16.3 Applying Energy Conservation, Fan Curves & Pump Curves
Key Takeaways
- Residential attic supply and return ducts 3 inches in diameter and larger must be insulated to R-8 under FBC Energy R403.3.1 (prescriptive); smaller attic ducts R-6. Other unconditioned portions: R-6 (≥3 in.) and R-4.2 (<3 in.). Ducts fully inside the thermal envelope are excepted. Building cavities shall not be used as ducts or plenums (R403.3.5).
- Commercial Table C403.2.9.1: exterior and ventilated-attic supply R-6 / return R-4.2; unconditioned-space supply and return R-4.2; none required in conditioned or indirectly conditioned space. C403.2.9.1.1 still lists exceptions (ΔT ≤ 15°F, short runouts, etc.).
- Fan and pump operating point is the intersection of the manufacturer's curve and the system curve (SP or head ≈ k × flow²). Affinity laws: flow ∝ speed, pressure ∝ speed², power ∝ speed³. A 20 percent speed increase multiplies power by 1.2³ = 1.728 (about 73 percent more power).
- Throttling a damper or valve rides up the system curve and wastes brake horsepower. A VFD that lowers speed follows the affinity laws and is the conservation move. SMACNA Energy Systems Analysis and Management (2nd Ed. 2014) is on the 2026 CBT list for this topic.
- Equipment efficiency (SEER2/EER/IEER, federal minimums in R403.7.1), right-sizing to 1.15, envelope R/U, duct location, and sealing (prescriptive total leakage 4 cfm/100 ft² CFA at 25 Pa with the air handler installed) are the same energy-conservation package — not separate hobbies.
16.3 Applying Energy Conservation, Fan Curves & Pump Curves
Trade Area A’s energy-conservation item is not a slogan. It is equipment efficiency, Florida energy codes, R and U factors, SEER, and pump/fan curves used together so the load you calculated in 16.2 is moved with the least kWh. Energy Efficient Building Construction in Florida (2024) and SMACNA Energy Systems Analysis and Management, 2nd Edition (2014), are the narrative references; FBC Energy Conservation 2023 is the enforceable one. Class B still tests this theory; a chilled-water pump on a 40-ton plant is Class A to install, but the affinity laws do not care which ticket is on the wall.
Quick Answer: Cut the load (envelope U, SHGC, ducts inside the envelope), then move the remaining air or water at the intersection of the fan/pump curve and the system curve. R403.3.1: attic ducts R-8 at 3 in. diameter and larger. Fan laws: CFM with RPM, static with RPM², horsepower with RPM³. Speeding a blower 20 percent to fix undersized duct multiplies power by 1.73 — usually the wrong conservation recommendation.
Conservation is a stack, not a SEER sticker
Rank the moves the way a load ranks the Btu:
- Envelope. Table R402.1.2 / R402.1.4 R and U, SHGC 0.25, air barrier, 7 ACH50 testing. Solar through west glass dwarfs ceiling conduction in Section 16.2’s example; low-SHGC glass and overhangs are conservation, not decoration.
- Duct location and leakage. A vented Florida attic can sit 20–40°F above outdoor in the afternoon. Supply air at 55°F in an R-4.2 leaky run is an unplanned reheat coil. Prescriptive R403.3.4 total leakage is 4 cfm per 100 ft² of conditioned floor area at 25 Pa with the air handler installed (rough-in 3 cfm/100 ft² if the air handler is not yet installed). Testing is ANSI/RESNET/ICC 380. R403.3.2.1: air handlers designated ≤ 2 percent leakage per ASHRAE 193. R403.3.5 (Mandatory): building framing cavities shall not be used as ducts or plenums.
- Equipment efficiency and size. R403.7.1 last sentence: meet the federal minimum for the location (SEER2/EER2/HSPF2 or IEER on larger commercial). Right-size to 1.15 so the coil runs long enough to pull latent. A high-SEER2 5-ton on a 3-ton latent-heavy load is not “efficient.”
- How the fan and pump are driven. Operating point, affinity laws, VFD versus throttling — the rest of this section.
