NATE Senior Efficiency Analyst flashcards

31 cards. Read the term and the definition here. The deck at the top of this page is optional.

Sensible capacity formula

Qs = 1.08 × CFM × ΔT for standard air at sea level. At altitude, multiply by the density ratio so thin air does not inflate the calculated capacity.

Total capacity formula

Qt = 4.5 × CFM × Δh, where Δh is the entering-to-leaving enthalpy difference. This captures both sensible and latent capacity removed by the coil.

Sensible heat ratio

SHR = sensible capacity ÷ total capacity. A measured SHR far from the rated value usually points to airflow or charge problems rather than a failed compressor.

EER versus SEER2

EER is a single-point Btu/h per watt rating at 95 °F outdoor. SEER2 is a seasonal rating with different test conditions and static allowances, so the two numbers are not interchangeable.

Oversizing symptoms

Short runtimes, high cycling rate, poor latent removal, and a low coil ΔT indicate an oversized system. Runtime fraction below about 0.5 supports the diagnosis.

Steady-state combustion efficiency

Efficiency = (CO2 max − CO2 measured) ÷ CO2 max. Alternatively use flue O2 and stack temperature; condensing appliances gain roughly one point per 2 °F stack reduction.

Heat pump COP from HSPF2

COP ≈ HSPF2 ÷ 3.412 applies only at the rating point. Seasonal COP varies with outdoor bin temperature, so do not use the ratio for annual energy estimates.

Fan laws

CFM varies with RPM, static pressure with RPM squared, and brake horsepower with RPM cubed. A 10% speed increase raises power about 33%.

External static pressure

ESP is measured across the air handler between the return and supply plenum taps. Whether filter and coil drops are included depends on the manufacturer's table.

Available static pressure

Available static = blower rated ESP minus the drops of coil, filter, and grilles. It is the budget that the duct system's total equivalent length must consume.

Equivalent length method

Each fitting is converted to an equivalent length of straight duct. Total effective length = straight runs plus fitting equivalents; friction rate = available static ÷ total effective length.

Duct leakage fraction

Measured with a duct blaster at 25 Pa. Leakage fraction = CFM25 ÷ system CFM; about 10% is a common threshold for existing residential systems.

Airflow from temperature rise

CFM = output Btu/h ÷ (1.08 × ΔT). Use the electric heat kit's rated output or the furnace's temperature rise table to solve for airflow.

Pitot traverse velocity

V (fpm) = 4005 × √VP, where VP is velocity pressure in inches of water column. Residential branch velocities above roughly 900 fpm raise noise and static.

Superheat definition

Superheat = suction line temperature − saturated suction temperature at the measured suction pressure. It confirms the evaporator is fully boiling refrigerant before the compressor.

Subcooling definition

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

Target superheat chart inputs

Fixed-orifice charging charts take indoor wet-bulb and outdoor dry-bulb. The system is correct when measured superheat matches the chart at those conditions.

TXV overcharge pattern

Low superheat with high subcooling suggests overcharge or a restricted, overfeeding valve. High superheat with low subcooling suggests undercharge or a starved evaporator.

Glide and PT columns

Zeotropic blends have separate bubble and dew points. Use the dew point for superheat and the bubble point for subcooling; R-407C glides about 10 °F.

Non-condensable symptoms

High discharge pressure and high subcooling with normal superheat indicate air or nitrogen in the system. Compare condensing temperature to outdoor ambient plus design TD.

Compression ratio

Ratio = absolute discharge pressure ÷ absolute suction pressure. Above roughly 4:1 on a single-stage system suggests a restriction, low load, or high condensing temperature.

Condensing temperature split

Subtract outdoor ambient from saturated condensing temperature. A split well above the design 10 to 20 °F points to dirty coils, low airflow, or overcharge.

Blower door leakage ratio

CFM50 ÷ conditioned floor area gives the leakage ratio. Natural infiltration is roughly CFM50 ÷ 20 for a typical two-story house using the LBL factor.

kWh savings calculation

kWh saved = (baseline watts − post watts) × annual operating hours ÷ 1000. Document the source of the hours: bin data, runtime log, or manufacturer data.

Degree-day normalization

Compare pre- and post-retrofit energy use against heating or cooling degree-days. Without normalization, weather variation can look like savings or a regression.

Simple payback

Payback in years = installed cost ÷ annual dollar savings. State the assumed equipment life and discount rate when presenting life-cycle cost instead.

Meter accuracy statement

Record the instrument model, calibration date, and stated accuracy as ±% of reading plus digits. A clamp reading without true-RMS and accuracy is not defensible.

Tariff-based cost savings

Cost savings = kWh saved × the customer's actual tariff, including demand charges where applicable. An average national rate understates or overstates the real bill impact.

Efficiency report contents

Include measured static pressures, airflow, superheat and subcooling, combustion readings, blower door CFM50, and the before/after calculation method with its assumptions.

Report evidence chain

State the measurement method and instruments, compare measured values to manufacturer or code references, then present calculated savings with every assumption disclosed.

Runtime fraction

Runtime fraction = actual compressor runtime ÷ total elapsed time. Values well below about 0.5 support an oversizing diagnosis when paired with short cycling.