16.2 Determining HVAC Heating & Cooling Loads

16.2 Determining HVAC Heating & Cooling Loads

Once you can turn an assembly into a U-factor, Trade Area A asks you to determine HVAC heating and cooling loads using Florida energy codes, those R and U values, and a real calculation method. Guessing “one ton per 400 square feet” is how Florida rooms go cold and wet. The code method is ACCA Manual J for dwellings and ASHRAE/ACCA Standard 183 for the commercial-style path in FBC Mechanical 2023 Section 312, both using the design conditions in Energy Conservation Chapter 3.

Quick Answer: Run Manual J (or approved equal) at R302.1 indoor conditions — 75°F cooling, 72°F heating — and the site’s outdoor design data. Add sensible and latent separately. Size with Manual S and R403.7.1.1: total cooling capacity between the calculated total load and 1.15 times that load (or the closest catalog size), with latent not less than the calculated latent load. No fudge factors.

Why the load is a code document, not a shop habit

R403.7.1 Equipment sizing (Mandatory) is the sentence the exam leans on:

• Size heating and cooling equipment to ACCA Manual S based on loads from ACCA Manual J or another approved methodology.
• Use building loads for the directional orientation of the building — a west glass wall is not an east glass wall.
• Submit manufacturer and model of outdoor and indoor units on a split, with sensible and total cooling at R302.1 design conditions.
• This Code does not allow designer safety factors, provisions for future expansion, or other factors that affect equipment sizing.
• Do not include loads from local intermittent mechanical ventilation such as standard kitchen and bathroom exhaust.
• New or replacement equipment shall meet the federal minimum efficiency for the geographic location (DOE regional SEER2/HSPF2 — Chapter 15.3).

R403.7.1.1 then caps cooling-only equipment: total capacity ≥ calculated total load and ≤ 1.15 × total load, or the closest available size in that manufacturer’s product line. Latent capacity shall not be less than the calculated latent load. The published AHRI total capacity is a nominal rating-test value and shall not be used for equipment sizing. Select from manufacturer’s expanded performance data at the outdoor design dry-bulb (or entering water temperature), the blower CFM from that data, and the design entering wet-bulb and dry-bulb, adjusted for return-side gains if the return duct is in unconditioned space.

R403.7.1.2.1 Heat pumps: size on the cooling requirements of R403.7.1.1. Heat-pump total cooling capacity shall not be more than 1.15 times the design cooling load even if the design heating load is 1.15 times greater than the design cooling load. That sentence exists because Florida is cooling-dominated. A Jacksonville heating day is real; it is not a reason to hang a 5-ton heat pump on a 3-ton cooling load.

R403.7.1.2.2: electric resistance furnaces shall be sized within 4 kW of the calculated design. R403.7.2 (prescriptive): electric resistance shall not be the primary heating system in Climate Zone 2. R403.7.1.3 extra capacity for parties and entertainment is a separate system or a variable-capacity system sized for the base load — not a bigger single-speed compressor.

Class B candidates still work this math. If the resulting machine is one system over 25 tons or 500,000 Btu, installation is Class A scope (F.S. 489.105). Splitting into independent systems, each with its own distribution, can be legitimate; relabeling one 30-ton loop as two 15-ton condensing units is still one system.

Indoor and outdoor design conditions

R302.1 (residential) and C302.1 (commercial) fix interior design temperatures for load calculations: maximum 72°F for heating and minimum 75°F for cooling. You may not run a “65°F cooling load” to fatten the tonnage. Outdoor design is not in that section as a single statewide number. Manual J lets you use Manual J Table 1A/1B or ASHRAE Handbook climatic data, but not a mix of the two (Manual J mixing rule). Typical Florida 1% cooling dry-bulbs sit in the low-to-mid 90s°F with coincident wet-bulbs near 76–78°F; 99% heating dry-bulbs range from the high 40s in the Keys/southeast to the low 30s in north Florida. AHRI AFull 95°F is a rating condition. Use it in a classroom ΔT example; do not paste it over Miami or Jacksonville when the approved weather table says otherwise.

Cooling ΔT at 95°F outdoor / 75°F indoor is 20°F. Heating ΔT at 35°F outdoor / 72°F indoor is 37°F. The heating number is larger, but Florida has far more cooling hours, high latent load, and a code that still sizes heat pumps on cooling. Envelope conduction Q = U × A × ΔT uses those ΔTs; solar, latent, and attic duct gains do not care about heating ΔT in August.

Anatomy of a Manual J cooling load

Treat the house as a set of named loads. Sensible load raises dry-bulb. Latent load is moisture that must be condensed at the coil. Total = sensible + latent. Sensible heat ratio (SHR) = sensible / total. A Florida dwelling often lands near 0.70–0.80 SHR; an oversized high-SHR unit short-cycles and leaves the space clammy.

Infiltration versus ventilation is a favorite mix-up. Infiltration is leakage through the thermal envelope — the blower-door 7 ACH50 cap in R402.4.1.2 is a test limit at 50 Pa, not the natural ACH in the load. A rough teaching conversion is that natural ACH is a small fraction of ACH50 (order of ACH50 divided by about 15–20, depending on climate and shielding). Ventilation is outdoor air you mean to bring in (balanced ERV, supply fan, or ASHRAE 62.2-style whole-house). Tightening the house without adding R403.6 ventilation trades one IAQ problem for another; adding both infiltration and the bath fan and whole-house CFM as if they were independent peak loads is how a 2.5-ton house becomes a 4-ton submittal.

