Study guide · General Building (B) · Core Trades
HVAC: The Rules the Exam Asks For
About 14 minutes · 7 sections
What this guide covers
HVAC is the third slice of Core Trades. Half test one definition or one rule from the California Mechanical Code. Where combustion air comes in. Where return air may not. How a vent connector runs. The rest are arithmetic: airflow from load, from tons, or from air changes. This guide carries both at the grain the exam tests them.
Key terms
- Combustion air
- The total air provided to the space holding fuel-burning equipment: for combustion, draft-hood dilution, and enclosure ventilation.
- Dilution air
- Air that enters a draft hood or draft regulator and mixes with the flue gases.
- Draft hood
- A fixed device on an appliance or its vent connector with three jobs: let flue gases escape during no draft, backdraft, or blockage; block a backdraft from entering; and neutralize the chimney's stack action on the appliance.
- Vent connector
- The pipe from the appliance to the vent or chimney.
The rules the exam tests
5 rules · 2 minDefinitions
include attics and crawl spaces used to move air.
A plenum is an air compartment or chamber other than the occupiable space being conditioned. One or more ducts connect to it, and it forms part of the supply-air, return-air, or exhaust-air system. Uninhabited crawl spaces above a ceiling or below a floor, spaces below raised computer-room floors, and attic spaces used that way are plenums. The conditioned room itself is never the plenum.
On the job
The exam tests the supply plenum as the box on the furnace that the branch ducts leave from. A joist space or attic used to move air is a plenum too, with a plenum's rules.
Exact wording
A plenum is an air compartment or chamber, other than the occupiable space being conditioned, to which one or more ducts connect and that forms part of the supply-air, return-air, or exhaust-air system. It includes uninhabited crawl spaces above a ceiling or below a floor, spaces below raised computer-room floors, and attic spaces used that way. The conditioned room itself is never the plenum.
Mechanical Code § 218.0
A draws all combustion air from outdoors and vents all flue gases outdoors.
Both the build and the installation must meet this: combustion air comes directly from the outdoors, so none comes from the room, and all flue gases discharge to the outdoors.
On the job
The tested consequence: a direct-vent appliance takes no combustion air from the room, so the room's combustion-air openings are not about it. Sealed in, sealed out.
Exact wording
A direct-vent appliance is built and installed so that all of its combustion air comes directly from the outdoors and all of its flue gases discharge to the outdoors.
Mechanical Code § 203.0
Dilution air enters the draft hood or draft regulator and mixes with the flue gases.
A draft regulator is the adjustable damper that admits dilution air on appliances with no draft hood. Dilution air is not the combustion air at the burner: it mixes in after the gases leave the appliance.
On the job
Dilution air is the air the draft hood or draft regulator handles: it mixes with the flue gases on the way out. The exam tests which air that device handles.
Exact wording
Dilution air is air that enters a draft hood or draft regulator and mixes with the flue gases.
Mechanical Code § 203.0
Combustion air is the total air provided to the space holding fuel-burning equipment.
Combustion air includes three things: air for fuel combustion, air for draft hood dilution, and air for ventilating the equipment enclosure. Add all three together, because that total is the air the space with fuel-burning equipment must be provided.
On the job
The code's combustion air is a space quantity, not just what burns: it covers combustion, dilution, and enclosure ventilation together. A definition question turns on that breadth.
Exact wording
Combustion air is the total amount of air provided to the space that contains fuel-burning equipment. It includes air for fuel combustion, draft hood dilution, and ventilation of the equipment enclosure.
Mechanical Code § 205.0
A draft hood neutralizes the chimney's stack action on the appliance.
Stack action is the draw a chimney or vent puts on the appliance. A draft hood is a nonadjustable device built into the appliance or made part of its vent connector. It does three things: lets flue gases escape readily if there is no draft, a backdraft, or a stoppage past the hood; keeps a backdraft from entering the appliance; and neutralizes that stack action.
On the job
The draft hood is where dilution air enters and where a blocked vent shows itself as spillage. The exam tests its three jobs, and neutralizing stack action is the one learners forget.
Exact wording
A draft hood is a nonadjustable device built into an appliance, or made part of its vent connector, designed to do three things: let flue gases escape readily if there is no draft, a backdraft, or a stoppage beyond the hood; prevent a backdraft from entering the appliance; and neutralize the effect of the chimney's or vent's stack action on the appliance.
