AABC Test and Balance Engineer (TBE) cheat sheet

Sensible heat ratio (SHR) = sensible heat / total heat; total = sensible + latent.

Apparatus dew point (ADP) is the coil surface condition where the coil curve intersects the saturation line at 100% RH.

Bypass factor = (entering DB - leaving DB) / (entering DB - ADP); a lower bypass factor means a deeper/wetter coil.

Standard air density = 0.075 lb/ft3 at 70 F, sea level, 29.92 in. Hg; sensible heat (Btu/h) = 1.08 x cfm x delta-T.

Latent heat (Btu/h) = 4,840 x cfm x delta-W (lb water/lb dry air) or 0.68 x cfm x delta-grains.

Total heat (Btu/h) = 4.5 x cfm x delta-h (Btu/lb dry air).

Fan total static pressure converts to velocity pressure via Vp = (V/4005)^2 for standard air; velocity in fpm = 4005 x sqrt(Vp in. w.c.).

Air density correction: actual density = 0.075 x (530/(460 + T F)) x (Pb/29.92).

Pitot-tube velocity pressure is the difference between total pressure and static pressure; read total and static separately, never subtract two independent gauge readings without noting uncertainty.

A manometer or differential pressure gauge should be zeroed at the test location before each reading to eliminate elevation and offset error.

Instrument accuracy is stated as % of reading or % of full scale; % of full scale error grows in absolute terms at low readings, so select a range near the measured value.

Combined standard uncertainty for independent sources: u_c = sqrt(u1^2 + u2^2 + ...); expanded uncertainty U = k x u_c with k = 2 for ~95% confidence.

Calibration traceability requires an unbroken chain to a national standard (NIST) with recorded as-found and as-left data.

A rotating-vane anemometer under-reads at low velocity and near duct edges; use a hood or pitot traverse for low-velocity or large-duct work.

Thermocouple and RTD readings require an ice-point or known-reference check; infrared thermometers are unreliable on reflective or low-emissivity surfaces.

Calibration interval should be shortened when as-found data drift approaches the instrument's tolerance limit.

A pitot traverse divides the duct into equal-area zones; take at least 16-20 readings (Log-Tchebycheff or equal-area method) and average the square roots of velocity pressures.

Traverse location should be at least 7.5 duct diameters downstream and 3 diameters upstream of disturbances where possible.

For a rectangular duct, divide each side into equal segments (minimum 5 per side, 25 points) per Log-Tchebycheff spacing.

Airflow (cfm) = area (ft2) x velocity (fpm); area for a round duct = pi x (diameter/2)^2.

Balancing sequence: set the fan to design total airflow first, then balance branches proportionally, then terminals, then recheck the fan.

Proportional balancing uses ratios of measured to design flow; the terminal with the lowest percentage is left fully open and others are throttled to match that ratio.

A duct leakage test per SMACNA measures leakage at a specified static pressure; report leakage class and test pressure.

Diversity factor reduces total connected load when not all terminals operate simultaneously; confirm the design diversity assumption before final airflow set.

Pump affinity laws: flow varies with speed (Q ~ N), head with speed squared (H ~ N^2), power with speed cubed (P ~ N^3).

Pump power (water horsepower) = (gpm x head in ft x specific gravity) / 3960; brake horsepower = water horsepower / pump efficiency.

System head varies with flow squared for turbulent flow (H ~ Q^2); the operating point is the intersection of the pump curve and system curve.

Balancing valves should be set by measuring differential pressure or flow; a circuit setter with a known Cv allows flow calculation from pressure drop.

Cv = gpm at 1 psi pressure drop with water; flow (gpm) = Cv x sqrt(delta-P in psi).

A three-way valve at a terminal maintains constant flow through the coil while bypassing excess; a two-way valve varies flow and requires a variable-speed pump or bypass.

Glycol solutions reduce heat capacity and increase viscosity; derate pump head and capacity for the specific concentration.

Differential pressure across a coil or valve should be measured with a single differential instrument, not two separate gauges, to avoid reference error.

Building pressure is controlled by the difference between outdoor air intake and exhaust/relief airflow; positive pressure limits infiltration, negative pressure controls exfiltration.

ASHRAE 62.1 ventilation rate procedure: Vbz = Rp x Pz + Ra x Az, where Rp is people outdoor air rate and Ra is area outdoor air rate.

System ventilation efficiency (Ev) and zone air distribution effectiveness (Ez) modify the required outdoor air; Vot = Vou / Ev.

Exhaust hood face velocity is measured at the hood opening; capture velocity and containment are verified against the hood's design and applicable standard.

A stairwell or elevator pressurization system is verified by measuring differential pressure across the door with the door closed and confirming the design pressure under fire-mode operation.

Smoke control system testing verifies fan operation, damper position, and pressure differentials per the approved design and applicable code.

Duct leakage in exhaust systems reduces capture; verify exhaust fan airflow at the fan and at the hood to identify leakage or blockage.

Room pressure differential is measured with a sensitive differential gauge (0.001 in. w.c. resolution) referenced to a stable adjacent space.

A DDC point list defines each point's type (AI, AO, DI, DO), range, units, and normal state; verification confirms each point against the design sequence of operations.

Analog inputs use 4-20 mA, 0-10 V, or resistive sensors; a 4-20 mA signal has a live zero at 4 mA, allowing broken-wire detection.

Sensor scaling: engineering value = (signal - signal_min) / (signal_max - signal_min) x (value_max - value_min) + value_min.

A control loop is verified by forcing the output and observing the controlled variable response, then confirming the loop returns to automatic control.

PID tuning is verified by observing response to a setpoint step; excessive proportional gain causes oscillation, excessive integral causes overshoot and slow settling.

Damper and valve actuators are verified for full stroke, fail-safe position on loss of power, and end-switch feedback to the DDC.

Trend logs and point-to-point checkout should be documented; a functional performance test confirms the sequence of operations under normal and failure modes.

Network protocols (BACnet, Modbus) require verification of device addresses, baud rate, and point mapping between the field device and the front end.

A test and balance report typically includes design vs. actual airflow, static pressures, fan RPM, motor amps, and final damper/valve positions.

Final readings should be recorded after the system has operated long enough to reach steady state; note the stabilization period in the report.

Instrument calibration certificates and traceability records should accompany the report to establish measurement validity.

NEBB and AABC procedural standards define report content, balancing tolerances, and instrument accuracy requirements.

ASHRAE Standard 62.1 and 90.1 govern ventilation and energy compliance; test results should reference the applicable standard and edition.

A deficiency list should separate items within the tester's scope from those requiring contractor correction, with the basis for each deficiency stated.

Report tolerances: airflow within +/-10% of design is a common balancing tolerance; confirm the project specification's stated tolerance.

As-built documentation should be reconciled with field conditions; discrepancies between drawings and installed systems are noted in the report.

A scope of work should define systems included, exclusions, tolerances, and deliverable format before fieldwork begins.

A safety plan identifies confined-space entry, lockout/tagout, fall protection, and electrical hazards; testers working near energized equipment follow NFPA 70E.

Confined-space entry requires atmospheric testing for oxygen, combustible gas, and toxics before and during entry, with an attendant present.

Lockout/tagout procedures isolate energy sources before servicing fans, pumps, or controls; verify zero energy state before work.

Ethical practice requires reporting measured data honestly, not adjusting readings to match design, and disclosing conflicts of interest.

Change orders should document scope changes, additional time, and cost impact; verbal direction is confirmed in writing.

Field data should be recorded contemporaneously and retained per the contract's record-retention requirements.

A professional should decline to certify results outside their competence or when instruments are out of calibration.