BAS-304 Visual Aid Library
Figure-use rule
Each figure is an orientation and reasoning aid, not a project drawing. Learners must pair it with the current sequence, point list, wiring/piping documents, manufacturer instructions, and as-found evidence.
Normal-rule anchor
A physically impossible energy balance is evidence that measurement, units, assumptions, or system boundary is wrong.
Case overlay
An AHU appears to remove more heat from the air than the chilled-water side can absorb.
Calculation/evidence callout: Use approved airside and waterside heat equations with units, density/fluid assumptions, and reasonableness checks.
Normal-rule anchor
One static reading cannot locate a restriction; compare pressure before and after components under known airflow and fan conditions.
Case overlay
Duct static is low while fan speed and motor current are high and filter differential pressure is elevated.
Calculation/evidence callout: Calculate pressure differences, percent change, fan affinity estimates, and airflow only from approved pickup/K data.
Normal-rule anchor
Mixed-air state must lie within the physical mixture of outdoor and return air when measurements and mass balance are valid.
Case overlay
Mixed-air temperature plots colder than both outdoor and return air without active cooling upstream.
Calculation/evidence callout: Calculate mixed-air fraction, humidity ratio, dew point, enthalpy comparison, and sensor/mass-balance residual.
Normal-rule anchor
Airside and control evidence can justify refrigeration escalation but cannot establish charge or internal refrigerant condition by assumption.
Case overlay
Cooling is commanded, compressor status is on, airflow is low, and the evaporator begins to ice.
Calculation/evidence callout: Calculate airside sensible/total capacity from supplied state points and compare stage/run-time expectations.
Normal-rule anchor
A BAS enable is not proof of safe ignition, combustion, heat transfer, or airflow.
Case overlay
The furnace command is on, the fan proof is intermittent, and high-limit trips recur.
Calculation/evidence callout: Calculate expected temperature rise or heat transfer from supplied airflow/input data only within stated assumptions.
Normal-rule anchor
Position is not flow; valve authority, close-off, pressure drop, linkage, system pressure, and coil load determine response.
Case overlay
A valve modulates from 20% to 80% with almost no change in flow until near full open.
Calculation/evidence callout: Calculate supplied pressure-drop ratios, valve authority concept, coil ΔT/capacity, and damper pressure effects; full sizing remains BAS-400.
Normal-rule anchor
Pump speed and proof do not establish delivered flow; pressure reference, system path, valve positions, and measurement matter.
Case overlay
Remote coils starve while plant differential pressure appears normal near the pumps.
Calculation/evidence callout: Use supplied pump/flow relationships, DP changes, terminal demand, and Q=flow×ΔT×factor reasonableness.
Normal-rule anchor
Low ΔT is a symptom family with many possible mechanisms, not a single valve or plant fault.
Case overlay
Chilled-water flow is high, return temperature is low, and many valves are fully open.
Calculation/evidence callout: Calculate system load, expected versus actual ΔT, flow required for load, and contribution of bypass or ineffective coils.
Normal-rule anchor
The loudest complaint location may be a downstream symptom of an upstream system or shared-source limitation.
Case overlay
One hybrid-VVT zone repeatedly wins the vote and drives the RTU away from the needs of the other zones.
Calculation/evidence callout: Compare zone demand, vote/priority, airflow, discharge conditions, runtime, and diversity across the same interval.
Normal-rule anchor
A defensible HVAC conclusion states what the evidence proves, what it only suggests, what requires another trade, and what remains unknown.
Case overlay
The BAS correction improves temperature temporarily, but the mechanical restriction and test override remain.
Calculation/evidence callout: Reconcile before/after energy and state-point evidence under comparable conditions and close all temporary states.