Air-side sensible and latent shortcuts (standard 0.075 lb/ft³ air):

(Q_s = 1.08 \times \text{CFM} \times \Delta T)

(Q_l = 0.68 \times \text{CFM} \times \Delta \text{grains})

Those formulas size airflow and moisture, not envelope conduction. Envelope still uses U × A × ΔT.

Worked load — 2,000 ft² Climate Zone 2 house

Orange County, Zone 2A, 2,000 ft², 8 ft ceilings, volume 16,000 ft³. Indoor 75°F. Example outdoor 95°F dry-bulb so envelope ΔT = 20°F (replace 95°F with the site 1% value on a real permit). Ceiling uses Table R402.1.4 U-0.030. Net walls 1,140 ft² at the code equivalent-U cap 0.084. Glass 300 ft² at U-0.40. Manual J glass tables (given for this example, west-heavy, SHGC 0.25) return 9,600 Btu/h solar. Natural infiltration modeled at 0.35 ACH after converting a code-limit envelope toward design leakage.

Ceiling conduction: (0.030 \times 2{,}000 \times 20 = 1{,}200) Btu/h
Wall conduction: (0.084 \times 1{,}140 \times 20 = 1{,}915) Btu/h
Glass conduction: (0.40 \times 300 \times 20 = 2{,}400) Btu/h
Glass solar (MJ tables): 9,600 Btu/h
Infiltration CFM: (0.35 \times 16{,}000 / 60 = 93) cfm
Infiltration sensible: (1.08 \times 93 \times 20 \approx 2{,}010) Btu/h
Internal (people + lights, sensible): 3,200 Btu/h
Attic duct gain (supply/return not in the envelope): 2,400 Btu/h
Sensible subtotal ≈ 22,725 Btu/h

Latent: people + wet infiltration/ventilation in this climate (example 8,200 Btu/h).
Total cooling = 22,725 + 8,200 = 30,925 Btu/h ≈ 2.58 tons.

1.15 × 30,925 = 35,564 Btu/h. Expanded-data total at the outdoor design dry-bulb and entering wet-bulb must land between 30,925 and 35,564 Btu/h, or the closest catalog size, and latent ≥ 8,200 Btu/h. A 4-ton whose expanded total is 44,000 Btu/h is 1.42 times the load — illegal as a “safety factor.” AHRI “36,000 Btu/h nominal” is not the selection column.

Notice the ranking: glass solar (9,600) beats ceiling conduction (1,200) by eight to one. In Florida, SHGC, overhangs, and west glass move the load more than arguing R-38 versus R-30 on a well-insulated ceiling. Ducts in a vented attic add a chunk that goes away if the air handler and ducts are brought inside the thermal envelope (and R403.3.6 restricts air handlers in attics on the prescriptive R402 path).

Heating load on the same house uses 72°F indoor and the 99% outdoor dry-bulb, envelope U × A × ΔT, infiltration, and no solar credit as a peak-heat reducer unless the approved method says so. In south Florida the heating load is often a fraction of cooling; the heat pump is still picked on cooling (R403.7.1.2.1). North Florida heating ΔT is larger, but the 1.15 cooling cap still binds the heat-pump compressor.

Commercial loads and replacement work

FBC Mechanical 312 requires heating and cooling load calculations for the design of HVAC systems, using ASHRAE/ACCA Standard 183 or an approved equivalent, with interior conditions from Energy Conservation C302.1 (same 72°F / 75°F). Commercial buildings add occupancy-driven ventilation (FBC Mechanical / ASHRAE 62.1), lighting power, and often a larger latent fraction from outdoor air. C403 then applies equipment efficiency, economizers where required, and fan-power limits. Do not run Manual J on a 40,000 ft² office and call it good — and do not run a block-load 183 calc without zone orientation when R403.7 would have required orientation on a house.

Replacements are not exempt from R403.7’s efficiency sentence or from a load when the building official requires it. “Match the old 5-ton because the breaker is already there” is how a tightened, re-windowed house fails latent. Recalculate with today’s U, SHGC, duct location, and orientation.

Florida HVAC scenario

A Class A qualifier in Duval County (Zone 2A) prices a 3,200 ft² two-story with a west glass wall, ducts in a vented attic, and a salesperson’s “500 square feet per ton” sheet that calls a 6.4-ton system. Manual J at 75°F indoor, site outdoor data, SHGC 0.25, and attic duct gain comes back 41,000 Btu/h total, 12,000 Btu/h latent (3.4 tons). 1.15 × 41,000 = 47,150 Btu/h. A 4-ton match whose expanded total is 45,000 Btu/h with 12,500 Btu/h latent is legal. The 6-ton is 1.76 times the load, will short-cycle, and will not wring 12,000 Btu/h of moisture if it only runs 10 minutes an hour. Kitchen hood exhaust stays out of the sizing load (R403.7.1). Electric resistance as primary heat is barred in Zone 2 (R403.7.2). The qualifier’s ticket covers the tonnage; the energy code still rejects the 6-ton “to be safe.”