R405.2 (performance path) still wants supply and return ducts not completely inside the envelope insulated to R-6, except site-wrapped supply ducts not completely inside the envelope at R-8. Do not mix that sentence with the prescriptive R403.3.1 attic R-8 table and call them identical.
R403.3.6 air handlers: not in the attic when the home complies by Section R402 (prescriptive envelope). Attic air handlers are allowed on R405 performance only with access within 6 ft, condensate alarm or shutoff, an opening large enough to replace the unit, and a panel notice. Putting the air handler in a vented attic on a prescriptive house is not a “means and methods” choice.
Duct insulation R-values you can cite
Residential prescriptive — R403.3.1 Insulation:
| Location and size | Minimum R |
|---|---|
| Supply and return in attics, diameter ≥ 3 in. | R-8 |
| Supply and return in attics, diameter < 3 in. | R-6 |
| Supply and return in other portions of the building, ≥ 3 in. | R-6 |
| Supply and return in other portions, < 3 in. | R-4.2 |
| Buried under a building | Same as this section, or listed thermal-distribution-efficiency equivalent |
| Portions completely inside the building thermal envelope | Exception — no R403.3.1 insulation |
Commercial — Table C403.2.9.1 (minimum duct insulation R-values, heating and cooling supply and return):
| Location | Supply | Return |
|---|---|---|
| Exterior of building | R-6 | R-4.2 |
| Ventilated attic | R-6 | R-4.2 |
| Unvented attic above insulated ceiling | R-6 | R-4.2 |
| Unvented attic with roof insulation | R-4.2 | None |
| Unconditioned spaces (including crawl spaces) | R-4.2 | R-4.2 |
| Indirectly conditioned spaces (including return plenums) | None | None |
| Conditioned spaces | None | None |
| Buried | R-4.2 | None |
C403.2.9.1.1 exceptions include ducts inside equipment, design ΔT across the duct wall ≤ 15°F, runouts < 10 ft to terminals not more than R-5, and some outlet-back insulation rules. A Florida RTU with 40 ft of outdoor supply duct is R-6 supply, not “unconditioned R-4.2.” A house with 10-inch attic flex is R-8, not the commercial attic R-6 row.
Fan curves, system curves, and the affinity laws
A fan curve (manufacturer) plots CFM versus static pressure at a stated RPM, belt drive, or tap, often with brake-horsepower contours. A system curve is the duct network: roughly
(\text{SP} = k \times \text{CFM}^2)
because fitting and duct friction rise with velocity squared. The operating point is where the two cross. You do not “pick 1,600 cfm” off a schedule if that point is not on the curve at the real external static (filter + coil + supply + return + grilles).
Fan laws (same fan, same duct geometry, air density constant):
- (\text{CFM}_2 / \text{CFM}_1 = N_2 / N_1) (flow proportional to speed)
- (\text{SP}_2 / \text{SP}_1 = (N_2 / N_1)^2) (pressure proportional to speed squared)
- (\text{bhp}_2 / \text{bhp}_1 = (N_2 / N_1)^3) (power proportional to speed cubed)
Worked fan-law example. A residential indoor blower delivers 1,200 cfm at 0.50 in. w.c. external static, 0.40 bhp. The installer finds 1,000 cfm at the boots and “turns it up 20 percent” with a pulley or a tap.
(N_2 / N_1 = 1.20)
New flow: (1{,}200 \times 1.20 = 1{,}440) cfm
New static if the system curve is unchanged: (0.50 \times 1.20^2 = 0.50 \times 1.44 = 0.72) in. w.c.
New power: (0.40 \times 1.20^3 = 0.40 \times 1.728 = 0.69) bhp — 73 percent more power for 20 percent more air.
If the duct was undersized, k is too large: the system curve is steeper. Speeding the fan rides out along that steep curve. You may still miss 1,200 cfm, now at higher noise, higher motor current, and a coil velocity that can blow condensate off the fins. The conservation recommendation is more duct area (Manual D), shorter flex, or a different blower — not a bigger pulley. That is the same recommendation sequence as Chapter 15’s equipment-type item; here the exam is asking why the kWh exploded.
