IRC M1401.3 requires heating and cooling equipment to be sized per Manual S based on Manual J loads - and the section has no exemption written in for like-for-like replacements. Whether your building department enforces that on a changeout is a local call, not a code exception.
Short answer: the base code requirement applies to a changeout the same as it applies to new construction. IRC Section M1401.3 requires heating and cooling equipment to be sized in accordance with ACCA Manual S, based on loads calculated in accordance with ACCA Manual J or another approved method.1 Nothing in that section says "except when replacing existing equipment." "The old unit was a 3-ton, so the new one should be too" is a common contractor shortcut, not a sizing methodology the code recognizes. Whether your local building department pushes back on a straightforward swap is a separate question of local discretion, covered further down.
The requirement text has been stable across recent editions. The 2021 International Residential Code, M1401.3: "Heating and cooling equipment and appliances shall be sized in accordance with ACCA Manual S or other approved sizing methodologies based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies."1 The 2018 edition carries the identical operative sentence,3 and the 2024 edition does too.2 The energy code says the same thing in its own words: IECC R403.7, Equipment sizing and efficiency rating, requires heating and cooling equipment to be sized per Manual S based on loads from Manual J or another approved method, and separately requires new or replacement equipment to meet the minimum federal efficiency rating for its location.4
M1401.3 does carry one exception, and it is worth reading carefully because it is not a changeout exemption. It says Manual S capacity limits do not apply where the specified equipment uses multistage or variable-refrigerant-flow technology and the calculated load falls within the manufacturer's published capacity range for that equipment, or where the manufacturer's next larger standard size is the only unit that can satisfy both the total and sensible heat gains.1 That exception is about matching real equipment capacity curves to a real calculated load - it still requires the load calculation. It does not say a replacement unit can skip Manual J because it is a replacement.
Section numbers move around by edition and your jurisdiction's amendments can change them further, so confirm the current section against your state's adopted code rather than assuming M1401.3 is untouched everywhere. Most states that adopt the International Residential Code adopt this section intact, but "most" is not "all," and a state or local amendment can narrow or widen how it applies to replacement work.
This is the question behind almost every changeout permit, and there are two separate reasons "just match the old one" is a bad assumption even before you get to code enforcement.
First, a lot of existing equipment was never sized correctly to begin with. Rule-of-thumb sizing - a fixed number of square feet per ton, without a room-by-room load calculation - has been standard contractor practice for decades and routinely produces equipment well above the actual load. If the old system was oversized, matching its tonnage on the replacement just carries the oversizing forward. Oversized equipment short-cycles, struggles to control humidity, and wears out faster; it is not a safe default just because it is what was already there.
Second, the house has usually changed since the original system went in. Added insulation, replaced windows, air sealing, a new roof - anything that touches the envelope changes the load a Manual J would calculate, almost always downward. A unit sized correctly for the house as it existed fifteen years ago is not necessarily sized correctly for the house today. The only way to know is to run the calculation again against current conditions, not to carry the old number forward on the assumption that nothing changed.
Manual S does give the installer room to work with once a current load is known - equipment does not have to match the calculated load exactly. It sets a tolerance window instead. Under the 2014 edition of Manual S, the one referenced by the 2021 IRC, single-speed air-to-air cooling equipment can run 90 to 115 percent of the calculated cooling load, multi-speed equipment 90 to 120 percent, and variable-speed equipment 90 to 130 percent.6 That range is deliberately wide enough to work with standard equipment increments - most residential equipment ships in half-ton or full-ton steps, so an exact percentage match is rarely possible - but it is a range built around a calculated number, not a guess. The 2024 IRC points to a newer, ANSI-approved 2023 edition of Manual S with its own revised tolerances, including expanded allowances specifically for variable-capacity heat pumps;57 which edition applies depends on which code edition your jurisdiction has adopted, and it is worth asking when you order so the Manual S selection matches the right table.
Yes. Swapping the heating source is installing new equipment with a different capacity profile than what it replaces, and M1401.3 applies to that new equipment on its own terms - it does not matter that the cooling side of the system is untouched. A gas furnace delivers roughly its rated output regardless of outdoor temperature. A heat pump's heating capacity drops as the outdoor temperature drops, so the unit has to be evaluated against its manufacturer's published capacity curve at your local winter design temperature, not just against a single nameplate number. Undersizing that curve leaves you short of heat on the coldest days of the year; oversizing it wastes money on equipment that short-cycles the rest of the time.
