Manual J vs Manual S vs Manual D, Explained

Manual J calculates a home's heating and cooling loads, Manual S picks equipment whose capacity at design conditions covers those loads, and Manual D designs ducts that deliver the selected airflow to each room. Each step takes numbers from the one before it, so a change anywhere (a new window, a different coil) has to be carried forward. This page runs one real house, the U.S. Department of Energy's modeled Orlando house, through all three.

Step 1, Manual J: Orlando's loads

The Building America guide models a one-story, 2,223 ft² slab-on-grade house in Orlando (IECC Zone 2) built to the 2009 IECC prescriptive path. Its Manual J results are below.

An SHR of 0.80 means about a fifth of the cooling load is moisture, which is why latent capacity gets its own check below. Before moving on, confirm the inputs: for example, 1,600 ft² of gross wall less 240 ft² of openings is 1,360 ft² of net wall, and entering all 1,600 ft² as wall while also listing the windows counts the openings twice. A measured ACH50 cannot go straight into a natural-infiltration entry, and these input checks are not a reason to add a blanket percentage to the finished load.

Step 2, Manual S: does the equipment cover it?

For cooling, the guide starts from the nominal size, 20,690 ÷ 12,000 = 1.72, so it begins at 2 tons. Manual S, as the guide summarizes it, keeps a heat pump in a cooling-dominated climate from exceeding the load by more than 15% in either cooling or heating mode, and allows up to 25% over the heating load in heating-dominated climates. Applied to Orlando that gives these windows.

Take a candidate reading 22,000 total, 17,000 sensible, and 3,800 latent at design conditions (assumed values for the arithmetic). Total is 22,000 ÷ 20,690 = 106%, and sensible is covered, but latent is 4,060 − 3,800 = 260 BTU/h short, so it passes on total and fails on moisture. The fix is a different supported selection or a documented humidity-control plan, not an arbitrary extra ton.

The heat pump needs a winter check

The guide builds a balance-point diagram: the capacity line comes from the manufacturer's data, and the load line runs from the design load at the design temperature down to zero load at 65°F outdoors. For Orlando that line drops from 23,640 BTU/h at 38°F to zero at 65°F, a slope of 23,640 ÷ 27 = 875.6 BTU/h per °F, so the load at 47°F is 875.6 × 18 = 15,760 BTU/h. Where the capacity line sits below the load line, the difference is supplemental heat; emergency heat is the entire 23,640 BTU/h design load, should the compressor fail. Record the shortfall, the backup equipment, and whether the compressor keeps running when backup comes on. Variable-capacity units also need a check of minimum output, because a large maximum does not prove mild-weather operation is suitable. The Manual S calculator helps organize the comparison.

Step 3, Manual D: from room loads to a pressure budget

Target airflow starts from the sensible load and the room-to-supply temperature difference (TD) that suits the SHR. Manual S uses CFM = sensible load ÷ (1.1 × TD) at low elevation. With 16,630 BTU/h and an assumed TD of 20°F, that is 16,630 ÷ 22 = 756 CFM, about 378 CFM per nominal ton. Room airflow then splits in proportion to each room's sensible load. The guide does not publish Orlando's room loads, so this split uses assumed room values that add to 16,630.

A primary bedroom branch sized at 109 CFM because it resembles Bedroom 2 would be about 15% short (129 − 109 = 20 CFM of 129, or 15.5%). Per-room airflow only works if each run is sized from its own load; see the duct sizing guide and static pressure guide.

Duct sizing then needs the blower data, available static pressure, straight lengths, fitting equivalent lengths, and component losses. The guide assumes 0.50 in. w.c. of external static pressure for its selection. If the filter and coil take 0.20 in., the duct budget is 0.30 in. With a critical-path effective length of 300 ft the friction rate is 0.30 × 100 ÷ 300 = 0.10 in. w.c. per 100 ft. That is a calculation for this layout, not a default for every house. Fittings count toward effective length, a closed bedroom door needs a return path, and changing TD from 20°F to 18°F lifts 16,630 ÷ (1.1 × 18) to 840 CFM, which is why room loads alone do not set a duct size. Manual T covers outlet selection.

Where the chain breaks

• Selecting from a nominal size. Manual S needs capacity at design conditions for the exact models.
• Sizing ducts at 400 CFM per ton. It ignores room loads and available static pressure.
• Swapping equipment without updating the load. A different coil or airflow setting changes sensible and latent capacity.
• Letting the report and the quote describe different houses. Both should cite the same revision, including planned window or insulation upgrades.

Match the symptom to the step

Ask for three handoff confirmations: that the load report and capacity comparison use the same conditions and revision (J to S), that the blower delivers design airflow at the calculated resistance (S to D), and that airflow, static pressure, furnace temperature rise, controls, and room balance were measured after installation. If the installed system differs from the selected models, request an updated comparison. The load estimator and duct size calculator are for planning checks, and the report checklist reviews the upstream inputs.

Sources: DOE Building America, Strategy Guideline: HVAC Equipment Sizing (Orlando house, limits, balance point, airflow equation); ACCA Manual J, Manual S, and Manual D.