15.2 Recommending Plan Changes & Equipment Types

15.2 Recommending Plan Changes & Equipment Types

Trade Area A does not stop at reading the sheet. The outline asks you to recommend changes based on costing, efficiency, and design requirements, and to recommend equipment types based on costing, efficiency, loads, SEER, airflow and duct design, psychrometric diagrams, and pump or fan curves. That is pre-installation judgment: the drawings are a proposal, the Florida Building Code is the floor, and your Class A or Class B ticket is the fence.

Quick Answer: Size to the load, not to a square-foot rule of thumb. Check Class B's 25-ton / 500,000 Btu per-system cap before you recommend a packaged rooftop. Then check FBC Energy R403.7 (Manual J + Manual S, 1.15 cooling cap, expanded data — not the AHRI nameplate), duct static versus the fan curve, and whether Florida's latent load will actually be wrung out of the air.

License scope is the first equipment filter

F.S. 489.105 is not only a Business & Finance topic. Class A air-conditioning contractors have unlimited capacity and may design (unless prohibited), install, and alter central AC, refrigeration, heating, ventilating, and the ductwork needed to complete an air-distribution system, plus boilers, unfired pressure vessels, and pneumatic control piping. Class B has the same trade skills but is limited to 25 tons cooling and 500,000 Btu heating in any one system. Class B does not pick up boilers, unfired pressure vessels, or pneumatic control piping. Class B candidates still need the theory of larger systems — the outline drops the 25–100 ton, over-100 ton, and centrifugal install items, not the idea that those machines exist.

Twenty-five tons is 300,000 Btu/h of cooling. A scheduled 30-ton rooftop (360,000 Btu/h) is Class A work if it is one system. Splitting a building into two independent 15-ton systems, each with its own distribution, can be legitimate Class B design. Relabeling one 30-ton air loop as “two 15-ton condensing units” to dodge the cap is still one system. F.S. 489.105(6) treats recommending, bidding, and negotiating that work as contracting. Recommend a change that keeps you legal, or walk away.

What “recommend a change” means on this exam

You recommend a change when the documents, as drawn, will not meet code, will not meet the load, will waste money, or will put the qualifier outside scope. Typical Florida flags:

• Specified efficiency is below the federal minimum for the installation location (FBC Energy R403.7.1 last sentence; DOE regional SEER2 — Section 15.3).
• Equipment is oversized past the 1.15 cooling cap, or selected from the AHRI nominal total-capacity column instead of manufacturer's expanded performance data at the actual outdoor dry-bulb and entering wet-bulb (R403.7.1.1).
• Duct velocity or external static on the drawing cannot be reached on the scheduled fan curve without overspeeding the blower.
• Outdoor unit is in a well or screen that starves condenser air, or the air handler is in an attic in a way R403.3.6 does not allow on a prescriptive-path house.
• Primary heat is electric resistance in Climate Zone 2 (R403.7.2 — not allowed as the primary system).
• Access, service space, or condensate (FBC Mechanical Chapter 3) is missing.
• The specified machine is a boiler, centrifugal chiller, or 40-ton rooftop on a Class B qualifier.

Costing is part of the recommendation. A smaller, correctly sized, higher-SEER2 unit with tight ducts often beats a larger cheap unit on first cost and on EnergyGauge performance. Changing the drawing after permit without the building official and the design professional is not a field option; the recommendation happens in submittal, value-engineering, or RFI time.

Loads, Manual S, and the 1.15 rule

FBC Mechanical 2023 Section 312 points commercial-style design loads to ASHRAE/ACCA Standard 183 (or an approved equivalent using Energy Conservation Chapter 3 [CE] design conditions). FBC Energy Conservation 2023 Section R403.7.1 is the residential rule the exam leans on: size heating and cooling equipment to ACCA Manual S based on ACCA Manual J (or another approved methodology), using the building's directional orientation. Submit manufacturer and model of outdoor and indoor units for a split, with sensible and total cooling at the R302.1 design conditions. The code does not allow designer safety factors, provisions for future expansion, or other fudge factors. Kitchen and bath exhaust intermittents stay out of the sizing load.

