Lesson 2: Heating and Cooling Equipment and Heat Pumps
IRC M1401-M1403, M1411-M1413 · 14 practice questions
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Reading
Before a furnace, heat pump, or air conditioner ever gets bolted down, the code asks one question first: is
it the right size for this house? Chapter 14 of the IRC sets that requirement, then covers the practical
installation rules for the equipment itself, heating, cooling, and heat pumps that do both. This lesson stays
at a practical level appropriate for reviewing a scope of work and a set of plans; the equipment's condensate
disposal, refrigerant piping, and outdoor placement are covered separately in Lesson 3, and combustion air and
venting for gas-fired equipment are covered in Lessons 7 through 9.
Sizing: Manual J and Manual S
Under M1401.3, heating and cooling equipment must be selected using ACCA Manual S, or
another approved sizing methodology, applied to loads calculated under ACCA Manual J, or
another approved load-calculation method. Manual J works from the building itself, insulation levels, window
area and orientation, air leakage, ceiling height, local climate design conditions, and room-by-room
distribution, to produce a heating and a cooling load in Btu/h. Manual S then takes that calculated load and
matches it against a specific piece of equipment's published performance data, rather than against a rule of
thumb like "a ton per 500 square feet." A replacement furnace or condenser sized only by matching the tonnage of
the unit being removed skips this step entirely, and is not, by itself, a code-compliant sizing method.
The sizing exceptions
Two exceptions in M1401.3 give this process some real-world flexibility. First, where the selected equipment
uses multistage or variable refrigerant flow (VRF) technology, sizing is not limited to the
Manual S capacities if the calculated loads fall within the manufacturer's published capacities for that
equipment, rather than needing to match a single fixed tonnage. Second, because equipment is manufactured in
standard capacity increments and a calculated load rarely lands exactly on one, where the manufacturer's
published capacities cannot satisfy both the total and the sensible heat gains calculated for the house, the
next larger standard size unit may be specified. That second exception only runs in one
direction: it lets a contractor round up to the next standard size when the exact number falls between two, it
does not license rounding down to whatever unit happens to be on the truck.
Key numbers
Loads: calculated under ACCA Manual J or another approved method (M1401.3)
Selection: matched to calculated loads under ACCA Manual S or another approved method, using the equipment's published performance data
Exception 1: multistage or VRF equipment is not limited to Manual S capacities where the calculated loads fall within its published capacities
Exception 2: where published capacities cannot satisfy both the total and sensible heat gains, the next larger standard size unit may be specified, never a smaller one
Furnace installation
A furnace, gas, oil, or electric, is installed the same way any other appliance covered in Lesson 1 is
installed: listed and labeled, set up per its manufacturer's installation instructions, with the access,
working space, and clearance-to-combustibles rules from Lesson 1 all applying in full. What is specific to a
furnace is the equipment that hangs off it, ducted supply and return air, and for a fuel-fired unit,
combustion air and a vent connector. Those get their own full treatment in Lessons 6 through 9; the sizing and
placement decisions in this lesson happen before any of that ductwork or venting is designed, because an
oversized or undersized furnace can force compromises in duct sizing and vent category that are hard to
correct later.
Heat pump equipment and the defrost cycle
A heat pump is installed under the same sizing and listing rules as a furnace, with one operating
difference worth understanding at the installation stage: in heating mode, the outdoor coil is colder than the
surrounding air, and under the right humidity and temperature conditions it accumulates frost. The equipment's
control system runs a periodic defrost cycle, briefly reversing the system to melt that frost
off the outdoor coil, per the manufacturer's controls and instructions. This is a normal part of listed heat
pump operation, not a fault, and it is one more reason the manufacturer's installation instructions, not a
generic furnace assumption, govern how a heat pump is actually set up and wired. The code itself asks that an
electric heat pump be listed and labeled (M1403.1) and installed per those instructions; the defrost and
supplementary-heat details below come from the manufacturer, not from a code number.
Supplementary heat and controls
A heat pump's heating capacity falls as the outdoor temperature drops, and at some point its own output can
no longer carry the full calculated heating load. Systems address this with supplementary (auxiliary)
heat, commonly electric resistance heat strips in the air handler, brought on by the equipment's
controls when the heat pump alone cannot keep up, or during the defrost cycle when heating from the outdoor
unit is briefly unavailable. At a practical level, an inspector or estimator reviewing plans should expect to
see a heat pump paired with a load-appropriate supplementary heat source and controls set up per the
manufacturer's instructions, sized and staged so that supplementary heat is a backstop for the coldest
conditions the Manual J calculation covers, not the system's everyday primary heat source.
Evaporative coolers: a pointer
The code also addresses evaporative (swamp) coolers, which cool by evaporating water into moving air rather
than through a refrigerant cycle. They are effective in hot, dry climates with low relative humidity and are
uncommon in Gulf Coast housing, where high ambient humidity limits how much cooling they can provide and adds
a moisture-management concern of its own. Where GCHS work does encounter one, typically in an older manufactured
or site-built home, it is installed per the manufacturer's instructions, on a level platform, with exterior wall openings flashed,
the potable water supply protected, and its air intake located as the code requires (M1413.1).