Mechanical Code § 206.0
Take away
4 rules · 2 minCombustion air and venting
You need two permanent openings when you combine indoor spaces for combustion air.
This rule covers indoor spaces on the same story combined to supply combustion air. Start one permanent opening within 12 inches of the top of the enclosure and one within 12 inches of the bottom. Each opening needs a free area of at least 1 square inch per 1,000 Btu/h of the total input of all appliances in the space, and never less than 100 square inches. No opening dimension is less than 3 inches.
On the job
Two openings, top and bottom, is the tested picture: the high one lets hot flue-side air out, the low one lets combustion air in. The 1-per-1,000 and 100-square-inch figures are the neighbors.
Exact wording
Where indoor spaces are combined to supply combustion air on the same story, each opening has a free area of at least 1 square inch per 1,000 Btu/h of the total input of all appliances in the space, and never less than 100 square inches. One permanent opening starts within 12 inches of the top of the enclosure and one within 12 inches of the bottom. No opening dimension is less than 3 inches.
Mechanical Code § 701.5
Use the known-air-infiltration method for combustion air when infiltration is known to be below 0.40 .
You size the indoor combustion-air volume by either the standard method or the known-air-infiltration method. Under the standard method the minimum volume is 50 cubic feet per 1,000 Btu/h. You must switch to the known-air-infiltration method whenever the air infiltration rate is known to be less than 0.40 air changes per hour. Total the volume for all appliances in the space. Rooms that communicate directly with the appliance space through doorless openings, and through combustion-air openings sized and located per Section 701.5, count toward the required volume.
On the job
A tight house changes the method: where the infiltration rate is known to be below 0.40 air changes per hour, the standard volume method no longer applies. The exam tests the trigger, and that it depends on the rate being known.
Exact wording
The required volume of indoor air for combustion is figured by the standard method or the known-air-infiltration method. Where the air infiltration rate is known to be less than 0.40 air changes per hour, the known-infiltration method must be used. The total is the sum for all appliances in the space. Rooms that communicate directly with the appliance space through doorless openings, and through combustion-air openings sized and located per Section 701.5, count as part of the required volume. Under the standard method the minimum volume is 50 cubic feet per 1,000 Btu/h.
Mechanical Code § 701.4
You size combustion, ventilation, and dilution air openings by .
Where the louver, grille, or screen's free area is known, use that figure. Where it is not known, assume wood louvers pass 25 percent of the opening and metal louvers and grilles 75 percent.
On the job
A louver eats most of an opening. The exam gives a louver type and asks what size opening delivers the required free area: divide by 0.25 for wood, 0.75 for metal.
Exact wording
Combustion, ventilation, and dilution air openings are sized on net free area. Where a louver, grille, or screen's free area is known, it is used; where it is not known, wood louvers are assumed to have 25 percent free area and metal louvers and grilles 75 percent.
Mechanical Code § 701.10
Slope the vent connector up toward the chimney at least 1/4 inch per foot.
Install the vent connector with no dips or sags. Slope it upward toward the vent or chimney at least 1/4 inch per foot. Exception: a connector on a mechanical draft system installed per the appliance and draft-system manufacturers' instructions.
On the job
Continuous rise is the tested rule: no sag, no dip, 1/4 inch per foot toward the vent. A sag traps flue gas and condensate.
Exact wording
A vent connector is installed without dips or sags and slopes upward toward the vent or chimney at least 1/4 inch per foot. The exception is a connector on a mechanical draft system installed per the appliance and draft-system manufacturers' instructions.
Mechanical Code § 802.10.7
Take away
6 rules · 4 minAir distribution
You can't pull return air from a garage, a bathroom, or a furnace room.
Don't take outside or return air from any of these: closer to vents than Sections 802.6.1 and 802.8 require, or closer than Section 402.4's separation distances; a hazardous or insanitary location, or a refrigeration machinery room; a closet, bathroom, toilet room, kitchen, garage, boiler room, furnace room, or unconditioned attic; or a space smaller than 25 percent of the whole volume served, unless a permanent opening connects it to a space of that size. In a dwelling, that opening may drop to half the required area when the rest of the return air comes from a room or hall with three or more doors to other rooms the furnace serves.
