Exam Cheat Sheet · Quick Reference

Denver Residential Mechanical (Class C) Exam (ICC F32, 2015 IRC)

Colorado  ·  Pearson Vue

Verified, not estimated. Every figure below is drawn from the official exam structure we maintain — question counts, passing standard and topic weighting. Practice questions are grounded in the source law with statute citations. We omit any figure we can't verify rather than guess at it.
Total questions
70
Passing score
70%
Exam time
180 min
Administered by
Pearson Vue
Format
Reference materials allowed

Colorado State Portion 70 questions

General Requirements 22 Q · 32%
Appliance access and working space clearances (attic, underfloor, alcove, and room installations)Clearances from combustible construction and clearance reduction methodsAppliance labeling requirements and nameplate informationAppliance installation requirements (anchorage, ignition source elevation, seismic strapping, garage protection)Mechanical administration: scope, existing system alterations, and maintenance responsibilitiesExhaust systems: clothes dryer exhaust ducts, range hoods, and mechanical ventilation ratesHydronic piping materials, support intervals, testing requirements, and floor heating systemsOil tank and oil piping installation, special piping storage systems, and solar thermal energy system requirementsGeneral Mechanical System RequirementsHydronic PipingExhaustSolar Thermal Energy Systems +2 more
Appliances and Equipment 4 Q · 6%
Equipment sizing requirements (ACCA Manual J and Manual S)Clearances, access, and serviceability requirements for heating and cooling equipmentCondensate disposal, auxiliary drain pans, and water-level monitoring devicesRefrigerant piping insulation, protection, and locking access port capsBoiler safety controls, pressure-relief valves, gauges, and low-water cutoffsExpansion tanks for hot water boiler systemsWater heater installation requirements, prohibited locations, and standardsSpecial appliances installation requirements (ranges, sauna heaters, fuel cell power plants)
Duct Systems 11 Q · 15%
Duct construction standards and material requirements (above-ground and underground)Duct sealing — joints, seams, and connection fastening methodsDuct insulation materials — flame spread/smoke-developed ratings and R-value labelingDuct support requirements by duct typeUse of stud wall cavities and joist spaces as air plenumsReturn air opening restrictions and prohibited source locationsUnder-floor plenum requirements and prohibitionsIndependent garage HVAC systems — supply and return air restrictions
Combustion Air 16 Q · 23%
Indoor combustion air volume requirements (standard method and known air-infiltration-rate method)Outdoor combustion air opening methods (two-permanent-openings and one-permanent-opening)Combination indoor and outdoor combustion air provisionsMechanical combustion air supply systems and appliance interlock requirementsCombustion air duct construction and installation requirementsLouvers, grilles, and screens for combustion air openings (net free area calculations)Combustion air requirements for solid-fuel, oil-fired, and gas-fired appliances by fuel typeFuel Gas
Chimneys and Vents 7 Q · 10%
Venting system types and applications (Type B, Type BW, Type L, single-wall metal pipe, special gas vent)Chimney and vent termination height requirementsVent connector installation requirements (slope, clearances, length, materials)Sizing of Category I appliance venting systems using IRC tablesMasonry chimney requirements (flue lining, cleanouts, connections, inspection)Factory-built chimney listing and installation requirementsDraft hoods, draft regulators, and automatically operated vent dampersFuel Gas
Fuel Supply Systems 10 Q · 14%
Gas piping materials and tubing types (steel, copper, CSST, polyethylene plastic)Gas pipe sizing methods and sizing tables (longest length, branch length, hybrid pressure)Piping system installation requirements (prohibited locations, underground burial depth, protection from physical damage)Pressure testing, leak detection, and purging proceduresShutoff valves and flow controls (appliance shutoff valves, pressure regulators, excess flow valves)Electrical bonding requirements for gas piping (CSST bonding, bonding jumper specifications)Appliance connections and connectors (maximum length, prohibited penetrations, union requirements)Fuel oil storage tanks and piping systems (tank types, installation, fill/vent piping, shutoff valves)Fuel Gas

Key Distinctions

Appliances using prescriptive combustion air methods (Sections G2407.5–G2407.9)vsAppliances using the manufacturer's instructions for combustion air

Category I natural-draft and draft-hood-equipped gas appliances are sized by the prescriptive indoor/outdoor methods, while direct-vent appliances, gas appliances of other than natural draft design, vented gas appliances not designated Category I, appliances with power burners, and solid fuel-burning appliances (M1701.1) get combustion, ventilation and dilution air per the manufacturer's instructions.