A worked example on the sizing exception: a Manual J calculation for a rehab house comes back with a total
cooling load of 27,400 Btu/h. Standard split-system equipment is manufactured in half-ton increments, 6,000
Btu/h apart, so the two standard sizes bracketing this load are a 2-ton unit at 24,000 Btu/h and a 2.5-ton unit
at 30,000 Btu/h. The 2-ton unit falls short of the calculated load, so the second exception applies; the 2.5-ton
unit is the next larger standard size unit and is the compliant selection, even though it runs about 9 percent above
the calculated number. The same load does not justify jumping to a 3-ton unit "for margin," because the
exception only reaches the next standard size up, not any larger unit that happens to be available.
In the field
GCHS reviews a lot of HVAC replacement bids where the proposed unit is simply the same tonnage as the failed
one, with no load calculation behind it. In an older Gulf Coast house that has since had insulation, windows,
or air sealing improved, that old tonnage is very often oversized for the house today. An oversized system
short-cycles, which means it satisfies the thermostat before it has run long enough to pull humidity out of the
air, leaving a house that reads cool but feels damp. Requiring a Manual J calculation as part of the bid package
is a durability and comfort decision, not just a paperwork one.
Sizing runs from calculated load to matched equipment before installation details are decided.Follow the sequence described in this lesson. A load calculation informs equipment selection, and the selected equipment still has to be installed as listed and instructed.
Practice questions
1. Under M1401.3, the calculated basis for sizing heating and cooling equipment is generally:
The size of the equipment being removed
Building loads calculated under ACCA Manual J (or another approved method), with equipment selected under ACCA Manual S (or another approved method)
The local utility's rate schedule
The manufacturer's published efficiency rating for the unit, weighed against the seasonal energy cost of running it and the number of occupants in the house
Answer: b. Manual J calculates the building's loads; Manual S then matches equipment to those loads. Matching the old unit's tonnage is not a substitute for this process.
2. Compared with Manual J, what does Manual S add to the sizing process?
A second, independent load calculation performed by a different method and averaged with the first one
A ductwork layout
A combustion air calculation
Matching the calculated load against a specific piece of equipment's published performance data
Answer: d. Manual S selects and verifies a specific model against the Manual J load, rather than a rule-of-thumb tonnage.
3. An inspector reviews a furnace replacement submittal and finds the new unit's capacity was matched only to the tonnage of the failed unit being removed, with no load calculation on file. This submittal is:
Noncompliant, because sizing must be based on a calculated load, not the tonnage of the previous unit
Compliant, because like-for-like replacement is always acceptable
Compliant if the new unit is a well-known brand
Noncompliant only if the house has been renovated since the original installation, when the load may have changed materially
Answer: a. Matching an old unit's size skips the Manual J/Manual S process entirely and is not itself a compliant sizing method.
4. A calculated cooling load falls between two standard equipment sizes, and the smaller unit's published capacities cannot satisfy both the total and sensible loads. A contractor proposes the smaller unit anyway, saying it will "run longer and dehumidify better." Under M1401.3's sizing exception, this selection is:
Compliant, because any size within one increment of the calculated load is acceptable
Compliant, because dehumidification performance overrides the sizing rule
Noncompliant, because the standard-size exception allows specifying the next larger size, not a smaller one that falls short of the load
Compliant only in a coastal county
Answer: c. The exception for standard sizes runs one direction: up to the next larger size when the exact load falls between increments, never down to an undersized unit.
5. A contractor proposes a variable refrigerant flow (VRF) heat pump system whose manufacturer-published capacities cover the entire calculated heating and cooling loads. Under M1401.3, this selection is:
Noncompliant, because VRF equipment cannot be used for code sizing
Compliant, because sizing is not limited to Manual S capacities where the calculated loads fall within that equipment's published capacities
Compliant only if paired with a fixed-capacity backup unit
Noncompliant without a separate Manual S worksheet signed by the manufacturer's representative and filed with the building department before the permit is issued
Answer: b. Under Exception 1 of M1401.3, multistage and VRF equipment can satisfy sizing by covering the calculated loads within its published capacities, rather than matching one fixed tonnage.
6. Beyond sizing, a furnace's installation itself is governed primarily by:
Its listing and the manufacturer's installation instructions, along with the general access and clearance rules covered in Lesson 1
The installer's personal preference for layout, provided the unit clears the ceiling, the supply plenum, the nearest wall studs, and the platform edge
Whichever clearance is easiest to achieve on site
The homeowner's request, regardless of the listing
Answer: a. Sizing decides which unit; the listing, the manufacturer's instructions, and the general access and clearance rules decide how it is actually installed.
