
Before sizing, write down four inputs: gas type, inlet pressure, allowable pressure drop, and total connected load with diversity. Then choose the matching table and apply the correction factor if the gas differs from the table's basis. Practise with both natural gas and propane so the correction step becomes automatic.
Sizing gas pipe by adding the nameplate inputs of all appliances with no diversity or demand factor, and by using the longest single run rather than the longest developed length from the meter/regulator to the most remote appliance.
Adding every appliance is the intuitive 'safe' approach, and measuring the run that looks longest on a sketch misses branches that add equivalent length.
What to do instead
Always compute total connected load, apply the permitted demand/diversity factor, then measure developed length including fittings and elevation to the furthest appliance. Practise on branched layouts where the visually longest path is not the hydraulically longest.
Misapplying combustion-air and venting rules for Category I versus direct-vent appliances, e.g., treating a fan-assisted appliance as naturally drafting, or ignoring the prohibition on combining vent systems of different categories.
Appliance category is often buried in the model number or installation manual, and candidates default to the atmospheric-draft rules they learned first.
What to do instead
For each venting question, first classify the appliance by category and draft type from the stem, then apply that category's venting rules (material, termination, slope, common-venting permission). Build a comparison grid of the categories and drill questions by category.
Ignoring gas piping installation details that are frequently tested: sediment trap placement, union and shutoff location, protection of concealed piping, and bonding/grounding of gas piping.
These are small installation details rather than calculations, so candidates focus on sizing and pressure and skim the installation section.
What to do instead
Make a short checklist of installation requirements (trap at the appliance connection, shutoff ahead of the union, sleeve/protection through masonry, bonding conductor sizing) and review it before every practice set. Ask yourself where each fitting sits relative to the appliance.
Testing gas piping for leaks with the wrong medium or pressure, or failing to distinguish the rough-in test from the final test after appliances are connected (including the allowable pressure drop and test duration).
Test pressures and durations differ by stage and by system pressure, and candidates remember one test pressure number and apply it to every scenario.
What to do instead
Memorise a two-row table: rough-in test versus final test, each with medium, pressure, and duration, plus the rule that the test pressure must not exceed the appliance rating. Practise questions that state the stage explicitly and require you to pick the correct test.