On the job
Garage, bathroom, and furnace room are the tested prohibitions: return air pulled from them drags fumes, moisture, or flue gases into the house. The 25-percent volume rule and its dwelling reduction are tested too. A dedicated furnace room is barred outright by this list; a room that merely contains an enclosed furnace is governed by its own rule.
Exact wording
Outside or return air for a heating or cooling system may not be taken from: closer to vents than Sections 802.6.1 and 802.8 require; closer than the separation distances Section 402.4 requires; a hazardous or insanitary location or a refrigeration machinery room; an area whose volume is less than 25 percent of the whole volume served, unless a permanent opening connects it to an area of that size; or a closet, bathroom, toilet room, kitchen, garage, boiler room, furnace room, or unconditioned attic. In a dwelling, the 25-percent opening may be reduced to half the required area where the rest of the return air comes from a room or hall with three or more doors leading to other rooms served by the furnace.
Mechanical Code § 311.3
Don't take return air from a room that holds a fuel-burning appliance.
Three exceptions let you take return air from a room or space that contains a fuel-burning appliance. First: the appliance installed in that room is a fireplace, fireplace appliance, residential cooking appliance, , enclosed furnace, or domestic clothes dryer. Second: the equipment is a gravity-type or listed vented wall heating or cooling system. Third: a blower system, where all three of these hold — the room's volume exceeds 1 cubic foot per 10 Btu/h of appliance input, at least 75 percent of the supply air returns to that room, and the return inlet sits at least 10 feet from any firebox or draft diverter in the same enclosed room or confined space.
On the job
This is the rule for a family room or laundry room that happens to hold an appliance, not for a furnace room, which the named-room list already bars. An enclosed furnace is an exception here; a furnace room is not.
Exact wording
Return air also may not come from a room or space containing a fuel-burning appliance, with three exceptions. First: fireplaces, fireplace appliances, residential cooking appliances, direct-vent appliances, enclosed furnaces, and domestic clothes dryers installed in that room. Second: gravity-type or listed vented wall heating or cooling systems. Third: a blower system where the room's volume exceeds 1 cubic foot per 10 Btu/h of appliance input, at least 75 percent of the supply air returns to that room, and the return inlet is at least 10 feet from any firebox or draft diverter in the same enclosed room or confined space.
Mechanical Code § 311.3
Install a listed appliance to its listing and the manufacturer's installation instructions.
A listed appliance is one a testing agency has listed; its listing and the manufacturer's instructions both apply. Or install it as this chapter specifies. Install an unlisted appliance as this chapter specifies. The chapter applies primarily to nonindustrial appliances.
On the job
902.1 makes the manufacturer's instructions binding, and the furnace rating plate's temperature-rise range is part of them, so running outside that range is a violation. As a practical matter, a high rise means too little airflow and is corrected with more blower speed.
Exact wording
Listed appliances are installed in accordance with their listing and the manufacturer's installation instructions, or as the chapter specifies. Unlisted appliances are installed as the chapter specifies. The chapter applies primarily to nonindustrial appliances.
Mechanical Code § 902.1
You verify charge by on a TXV or EXV system. You still check .
Reference Appendix RA3.2 sets the standard charge verification procedure for residential air-cooled air conditioners and air-source heat pumps, and it is written for ducted split systems. On an expansion-valve system, hold subcooling within ±3°F of the manufacturer's subcooling target, and have the metering device verified: superheat must meet the manufacturer's specification or fall between 4°F and 25°F. With a , use the superheat method, within ±5°F of the superheat target. Minimum airflow across the coil is verified with the charge.
On the job
Fixed metering device means superheat; TXV or EXV means subcooling PLUS a superheat check of the metering device. The exam trap is 'subcooling instead of superheat'; the procedure requires both on an expansion-valve system.
Exact wording
Under the Energy Code's Reference Appendix RA3.2, refrigerant charge on a residential air-cooled air conditioner or air-source heat pump is verified by the standard charge verification procedure, written for ducted split systems. Systems with fixed metering devices use the superheat method. Systems with thermostatic or electronic expansion valves use the subcooling method and, in addition, have their metering device verified: superheat must meet the manufacturer's specification or fall between 4°F and 25°F. Installer tolerances are ±5°F of the superheat target for fixed metering and ±3°F of the manufacturer's subcooling target for expansion-valve systems, and minimum airflow across the coil is verified with the charge.