Closing a supply damper to “balance” a hot room increases k, slides the operating point left (less CFM, more SP), and converts fan energy to heat in the duct. A VFD or ECM that lowers speed 10 percent at part load:
Power ratio ( = 0.90^3 = 0.729) — about 27 percent less power, not 10 percent. That is energy conservation you can draw on a curve.
Pump curves — same picture, water instead of air
A pump curve plots flow (gpm) versus head (feet of water) at a stated impeller diameter and speed, with bhp or efficiency islands. The hydronic system curve is head ≈ k × gpm² (pipe, fittings, coil, control valves). Operating point is the intersection. Affinity laws for a centrifugal pump are the same three ratios: flow with N (or impeller diameter, within limits), head with N², power with N³.
Worked pump example. A condenser-water pump is 80 gpm at 40 ft, 1.20 bhp, 1,750 rpm. Someone throttles a discharge valve until flow is 72 gpm (10 percent less) because the tower is noisy.
Throttling does not follow N³. Speed is still 1,750 rpm. The system curve steepens; the pump rides up its published curve to a higher head and only a small power drop — most of the “saved” flow energy becomes heat and noise in the valve. A VFD that reduces speed so that flow is 72 gpm on the original system curve:
(N_2 / N_1 = 72/80 = 0.90)
Head: (40 \times 0.81 = 32.4) ft
Power: (1.20 \times 0.729 = 0.87) bhp
about 27 percent less power, same 10 percent flow cut.
Class A work on boilers, condenser water, and chilled water uses this daily. Class B hydronic exposure is lighter, but the curve question on the outline is not labeled “Class A only.” Trimming an impeller is a permanent diameter change (new pump curve family). Balancing valves after a VFD is commissioned are for trim, not for creating the design head.
Putting SEER, ducts, and curves on one job
SEER / SEER2 (Chapter 15.3) is seasonal Btu of cooling per watt-hour. It assumes reasonable runtime and airflow. A throttled, leaky, R-4.2 attic system pulls the real operating SEER down even if the AHRI match is 16.0 SEER2. Conservation that actually shows up on the EPL card and on the owner’s bill:
- Meet R402 U/R and SHGC so Manual J total stays inside a 1.15-legal outdoor unit.
- Put ducts inside or insulate them to R403.3.1 / C403.2.9.1, seal to R403.3.4, and never use stud cavities as plenums.
- Select from expanded data, not AHRI nominal.
- Set the fan/pump operating point on the curve at design; use VFD/ECM for part load instead of dampers and discharge valves as horsepower controls.
- Keep electric resistance out of the primary heat role in Climate Zone 2 (R403.7.2).
Florida HVAC scenario
A certified Class B shop in Hillsborough County (Zone 2A) changeouts a 3-ton split. The air handler stays in the vented attic on a house pulling prescriptive R402 compliance. Flex is labeled R-6. After start-up the rooms at the end of the run are 2–3°F hot, so the tech moves the PSC blower from the middle tap to high. Amp draw jumps, the owner hears the return, and the coil blows a mist at the supply plenum.
Three code-and-curve corrections, in order: (1) R403.3.6 does not allow that attic air handler on a Section R402 house — move it into conditioned space or change the compliance path and meet the R405 attic conditions; (2) R403.3.1 wants R-8 on those ≥3 in. attic ducts, not R-6; (3) the fan is off the intended operating point — high tap cubed the motor load instead of fixing k (larger trunk, less flex, sealed returns). Speeding the fan was not energy conservation. Matching a legal 3-ton expanded-data unit to a Manual J total, insulating R-8, and leaving the blower on the tap that hits design CFM at the measured static is.
An indoor blower is delivering 1,200 cfm at 0.50 in. w.c. and 0.40 bhp. If speed is increased 20 percent on the same duct system, what do the fan laws predict for power, and what is the usual Florida field implication?
Which duct-insulation statement is consistent with FBC Energy Conservation 2023?
A condenser-water pump is selected at 80 gpm and 40 ft head. The tower is noisy, so the operator wants about 10 percent less flow. Which energy-conservation statement is accurate?