NATE Senior Efficiency Analyst cheat sheet

Sensible capacity check: Qs = 1.08 × CFM × ΔT (standard air, sea level); at altitude multiply by the density ratio (ρ/0.075).

Total capacity check: Qt = 4.5 × CFM × Δh, where Δh is the entering-air to leaving-air enthalpy difference in Btu/lb.

Sensible heat ratio SHR = Qs/Qt; a measured SHR far from the equipment's rated SHR indicates airflow or charge problems, not a capacity fault.

EER = Btu/h output ÷ watts input at 95 °F outdoor; SEER2 uses different test conditions and static-pressure allowances, so never compare EER to SEER2 directly.

For heat pumps, HSPF2 and COP relate by COP ≈ HSPF2/3.412 only at the rating point; seasonal COP varies with bin temperature.

Equipment oversizing shows as short runtime, high cycling rate, elevated latent load removal failure, and low ΔT across the coil; verify with a runtime fraction below ~0.5.

Combustion efficiency: steady-state efficiency = (CO2 max − CO2 measured)/CO2 max, or use flue O2 and stack temperature; condensing units gain ~1 point per 2 °F of reduced stack temperature.

Boiler/furnace derate check: gross to net output accounts for jacket and piping losses; compare measured stack temp and O2 against the manufacturer's setup table, not a generic target.

Fan law 1: CFM varies directly with RPM; fan law 2: static pressure varies with RPM squared; fan law 3: brake horsepower varies with RPM cubed.

External static pressure (ESP) is measured across the air handler only - between the return plenum and supply plenum taps - and excludes filter and coil drop only if the manufacturer's table says so.

Duct static pressure budget: typical residential supply trunk 0.1 in. w.c., branch runs 0.05 in. w.c., return 0.05 in. w.c.; total available static = blower rated ESP minus coil, filter, and grille drops.

Equivalent length method: each fitting has an equivalent length in feet of straight duct; total effective length = straight runs + fitting equivalents, and friction rate = available static ÷ total effective length.

Duct leakage: measure with a duct blaster at 25 Pa; leakage fraction = CFM25 ÷ system CFM; a common threshold is 10% of system airflow for existing systems.

Airflow by temperature rise (electric heat or furnace): CFM = output Btu/h ÷ (1.08 × ΔT); by sensible capacity: CFM = Qs ÷ (1.08 × ΔT).

Velocity check: V (fpm) = 4005 × √VP (in. w.c.) for a pitot traverse; duct velocity above ~900 fpm in residential branch runs raises noise and static.

Balance check: total supply CFM should be within 10% of total return CFM plus outdoor air; room-by-room airflow should match the Manual J room load within 10–20%.

Superheat = suction line temperature − saturated suction temperature at the measured suction pressure; target superheat for fixed-orifice systems depends on indoor wet bulb and outdoor dry bulb.

Subcooling = saturated condensing temperature − liquid line temperature; TXV systems are charged to the manufacturer's target subcooling (commonly 8–12 °F).

Superheat charging chart inputs are indoor wet-bulb and outdoor dry-bulb; a fixed-orifice system is correct when measured superheat matches the chart at those conditions.

TXV diagnosis: low superheat with high subcooling suggests overcharge or a restricted/overfeeding valve; high superheat with low subcooling suggests undercharge or a starved evaporator.

Pressure–temperature relationship: use the refrigerant's PT chart; a suction pressure reading alone is meaningless without the corresponding saturation temperature.

Refrigerant glide: zeotropic blends (e.g., R-410A ~0.3 °F, R-407C ~10 °F) require dew-point and bubble-point columns; use dew point for superheat and bubble point for subcooling.

Non-condensables show as high discharge pressure and high subcooling with normal superheat; compare condensing temperature to outdoor ambient plus the design TD (typically 10–20 °F).

Compressor ratio = absolute discharge pressure ÷ absolute suction pressure; a ratio above roughly 4:1 on a single-stage system signals a restriction, low load, or high condensing temperature.

Blower door result: CFM50 ÷ conditioned floor area gives the leakage ratio; natural infiltration is roughly CFM50 ÷ 20 (LBL factor) for a typical two-story house.

Energy savings estimate: kWh saved = (baseline watts − post watts) × annual operating hours ÷ 1000; document the hours source (bin data, runtime log, or manufacturer's data).

Measurement uncertainty: a clamp meter reading should be paired with a true-RMS meter and a stated accuracy (±% of reading + digits); record the meter model and calibration date.

Baseline normalization: compare pre- and post-retrofit energy use against heating/cooling degree-days so weather variation does not masquerade as savings.

Cost savings = kWh saved × utility rate; use the customer's actual tariff including demand charges, not an average national rate.

Documentation set for an efficiency report: measured static pressures, airflow, refrigerant superheat/subcooling, combustion readings, blower door CFM50, and the before/after calculation method.

Simple payback (years) = installed cost ÷ annual dollar savings; state the assumed equipment life and discount rate if presenting life-cycle cost.

Report structure: state the measurement method and instruments, present measured values against the manufacturer's or code reference, then the calculated savings and the assumptions behind them.