R403.7.1.1 Cooling equipment capacity. Total capacity shall be not less than the calculated total load and not more than 1.15 times that load, or the closest available size in the 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 expanded 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 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. Florida is a cooling-dominated peninsula; that sentence exists so you do not hang a 5-ton heat pump on a 3-ton cooling load just because a Panhandle heating-day looks large on paper.

Worked check: Manual J total cooling 31,200 Btu/h, latent 7,400 Btu/h. 1.15 × 31,200 = 35,880 Btu/h. A unit whose expanded-data total at 95°F outdoor / design indoor WB is 34,000 Btu/h with 8,000 Btu/h latent is legal. A 4-ton whose expanded total is 44,000 Btu/h is 1.41 times the load — too big unless it is literally the closest size and you can still defend latent (the 1.15 “or closest size” clause is not a license to jump a whole ton when a 2.5- or 3-ton match exists).

Equipment types, airflow, psychrometrics, and curves

Recommend the type that matches the load profile, the duct system, and the license.

Airflow and duct design. A planning number of 350 to 450 cfm per ton is a starting point, not a code. Florida latent loads often want the lower end so coil temperature and run-time can dehumidify. The scheduled CFM must be deliverable at the external static pressure of the real duct system (filter, coil, supply, return). FBC Energy R403.3 still wants attic ducts insulated (R-8 at 3 inches diameter and up), ducts sealed, and building cavities not used as ducts. If the drawing shows 1,600 cfm through 50 feet of 8-inch flex, recommend a change: velocity and friction will miss the fan chart.

Psychrometric diagrams. Plot entering air (often near 80°F dry-bulb / 67°F wet-bulb at AHRI indoor, or the Manual J indoor 75°F with a stated relative humidity) and leaving air. The process line runs toward the coil apparatus dew point. Sensible heat ratio (SHR) is sensible divided by total. An oversized unit in Sarasota short-cycles, stays on the high-SHR end of the chart, and leaves the space cold and clammy. Recommending a two-stage or variable-speed match that hits latent at part load is an efficiency and comfort recommendation.

Fan curves. The manufacturer plots CFM versus static pressure for each RPM or tap. The system curve is roughly static pressure proportional to CFM squared. The operating point is the intersection. Fan laws: CFM varies with RPM; static varies with RPM squared; horsepower varies with RPM cubed. Need 20 percent more air by speeding the fan? Horsepower rises by (1.2^3 = 1.728) — about 73 percent more power, more noise, and a motor that may leave the nameplate. The better recommendation is usually more duct area or a different blower, not a bigger pulley.

Pump curves (chilled water, condenser water, hydronic heat — Class A territory on boilers) work the same way: flow versus head, intersection with the pipe system curve. Trimming an impeller or adding a VFD moves you along or across curves; closing a balancing valve just rides up the system curve and wastes brake horsepower.

Florida HVAC scenario

A Volusia County (Climate Zone 2) dentist office is issued with a 30-ton packaged rooftop, 5-ton of electric resistance as “primary heat,” 1,200 cfm per ton through undersized rectangular duct, and a Class B qualifier invited to bid. Three recommendations, in order: (1) the 30-ton unit exceeds Class B's 25-ton cap as one system — either the owner hires Class A, or the design splits into independent systems each ≤25 tons if that still meets the load; (2) R403.7.2 does not allow electric resistance as the primary heating system in Climate Zone 2 — recommend a heat pump or fossil-fuel heat sized to Manual S; (3) 1,200 cfm/ton will not sit on a typical packaged fan curve without a static-pressure fantasy — reduce airflow toward the coil's expanded-data CFM, enlarge duct, and check the psychrometric latent load so the waiting room does not sweat. Costing follows the code-legal design, not the first unit that fits the existing curb.