7. In heating mode, a heat pump's outdoor coil periodically runs a defrost cycle. The purpose of this cycle is to:
Increase the compressor's rated efficiency
Reduce refrigerant charge for the next heating cycle and lower the discharge pressure at the compressor between cycles
Test the indoor thermostat wiring
Clear frost that accumulates on the outdoor coil under certain temperature and humidity conditions
Answer: d. The outdoor coil runs colder than the surrounding air in heating mode and can frost over; the defrost cycle periodically clears it per the manufacturer's controls.
8. The general purpose of a heat pump system's supplementary (auxiliary) heat is to:
Replace the heat pump as the everyday primary heat source whenever outdoor temperatures stay below freezing for several days in a row each winter
Cool the air handler during the cooling season
Cover the portion of the calculated heating load the heat pump alone cannot meet at low outdoor temperatures, or during a defrost cycle
Reduce the size of the outdoor unit required
Answer: c. Supplementary heat is a backstop for the coldest conditions and for defrost, staged in by the system's controls, not the system's everyday primary heat source.
9. An inspector reviewing a rehab estimate finds an air conditioner sized well above the Manual J calculated cooling load "for a safety margin." In a humid Gulf Coast climate, the most likely practical consequence is:
Improved comfort with no downside
Short cycling that satisfies the thermostat before the system removes enough humidity from the air
A lower electric bill in every case
No effect, because equipment tonnage does not change how long the system runs in any climate or in any season of the year
Answer: b. An oversized system satisfies the thermostat quickly and shuts off before it has run long enough to dehumidify, leaving a house that reads cool but feels damp.
10. Regarding evaporative (swamp) coolers, which statement is accurate for Gulf Coast residential work?
They are effective mainly in hot, dry climates and are uncommon in Gulf Coast housing because of high ambient humidity
They are the standard cooling equipment for new Gulf Coast construction
They require no listing or manufacturer's instructions
They replace the need for a Manual J calculation
Answer: a. Evaporative cooling depends on dry air to work well; Gulf Coast humidity limits its effectiveness, so it is rarely seen in this region's housing stock.
11. A contractor bidding a GCHS rehab proposes "doubling the old unit's tonnage for good measure" on a house with no current load calculation. The correct response is:
Accept the bid, since more capacity is always safer
Accept the bid only if the homeowner agrees in writing and the contractor provides a manufacturer warranty on the larger unit
Accept the bid if the house is under 1,500 square feet
Require a Manual J load calculation and Manual S equipment selection before accepting a proposed unit size
Answer: d. Sizing decisions are not a matter of adding margin by feel; the code requires a calculated load and a matched selection, and a grossly oversized unit creates its own comfort and durability problems.
12. A multi-stage furnace's lowest and highest firing rates both fall within the range needed to cover a calculated heating load at various outdoor temperatures. Under Exception 1 of M1401.3 for multistage equipment, this selection is:
Noncompliant, because staged equipment is never permitted for code sizing
Compliant only if the manufacturer certifies it in writing to the local jurisdiction and the code official signs off on the submittal
Compliant, because the calculated load falls within the equipment's published capacities
Noncompliant unless paired with a variable-speed air handler
Answer: c. The multistage exception looks at whether the calculated loads fall within the equipment's published capacities, not at a single fixed capacity number.
13. Equipment selection under Manual S compares the calculated building load against:
The square footage of the house only
The specific equipment model's published performance data at local design conditions
The equipment's retail price
The previous occupant's utility bills
Answer: b. Manual S is a model-specific comparison against published performance data at the applicable design conditions, not a square-footage rule of thumb.
14. A Manual J calculation returns a total cooling load of 34,000 Btu/h, and the standard equipment sizes available are 30,000 Btu/h and 36,000 Btu/h. Under the sizing exception, the compliant selection is:
The 36,000 Btu/h unit, the next larger standard size above the calculated load
The 30,000 Btu/h unit, because it is closer to the calculated load
Either unit, at the installer's discretion, provided the larger one is not more than one size above the calculated load
Neither; a custom-built unit must be ordered
Answer: a. Because the 30,000 Btu/h unit's capacity falls short of the calculated load, the code-compliant selection is the next larger standard size unit, 36,000 Btu/h.
Glossary
Manual J
The ACCA methodology for calculating a building's room-by-room heating and cooling loads from its construction, orientation, and local design conditions.
Manual S
The ACCA methodology for selecting specific equipment matched to a Manual J calculated load, using the equipment's published performance data.
Multistage or VRF equipment
Equipment able to run at more than one output level, or variable refrigerant flow systems; sizing is not limited to Manual S capacities where the calculated loads fall within its published capacities.
Defrost cycle
A periodic reversal of a heat pump's cycle in heating mode to clear frost that accumulates on the outdoor coil.
Supplementary (auxiliary) heat
A backup heat source, commonly electric resistance strips, that engages when a heat pump's own output cannot meet the load or during defrost.
Evaporative cooler
Cooling equipment that lowers air temperature by evaporating water into the airstream; effective mainly in hot, dry climates.