HVAC 403A Complete Practice Exam 2026

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Which statement best describes 'lead/lag sequencing' in a multi-chiller plant?

Running all chillers at the same load regardless of demand

Sequencing based on the oldest chiller first and never shut off

Always using the largest chiller first

Starting and staging chillers in a preferred order to match demand and energy efficiency, with a defined lead chiller and lagging backups

Lead/lag sequencing is about running chillers in a planned order so the plant meets demand efficiently and reliably. In normal operation, a primary chiller handles the bulk of the load, acting as the lead. As cooling needs rise or the lead reaches its optimal efficiency or capacity limit, additional chillers come online in a predefined sequence (lagging units). This approach minimizes energy use by matching the least number of chillers to the required load, reduces wear from unnecessary starts and stops, and provides redundancy for maintenance or unexpected outages.

Avoiding the other patterns helps explain why this is preferred. Running all chillers at the same load wastes energy and isn’t needed for typical demand. Never shutting off the oldest chiller ignores efficiency opportunities and can shorten equipment life. Always using the largest chiller first ignores efficiency at partial loads and can lead to overkill for small cooling demands. The lead/lag method specifically balances demand with energy efficiency and reliability by staging chillers in a defined order.