Energy Code § ra3.2
Use the standard charging procedure at 55°F or above outdoors and weigh in below that.
The standard procedure needs outdoor air at 55°F or above — up to 115°F with fixed metering, 120°F with expansion valves — return air at 70°F or above, and, for fixed metering, return wet-bulb at 76°F or below. Below 55°F, where no Energy Commission-approved alternative protocol in Reference Appendix RA1 applies to the system, the installer charges by weight: the weigh-in procedure. The rater verifies by the standard procedure unless an approved RA1 alternative applies, or the Standards make observing weigh-in mandatory or offer it as an option the installer elects.
On the job
The temperature limits decide which procedure runs. Weigh-in is the cold-weather route, but only where no approved alternative protocol fits the system.
Exact wording
The standard procedure is used at outdoor temperatures of 55°F or above, up to 115°F for fixed metering and 120°F for expansion-valve systems, with return air at 70°F or above and, for fixed metering, return wet-bulb at 76°F or below. Below 55°F, where no Energy Commission-approved alternative protocol in Reference Appendix RA1 applies to the system, the installer uses the weigh-in charging procedure. The rater verifies by the standard procedure unless an approved RA1 alternative applies, or the Standards make observation of weigh-in mandatory or offer it as an option the installer elects.
Energy Code § ra3.2
Move more air when climbs above the rating plate range.
More air across the heat exchanger narrows the gap between supply-air and return-air temperature, so the rise falls. With the filter, grilles, and ducts already clear, correct a rise above the top of the rating plate range by moving more air, usually by increasing blower speed.
On the job
The burner fixes the heat input; airflow is the installer's variable, and too little of it overheats the exchanger.
Exact wording
Temperature rise across a furnace is the supply-air temperature minus the return-air temperature, and it falls when airflow rises; with the filter, grilles, and ducts already clear, a rise above the top of the range printed on the rating plate is corrected by moving more air across the heat exchanger, typically by increasing the blower speed.
Standard trade practice
Take away
3 rules · 1 minHVAC arithmetic
Divide by 1.08 times the temperature difference to get supply CFM.
Use sensible heat in Btu per hour and the supply-to-return difference in °F, at standard air; the answer is supply airflow in CFM. Two of the three quantities are given; the third is found from them. Example: 24,000 Btu/h at a 20°F difference needs 24,000 ÷ (1.08 × 20) = about 1,111 CFM.
On the job
The exam gives a load and a temperature difference and asks for CFM, or gives CFM and the difference and asks for the load. One formula, three variables.
Exact wording
Supply airflow in CFM equals sensible heat in Btu per hour divided by 1.08 times the supply-to-return temperature difference in °F. Two of the three quantities are given; the third is found from them. Example: 24,000 Btu/h at a 20°F difference needs 24,000 ÷ (1.08 × 20) = about 1,111 CFM, at standard air.
Standard trade practice
One means 12,000 Btu per hour. Airflow per ton is only a convention.
The usual design convention supplies about 400 CFM per ton, so a 3-ton system moves about 3 × 400 = 1,200 CFM and delivers 3 × 12,000 = 36,000 Btu/h. If a stem gives a different CFM-per-ton figure, use it.
On the job
Tons to Btu is a definition; tons to CFM is a convention. The exam uses 400 unless it says otherwise, and the 12,000 figure is the one to memorize.
Exact wording
One ton of refrigeration is 12,000 Btu per hour. The usual design convention supplies about 400 CFM per ton, so a 3-ton system moves about 3 × 400 = 1,200 CFM and delivers 36,000 Btu/h. If a stem gives a different CFM-per-ton figure, use it.
Standard trade practice
Air changes come per hour but CFM is per minute. Divide by 60.
Room volume in cubic feet is length × width × height. CFM equals that volume times , divided by 60. Example: 20 × 15 feet with a 10-foot ceiling is 3,000 cubic feet; at 6 ACH, 3,000 × 6 ÷ 60 = 300 CFM.
