Chilled water plant control sequence

A chilled water plant has chillers making cold water, pumps moving it to the building’s coils and back, and cooling towers (on water-cooled plants) rejecting the heat. The control sequence orchestrates all of it: how many chillers and pumps run, at what temperatures and speeds, to match a building load that changes hour by hour.

Because the plant is often the single biggest energy consumer in a large building, small efficiency gains in the sequence translate to large dollar savings.

Staging decides when to start and stop chillers as load rises and falls. Bring chillers on too late and the building loses temperature; too early and you run machines inefficiently at low load. Good staging adds and sheds chillers at the points where overall plant efficiency is best, often using measured plant load (flow times temperature difference) rather than a single temperature.

Staging also has to respect minimum run and off times so chillers are not short-cycled, which wears them and wastes energy.

The colder the chilled water, the harder the chiller works. When the building is not at peak load, the water does not need to be as cold — so resetting the chilled-water setpoint upward saves chiller energy. The trick is that warmer water means the air-side coils and pumps must work a little harder, so the reset is a balance.

A good reset raises the chilled-water temperature as high as the most-demanding air handler will tolerate while still holding comfort — capturing chiller savings without starving the building. It is a plant-level reset strategy.

How water is pumped matters. Older plants use constant-speed primary-secondary pumping; modern efficient plants use variable-primary pumping, where variable-speed pumps slow down as the building’s control valves close and demand drops. Since pump energy falls steeply with speed, slowing pumps at part load saves significantly.

The sequence has to maintain minimum flow through the chillers (they need it to operate safely) while otherwise letting flow follow demand — a balance the controls manage continuously with VFDs.

On water-cooled plants, the condenser-water loop and cooling towers reject the chiller’s heat. Colder condenser water makes the chiller more efficient, so the sequence runs the tower fans to produce the coldest practical condenser water — down to a limit set by the outdoor wet-bulb and a balance against tower-fan energy.

Condenser-water reset, tower-fan staging, and (where used) waterside economizing all live here. In humid Tampa Bay, the high wet-bulb limits how cold the tower can get the water, which the sequence accounts for. See cooling tower controls.

The real efficiency comes from optimizing these levers together, not separately. The lowest-energy operating point balances chiller, pump, and tower energy at once — sometimes running an extra pump to allow warmer water, or an extra tower cell to lower condenser temperature, because the trade saves more than it costs.

Advanced plant-optimization sequences do this continuously. It is sophisticated supervisory logic — the kind of work we specify and, where certified integration is required, coordinate — while installing and servicing the plant equipment itself within our Class A scope.