IRC 2015 Section G2407.1 (304.1)
Two outdoor openings through horizontal ductsvsTwo outdoor openings communicating directly or through vertical ducts

Horizontal-duct openings each require 1 square inch of free area per 2,000 Btu/h of total input, while direct openings or vertical ducts each require only 1 square inch per 4,000 Btu/h.

IRC 2015 Section G2407.6.1 (304.6.1)
One-permanent-opening method (outdoor air)vsTwo-permanent-openings method (outdoor air)

The single opening requires 1 square inch per 3,000 Btu/h of total input and not less than the sum of the areas of all vent connectors, whereas the two-opening method uses 1 square inch per 2,000 Btu/h (horizontal ducts) or per 4,000 Btu/h (direct/vertical) for each opening.

IRC 2015 Section G2407.6.2 (304.6.2)
Wood louvers of unknown design/free areavsMetal louvers and grilles of unknown design/free area

Where design and free area are not known, wood louvers are assumed to have 25-percent free area and metal louvers and grilles are assumed to have 75-percent free area.

IRC 2015 Section G2407.10 (304.10)
Louver with known free areavsLouver with unknown design and free area

Where the free area through a louver, grille or screen design is known, that known free area is used to size the opening; the 25-percent (wood) and 75-percent (metal) assumptions apply only where the design and free area are not known.

IRC 2015 Section G2407.10 (304.10)
Equation 24-1 (appliances other than fan-assisted)vsEquation 24-2 (fan-assisted appliances)

Under the known-air-infiltration-rate method, other-than-fan-assisted appliances use a 21/ACH factor while fan-assisted appliances use a 15/ACH factor, each multiplied by input in thousands of Btu/h.

IRC 2015 Section G2407.5.2 (304.5.2)
Standard method for indoor combustion air volumevsKnown-air-infiltration-rate method

The standard method requires 50 cubic feet per 1,000 Btu/h of total appliance input, while the known-air-infiltration-rate method (which must be used where the known infiltration rate is less than 0.40 ACH) uses the 21/ACH or 15/ACH equations per appliance type.

IRC 2015 Section G2407.5.1 (304.5.1)
Indoor combining openings between spaces on the same storyvsIndoor combining openings between spaces on different stories

Same-story openings each need 1 square inch per 1,000 Btu/h of total input but not less than 100 square inches, while spaces on different stories are considered communicating where openings in doors or floors provide a total minimum free area of 2 square inches per 1,000 Btu/h.

IRC 2015 Section G2407.5.3.1 (304.5.3.1)
Radiant piping in slab-on-grade applicationsvsRadiant piping in suspended floor joist bay applications

Slab-on-grade installations require insulating material with a minimum R-value of 5 beneath the piping (M2103.2.1), while suspended floor applications require minimum R-11 insulation in the joist bay cavity serving the heated space above (M2103.2.2).

IRC 2015 Section M2103.2.1
Attic passageway limited to 20 feetvsAttic passageway permitted up to 50 feet

The attic passageway to an appliance is limited to not more than 20 feet measured along the centerline, but 50 feet is permitted where the passageway is unobstructed and not less than 6 feet high and 22 inches wide for its entire length.

IRC 2015 Section M1305.1.3
Floor furnace access through a foundation openingvsFloor furnace access through a trap door

A foundation opening must be not less than 18 inches by 24 inches, while a trap door must be not less than 22 inches by 30 inches; either way the opening and passageway must allow replacement of any part of the equipment.

IRC 2015 Section M1408.4
Draft hoodvsBarometric draft regulator

A draft hood is a nonadjustable device built into an appliance or vent connector that relieves flue gases, prevents backdraft and neutralizes stack action, whereas a barometric draft regulator is a balanced damper device attached to a chimney, vent connector, breeching or flue gas manifold that controls chimney draft (double-acting when free to move both directions against excessive draft and backdraft).

