Controls
This domain tests control sequences and the logic behind them: thermostats, economizers, demand-controlled ventilation, static pressure reset and the point types used in building automation systems.
Core rules
- A thermostat has a deadband; proportional and PID loops trade offset against stability. Integral action removes offset but can wind up if not limited.
- Economizer changeover can be dry-bulb or enthalpy based. Enthalpy compares total heat, so humid outdoor air can be rejected even when it is cooler than return air.
- Minimum outdoor damper position must account for demand-controlled ventilation and building pressurization, not just code minimum airflow.
- Typical BAS points: analog input, analog output, binary input, binary output. Common protocols include BACnet and Modbus.
- Safety devices that must never be defeated or jumpered include freeze stats, high limits and low-pressure cutouts.
Worked example
Return air is 75F at 50 percent RH, about 28.3 Btu/lb enthalpy. Outdoor air is 68F at 90 percent RH, about 30.5 Btu/lb. A dry-bulb economizer sees 68F below 75F and opens to full outdoor air. An enthalpy economizer sees the outdoor air is more energetic than return air and keeps the damper at minimum, with mechanical cooling available. The enthalpy control saves energy here; the dry-bulb control imports moisture and increases latent load.
Traps that cost marks
- Setting minimum damper position without coordinating with DCV sequences.
- Allowing integral windup in a PID loop, causing overshoot on startup.
- Placing sensors where solar gain or stratification biases the reading.
- Omitting a freeze stat on a unit with a waterside coil or an economizer.
- Assuming a written sequence is functioning without performing a functional test.