12.2 Control Loops: Proportional Offset, PID Action, Reset Strategies & Standard Sequences

12.2 Control Loops: Proportional Offset, PID Action, Reset Strategies & Standard Sequences

Sub-topic 2D names two loop concepts explicitly: temperature reset and PID. This section develops both, plus the standard sequences that a mechanical PE is expected to be able to write and critique.

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1. Control Modes, From Simplest to Most Capable

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2. Proportional Control and Its Unavoidable Offset

Proportional gain and proportional band are reciprocals of each other:

$K = \frac{100\%}{PB}$

Output is then:

$\text{Output} = \text{Bias} + K \times \text{Error}$

Worked Example - Calculating Offset

A hot water reheat valve is controlled by a proportional-only loop with a 10 F proportional band and a 72 F space setpoint. The controller is calibrated so that 50% valve position occurs at zero error. Gain is 100% / 10 F = 10% per F.

Suppose the actual heating load requires only 30% valve position to hold steady state:

• 30 = 50 + 10 x (72 - T)
• -20 = 10 x (72 - T)
• 72 - T = -2, so T = 74 F

The space settles 2 F above setpoint and stays there. This is offset, and no amount of patience fixes it: the loop is at equilibrium and reports no fault. Generalized:

$\text{Offset} = \frac{\text{Output required} - \text{Output at zero error}}{K}$

Narrowing the proportional band raises the gain and shrinks the offset - but at some point the loop becomes unstable and hunts. Integral action resolves the dilemma by continuing to shift the output for as long as any error persists, driving offset to zero without requiring a high proportional gain. Integral is expressed in repeats per minute or as an integral time; too much integral produces overshoot and slow oscillation, too little leaves a sluggish approach to setpoint.

Direct vs. Reverse Acting

A direct-acting loop increases output as the measured variable rises (cooling valve on space temperature). A reverse-acting loop increases output as the measured variable falls (heating valve on space temperature). Getting this backwards produces a loop that drives to a hard stop and stays there - one of the most common commissioning findings, and easy to spot on an exam because the described behavior is always monotonic runaway rather than oscillation.

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3. Reset Strategies

Reset means moving a setpoint in response to a second variable so the plant does no more work than the load requires.

Outdoor Air Reset of Hot Water Supply Temperature

Three benefits stack:

1. Distribution losses fall roughly in proportion to the difference between pipe and ambient temperature.
2. Control valve authority improves, because a lower supply temperature means the valve must be more open to deliver the same heat, moving operation off the top of the stroke.
3. Condensing boilers actually condense. Return water must fall below the roughly 130 F flue gas dew point for the latent heat to be recovered, so reset is not an optimization for a condensing plant - it is a precondition for the efficiency you paid for.

Chilled Water and Condenser Water Reset

Raising chilled water supply temperature in mild weather reduces the chiller lift and therefore its kW/ton. Lowering condenser water temperature does the same thing from the other end. Both must be bounded: raising chilled water temperature too far compromises dehumidification, and lowering condenser water below the chiller's minimum starves the expansion device and can trip the machine.

Supply Air Temperature and Static Pressure Reset - Trim and Respond

Modern sequences do not use a fixed relationship to outdoor temperature. Instead, each terminal unit generates a request when it cannot meet its zone setpoint, and the air handler trims its setpoint in the efficient direction each cycle until it receives requests, then responds by backing off:

  every T seconds:
     if requests <= ignored_count:   setpoint = setpoint - trim_amount     (efficient direction)
     else:                           setpoint = setpoint + respond_amount * (requests - ignored)
     clamp setpoint between min and max

Applied to duct static pressure, this drives the fan to the lowest pressure that still satisfies every box. Because fan power varies with roughly the cube of flow and static pressure setpoint drives the whole curve, the savings routinely exceed those of any other single control measure. This is the core of the ASHRAE Guideline 36 high-performance sequences.

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4. Economizer Control

An airside economizer uses outdoor air as free cooling. The control question is the high-limit shutoff - when to stop.

The engineering trade is honest and appears on exams: enthalpy control is thermodynamically correct but sensor-dependent. Two drifting humidity sensors can produce worse real-world performance than a robust differential dry-bulb changeover, which is why several climate zones in ASHRAE Standard 90.1 prohibit or restrict certain high-limit types.

Integrated economizer operation - running the economizer simultaneously with mechanical cooling rather than locking out one or the other - is required by 90.1 for most equipment, because partial free cooling still displaces compressor work.

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5. Sequences Worth Memorizing

• AHU cooling sequence: in order of increasing energy cost, heating valve closed, then economizer dampers modulate, then mechanical cooling stages - with a single sequenced output and no overlap between heating and cooling.
• Deadband: a zone must have a temperature range in which neither heating nor cooling operates. Simultaneous heating and cooling in a VAV box is an energy defect that a deadband exists to prevent.
• Optimum start: the controller learns the building's warm-up and cool-down rates and starts equipment only as early as needed to reach setpoint at occupancy, rather than at a fixed hour.
• Night setback and unoccupied cycling: setpoints widen, outdoor air dampers close entirely, and equipment cycles only to hold the setback limits.
• Minimum on and off times: protect compressors from short-cycling, at the cost of some control precision.