IRC 2015 Section G2403 (202) — definition of DRAFT HOOD
Factory-made ducts listed and labeled to UL 181vsField- and shop-fabricated rigid fibrous glass board ducts

UL 181-listed factory-made ducts are supported per the duct manufacturer's installation instructions, while fibrous glass ducts are supported per the SMACNA or NAIMA Fibrous Glass Duct Construction Standards.

IRC 2015 Section M1601.4.4
Shield plates for concealed mechanical pipingvsDryer exhaust duct metal and shield plate thicknesses

Concealed mechanical piping shield plates must be steel not less than 0.0575 inch (No. 16 gage), while clothes dryer exhaust duct shield plates are 0.062 inch (M1502.5) and the dryer duct itself is minimum 0.0157 inch (No. 28 gage).

IRC 2015 Section M1308.2.3

Key Terms

Draft hood IRC 2015 Section G2403 (202) — definition of DRAFT HOOD
A nonadjustable device built into an appliance, or made part of the vent connector, that permits ready escape of flue gases in the event of no draft, prevents backdraft from entering the appliance, and neutralizes the effect of stack action of the chimney or gas vent on appliance operation (G2403/202).
Barometric draft regulator IRC 2015 Section G2403 (202) — definition of BAROMETRIC DRAFT REGULATOR
A balanced damper device attached to a chimney, vent connector, breeching or flue gas manifold to protect combustion appliances by controlling chimney draft; the double-acting type has a balancing damper free to move in either direction to protect against both excessive draft and backdraft (G2403/202).
Flue collar IRC 2015 Section G2403 (202) — definition of FLUE COLLAR
That portion of an appliance designed for the attachment of a draft hood, vent connector or venting system (G2403/202).
Venting system IRC 2015 Section G2403 (202) — definition of VENTING SYSTEM
A continuous open passageway from the flue collar or draft hood of an appliance to the outdoor atmosphere for removing flue or vent gases, usually composed of a vent or chimney and a vent connector, if used (G2403/202).
Mechanical or induced draft IRC 2015 Section G2403 (202) — definition of DRAFT (Mechanical or induced draft)
The pressure difference created by the action of a fan, blower or ejector located between the appliance and the chimney or vent termination (G2403/202).
Working space (appliance control side) IRC 2015 Section M1305.1
A level working space not less than 30 inches deep and 30 inches wide required in front of the control side to service an appliance (M1305.1).
Furnace/air handler compartment or alcove IRC 2015 Section M1305.1.1
An enclosing space requiring a minimum 3-inch working space along the sides, back and top, with total width not less than 12 inches wider than the furnace or air handler (and 6 inches at the front combustion chamber side where the firebox is open to the atmosphere) (M1305.1.1).
Appliance access passageway (compartment, alcove or basement) IRC 2015 Section M1305.1.2
An unobstructed passageway not less than 24 inches wide and large enough to remove the largest appliance, served by an opening or door, with a level service space not less than 30 inches deep at the front or service side (M1305.1.2).
Transition duct (clothes dryer) IRC 2015 Section M1502.4.3
A single length listed and labeled to UL 2158A connecting the dryer to the exhaust duct, not greater than 8 feet long and not concealed within construction (M1502.4.3).
Dryer exhaust duct IRC 2015 Section M1502.4.1
Duct with a smooth interior finish constructed of metal not less than 0.0157 inch (No. 28 gage) thick with a 4-inch nominal diameter, secured in place and supported at intervals not exceeding 12 feet, with insert ends extending into the adjoining duct or fitting in the direction of airflow (M1502.4.1, M1502.4.2).
Makeup air (kitchen exhaust) IRC 2015 Section M1503.4
Air required for exhaust hood systems capable of exhausting in excess of 400 cfm, supplied at a rate approximately equal to the exhaust rate with at least one gravity or automatic electrically operated damper (M1503.4).
Air exhaust opening termination IRC 2015 Section M1506.3
Exhaust openings shall terminate not less than 3 feet from property lines, 3 feet from operable and nonoperable openings into the building, and 10 feet from mechanical air intakes, except where the exhaust opening is located 3 feet above the air intake (M1506.3).
Intermittent whole-house ventilation IRC 2015 Section M1507.3.3
Permitted by exception where controls enable operation for not less than 25 percent of each 4-hour segment and the required ventilation rate is multiplied by the factor from Table M1507.3.3(2) (M1507.3.3).
Thermal break (heated slab) IRC 2015 Section M2103.2.3
Asphalt expansion joint material or similar insulating material required where a heated slab meets a foundation wall or other conductive slab (M2103.2.3).
Prohibited tee application (hydronic) IRC 2015 Section M2101.7
Fluid in the supply side of a hydronic system shall not enter a tee fitting through the branch opening (M2101.7).
Maintenance of required safety devices IRC 2015 Section M1202.3
Devices or safeguards required by the code shall be maintained in compliance with the code edition under which they were installed, and mechanical systems maintained in proper operating condition in accordance with the original design (M1202.3).
Installed insulation thickness, factory-made flexible air duct IRC 2015 Section M1601.3
For R-value purposes the installed thickness equals the difference between the actual outside diameter and the nominal inside diameter divided by two; nominal thickness is used for duct board and rigid ducts and a 75-percent (25-percent compression) assumption applies to ductwrap (M1601.3, item 4).