This is also where Manual S includes a heat-pump-specific option worth knowing about if you are converting in a heating-dominated climate: an alternate sizing method that allows heating capacity up to 15,000 Btu/h above the Manual J cooling load, but only where the home's sensible heat ratio is 0.95 or higher (a dry load) and the ratio of heating degree days to cooling degree days is 2.0 or higher (a heating-dominated location).6 That combination fits a lot of Inland Northwest and mountain-climate houses converting off gas heat. It is a Manual S calculation choice, not a substitute for running the numbers - the load calculation still has to happen first.
For the broader question of when Manual J is required across different project types - new construction, additions, ADUs, straight like-for-like swaps - see our companion guide, Do I Need a Manual J? This page goes deeper specifically on the changeout and conversion scenario.
Generally yes - a ductless mini-split is new heating and cooling equipment, and it needs its own load-based sizing the same as any other new unit, whether it is replacing window units in one room or supplementing a whole-house system. Mini-splits raise a few questions of their own around zoning, how much of the existing house load a single head is expected to cover, and how permitting offices treat a one-room retrofit differently from a whole-system changeout. We cover that in a dedicated guide: Mini-Split Manual J and Permit Requirements.
Code sections tell you what is technically required; permit checklists tell you what a specific building department actually collects. Alachua County, Florida is a useful real-world example because its residential HVAC replacement checklist spells out the trigger in plain terms: required submittal documents include the AHRI certificate for the equipment being replaced, and then - "If and ductwork is being replaced, altered, or added, manual J and Manual D forms are also required and a 3000 - Rough-in Mechanical inspection should be scheduled as well."8 In other words, that department's practical line for a straight equipment swap with no duct changes is lighter documentation, but the moment ductwork is touched, both a Manual J load calculation and a Manual D duct design are required submittals, plus an added rough-in inspection.
That is one county's checklist, not a national rule, and it illustrates the point rather than proving what your jurisdiction does. Some building departments draw the line at ductwork the way Alachua County does; others require a Manual J on every mechanical permit regardless of scope; others accept a like-for-like swap with minimal paperwork as a matter of practice even though the base code text does not grant that exception. This is exactly the kind of local-discretion variation that makes "check with your building department" a real instruction here, not boilerplate.
Most states that have adopted the International Residential Code have adopted M1401.3 intact, and IECC R403.7 alongside it. That is a general pattern, not a state-by-state confirmation - amendments happen, effective dates lag adoption, and a handful of states use their own residential energy code entirely rather than the IECC as written. Look up your state for the edition currently in force before you assume the base code language applies unmodified to your permit.
For a changeout or conversion Manual J, send us the address and, where available, the existing equipment's nameplate or AHRI certificate, the square footage and rough layout of the conditioned space, and window and insulation information if you know it - what's on the plans if this is part of a larger remodel, or a reasonable description of the existing envelope if it is a standalone HVAC permit. If ductwork is changing at all, tell us; that affects what your building department is likely to ask for alongside the load calculation. We build the report from what you send and note anything we estimated so your building department can see exactly where each number came from.
The base code requirement doesn't carve out an exception for replacement equipment - M1401.3 applies to a changeout the same as new construction. Whether your building department enforces it on a like-for-like swap with minimal review is a local practice question, separate from what the code text says.
That is not a code-recognized sizing methodology on its own. Old equipment is often oversized to begin with, and envelope changes since the original install usually lower the load. A current Manual J tells you whether the old tonnage is still right; assuming it is not.
Yes. New heating equipment with a different capacity profile than what it replaces needs its own Manual J-based sizing and Manual S equipment selection, regardless of whether the cooling side is untouched.
Manual S sets a tolerance window, not an exact match. Under the 2014 edition referenced by the 2021 IRC: single-speed 90-115 percent of the cooling load, multi-speed 90-120 percent, variable-speed 90-130 percent. The 2024 IRC references a revised 2023 edition with its own heat pump tolerances - ask which edition applies when you order.
Generally yes, since a mini-split is new equipment being sized for the space it serves. See our mini-split-specific guide for the zoning and scope questions that come up on single-head installs.
Most states adopt the International Residential Code with M1401.3 intact, but enforcement on replacement-only permits varies by building department. Check your state's adopted edition rather than assuming.
This page explains the base code requirement and one real jurisdiction's checklist as an example of how it plays out in practice. It is not a survey of every state's amendments or every building department's changeout enforcement policy; the state finder tells you which code edition applies where you're permitting, and your building official has final say on what your specific permit requires.
Travis Smith, ICC-Certified Residential Energy Inspector / Plans Examiner. The same certification your building department's plan reviewer holds - working for you.