Free HVAC Design Checklist (Manual J, S & D)

ACCA Manual S puts an air conditioner between 95% and 115% of the Manual J design total heat gain. The 115% ceiling is also in code — IECC R403.7 limits cooling equipment to "not more than 1.15 times greater than the total load calculated". A heat pump gets 100% to 125%, and that wider range exists only because the heating side has to come out of the same selection, not because heat pumps tolerate oversizing better. Note that some jurisdictions amend the heat pump allowance back down to 115%, so check the AHJ before you lean on the 125%. This builder grades your entry against whichever range applies and treats the ceilings as inclusive, because "not more than 1.15 times" means exactly 115.0% is compliant.

No, and it is not a rule this builder will grade anything against. It is not published anywhere as a limit, it is not an approved methodology under IRC M1401.3, and it is wrong by a large margin on real houses. Across 40 published load calculations, none landed inside the 400 to 600 square feet per ton band, the tightest was 624 and the average was 1,431. The builder computes square feet per ton from your selected capacity and prints it as an outcome so you can see what the design actually worked out at — a well-designed house frequently lands somewhere the rule of thumb would call impossible.

Because total capacity is the check that misses the failure. A unit’s rated total capacity splits between sensible and latent, and that split moves with entering wet bulb and with airflow — so a selection can cover the total load and still fall short on latent. In a dry climate you never notice. In a humid one the system satisfies the thermostat, stops, and leaves the house clammy, and no amount of thermostat fiddling fixes it. Manual S requires the selected sensible capacity to cover the sensible load and the selected latent capacity to cover the latent load, both read from the manufacturer’s expanded performance data at your design conditions. This builder checks both, and it will tell you a selection is short on latent even when the total ratio reads comfortably inside 115%.

In every jurisdiction on the IRC, yes. Section M1401.3 of both the 2021 and 2024 IRC requires heating and cooling equipment to be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCA Manual J or other approved methodologies, and IECC R403.7 says the same for energy-code compliance. Most jurisdictions that adopt it require the load report on submission, and a completed form with no report behind it is the failure inspectors see most often. IRC M1401.3 does carry an exception permitting capacity beyond the Manual S ranges for multistage and variable refrigerant flow equipment where the calculated loads fall inside the manufacturer’s specifications — but that is a documentation burden, not a blanket permission, so the builder still flags an over-range selection and names the exception in the finding.

The ASHRAE 99% heating dry bulb and 1% cooling dry bulb for the weather station nearest the house — the pair EPA and ENERGY STAR require for Manual J 8th edition. The 99% heating value is the temperature the location stays above for 99% of the year, so roughly 88 hours a year are colder; the 1% cooling value is exceeded for about the same 88 hours. Designing to the record low instead is one of the quieter routes to an oversized system: you size for a condition that occurs for a handful of hours a decade and then run the equipment badly for the other 8,700 hours a year. Indoors, Manual J uses 70°F for heating and 75°F for cooling at 50% relative humidity, with 45% as the dry-climate default and 55% reserved for genuinely humid climates.

Available static pressure times 100, divided by total effective length. Available static pressure is the blower’s rated external static pressure less the pressure drop of everything that is not duct or fitting — the coil at its wet-coil figure, the filter at its rated drop, and any balancing dampers, registers and grilles. Total effective length is the straight duct plus the equivalent lengths of the fittings on the critical path: the longest supply run plus the longest return run, not a tape-measure figure. A blower rated at 0.50 in. w.c. behind a 0.20 coil, a 0.10 filter and 0.03 of accessories leaves 0.17 in. w.c. available; over a 200 ft total effective length that is a friction rate of 0.085 in. w.c. per 100 ft, and every duct on the job is sized from that number.

Because Manual D does not publish a pass/fail band for it, and inventing one would be the same error as the sizing rules of thumb. The friction rate is an output of your specific blower, coil, filter and layout — the widely quoted 0.10 in. w.c. per 100 ft is the figure designers assume rather than compute, and ACCA’s own guidance calls that assumption out as the mistake. The builder computes the real number and prints it so you can size from it. The one duct verdict it does issue is unambiguous: when the coil, filter and accessory drops total more than the blower is rated for, there is no static pressure left to move air through any duct system at all, and no duct size can rescue that.

ASHRAE 62.2 sets it at Qtot = 0.03 × conditioned floor area + 7.5 × (bedrooms + 1), in cfm, with the bedroom count never taken below one. A 2,400 square foot house with three bedrooms needs 102 cfm. Worth knowing: the floor-area coefficient was 0.01 before addendum m to 62.2-2013, so an old spreadsheet gives 54 cfm for that same house and undersizes badly. Qtot is the total from infiltration plus the fan, and 62.2-2019 onward allows an infiltration credit that can reduce the fan rate below it — so the builder flags a shortfall as a design that owes a documented credit calculation rather than as a flat failure.

No. It is the designer’s record of the review, and every export says so. The citations are there to show why each figure was checked, not to certify the design — plan review and approval rest with the authority having jurisdiction, and the Manual J, S and D reports themselves still have to go with the permit application.