Formulas to Know

Indoor combustion air volume — standard methodRequired volume = 50 cubic feet per 1,000 Btu/h of total appliance input (example: 100,000 + 50,000 = 150,000 Btu/h; 150 × 50 = 7,500 cu ft)
Indoor volume — known air infiltration rate, appliances other than fan-assisted (Equation 24-1)Required volume ≥ (21/ACH) × (I ÷ 1,000 Btu/h) (example: (21/0.30) × 40 = 2,800 cu ft)
Indoor volume — known air infiltration rate, fan-assisted appliances (Equation 24-2), and multiple appliancesRequired volume ≥ (15/ACH) × (I_fan ÷ 1,000 Btu/h); where more than one appliance is in the space the required volumes for all appliances are added (example: (15/0.30) × 60 = 3,000 plus (21/0.30) × 30 = 2,100, total 5,100 cu ft); this method must be used where the known infiltration rate is less than 0.40 ACH
Outdoor combustion air — two permanent openingsEach opening through a horizontal duct: 1 sq in free area per 2,000 Btu/h of total input (120,000 ÷ 2,000 = 60 sq in each); each direct opening or opening through a vertical duct: 1 sq in per 4,000 Btu/h (100,000 ÷ 4,000 = 25 sq in each)
Outdoor combustion air — one permanent openingOpening free area = 1 sq in per 3,000 Btu/h of total input of all appliances in the enclosure, and not less than the sum of the areas of all vent connectors (example: 120,000 ÷ 3,000 = 40 sq in, which exceeds 28 sq in of vent connector area)
Openings combining indoor spaces for combustion airSame story: each of two openings = 1 sq in per 1,000 Btu/h of total input, not less than 100 sq in (150,000 ÷ 1,000 = 150 sq in each). Different stories: openings in doors or floors with total minimum free area of 2 sq in per 1,000 Btu/h (2 × 75 = 150 sq in total)
Combination indoor/outdoor combustion air — outdoor opening reductionRatio of interior spaces = available volume of all communicating spaces ÷ required volume; reduction factor = 1 − ratio; required outdoor opening = full opening size from G2407.6 × reduction factor (example: 2,400 ÷ 6,000 = 0.4; 1 − 0.4 = 0.6; 120,000 ÷ 2,000 = 60 sq in; 60 × 0.6 = 36 sq in each). Minimum opening dimension is still not less than 3 inches
Mechanical combustion air supply rateOutdoor air not less than 0.35 cfm per 1,000 Btu/h of total input of all appliances in the space (150,000 Btu/h → 150 × 0.35 = 52.5 cfm; 35 cfm ÷ 0.35 = 100,000 Btu/h maximum served); where supplied by the building's mechanical ventilation system the combustion air rate is in addition to the required ventilation air (60 + 35 = 95 cfm), and each appliance must be interlocked with the air supply system
Gross opening size for louvers, grilles and screensGross opening = required net free area ÷ free area fraction; use the known free area where known (44 ÷ 0.55 = 80 sq in); where design and free area are not known use 25 percent for wood louvers (60 ÷ 0.25 = 240 sq in) and 75 percent for metal louvers and grilles (90 ÷ 0.75 = 120 sq in)
Hydronic piping hydrostatic test pressureTest at 1.5 × maximum system design pressure, but not less than 100 psi (50 psi design → 75 psi, so the 100 psi minimum governs); test duration not less than 15 and not more than 20 minutes
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