On the job
The exam gives room dimensions and an air-change rate and asks for CFM. Divide by 60 because the rate is per hour and airflow is per minute.
Exact wording
Airflow in CFM equals room volume in cubic feet times air changes per hour, divided by 60. Volume is length × width × height. Example: a 20-by-15-foot room with a 10-foot ceiling is 3,000 cubic feet; at 6 air changes per hour it needs 3,000 × 6 ÷ 60 = 300 CFM.
Standard trade practice
Take away
Important numbers to know
Where people go wrong
Sounds right: "A direct-vent furnace still needs combustion-air openings in the closet."
Where’s the catch?
Sounds right: "The vent connector just needs to reach the vent."
Where’s the catch?
Sounds right: "Return air from the garage is fine if the furnace is in the garage."
Where’s the catch?
Sounds right: "A 100-square-inch opening covered by a wood louver delivers 100 square inches of combustion air."
Where’s the catch?
Sounds right: "Combustion air and dilution air are the same thing."
Where’s the catch?
Sounds right: "Combustion-air figures: 1 per 1,000 and 50 per 1,000 are the same rule."
Where’s the catch?
Sounds right: "A crawl space or joist bay is just a space; only sheet-metal boxes are ."
Where’s the catch?
Where to look it up
- Definitions (plenum, direct-vent appliance, dilution air, draft hood)
- CMC Chapter 2, alphabetical by section (203.0 A, 206.0 D, 218.0 P)
- Return-air prohibited sources
- CMC 311.3
- Combustion air — volume, openings, louvers
- CMC Chapter 7: 701.4, 701.5, 701.10
- Vent connectors
- CMC 802.10 (802.10.7 slope)
- Appliance installation per listing
- CMC Chapter 9 (902.1)
Glossary
Every term this guide defines, in one place. Each is also defined where it first appears.
- Air changes per hour (ACH)
- How many times a room's volume of air is replaced in an hour.
- Direct-vent appliance
- An appliance that takes all combustion air from outdoors and discharges all flue gases outdoors.
- Fixed metering device
- A fixed orifice, piston, or capillary tube that meters refrigerant without adjusting to load.
- Net free area
- The open area of a louver or grille that air can actually pass through.
- Plenum
- An air chamber, other than the conditioned room itself, that ducts connect to and that forms part of the supply, return, or exhaust system; a crawl space or attic used that way is one.
- Sensible heat
- Heat that changes air temperature, as opposed to latent heat that changes its moisture.
- Subcooling
- How far the liquid refrigerant leaving the condenser is below its saturation temperature; the charging measure for TXV and EXV systems.
- Superheat
- How far the refrigerant vapor leaving the evaporator is above its saturation temperature; the charging measure for fixed metering devices.
- Temperature rise
- Supply-air temperature minus return-air temperature across a furnace; the rating plate gives the allowed range.
- Ton of refrigeration
- 12,000 Btu per hour of cooling.
- TXV / EXV
- Thermostatic or electronic expansion valve: a metering device that adjusts refrigerant flow to load.
Keep going
- Practice questions for General Building (B) — Core Trades is 30% of the exam.
- Job scenario: Six Circuits and a Closet
- Job scenario: The Closet Heater
- Also in Core Trades: Core Trades: Seven Trades, One Section
- Also in Core Trades: Electrical: The Rules the Exam Asks For
- Also in Core Trades: Plumbing: The Rules the Exam Asks For
- Also in Core Trades: Building Envelope: The Rules the Exam Asks For
- Every number on one page — this guide’s figures alongside every other General Building (B) guide’s.
Test yourself: 12 questions for this guide
A paid account adds more ways to practice and prepare: study questions after every chapter, practice questions for every topic, timed practice exams, and job scenarios drawn from real jobs. A free account gets you one timed practice exam and saves your progress across devices. Here is one of this guide's questions:
A furnace closet gets its combustion air from the adjoining room on the same story through two openings. Where do they go, and how big?
AnswerOne permanent opening starting within 12 inches of the top of the closet and one permanent opening within 12 inches of the bottom. Each has at least 1 square inch of free area per 1,000 Btu/h of the total input of all appliances in the space, and never less than 100 square inches, with no dimension under 3 inches.
Mechanical Code § 701.5
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