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COLUMN ENGINEER’S NOTEBOOK

Steven T. Taylor

Steven T. Taylor, P.E., is a principal of Taylor Engineering in Alameda, Calif. He is a mem-ber of SSPC 90.1 and chair of TC 4.3, Ventilation Requirements and Infiltration.

BY STEVEN T. TAYLOR, P.E., FELLOW ASHRAE

Resetting Setpoints Using Trim & Respond LogicFor many years, ASHRAE/IES Standard 90.11 has required that variable air volume (VAV) duct static pressure setpoint be reset based on damper position, and that vari-able speed pump differential pressure setpoint* be reset by valve position for systems with direct digital controls (DDC).

Specifically:

6.5.3.2.3 VAV Setpoint Reset For systems with direct digital

control of individual zone boxes reporting to the central control

panel, static pressure setpoint shall be reset based on the zone

requiring the most pressure; i.e., the setpoint is reset lower until

one zone damper is nearly wide open.

6.5.4.1 Hydronic Variable Flow Systems Where dif-

ferential pressure control is used to comply with this section

and DDC is used to control valves, the setpoint shall be reset

downward based on valve positions until one valve is nearly

wide open.

California’s Title 24 Energy Standards have similar

requirements.

The 2013 version of Standard 90.1 adds new require-

ments for DDC systems including these mandatory

capabilities:

6.4.3.11 DDC Requirements The DDC system shall be capa-

ble of all of the following as required to provide the control logic

required in Section 6.5:

a. Monitor zone and system demand for fan pressure, pump

pressure, heating, and cooling;

b. Transferring zone and system demand information from

zones to air-distribution system controllers and from air-distribu-

tion systems to heating and cooling plants controllers;

c. Automatically detect those zones and systems that may be

excessively driving the reset logic and generate an alarm or other

indication to the system operator; and

d. Readily allow operator removal of zone(s) from the reset

algorithm.

One of the key requirements is Item C, which allows sys-

tem operators to detect what we call “rogue zones,” zones

that are driving the reset algorithm to setpoint extremes

and thus defeating their energy savings. Once identified,

the zones can either be modified (e.g., increasing HVAC

capacity or reducing loads) or, if noncritical, they can be

ignored using the feature required by Item D.

Design engineers and control system programmers

have implemented these performance requirements

in disparate ways, some more successfully than oth-

ers. To improve performance and eliminate “reinvent-

ing the wheel” on every project, ASHRAE sponsored

Research Project 1455-RP2 to develop best-of-class

sequences for VAV systems; these sequences are being

further developed into proposed ASHRAE Guideline 36P,

High Performance Sequences of Operation for HVAC Systems,

expected to be open for public review in early 2016. This

month’s column discusses the 1455-RP/Guideline 36P

sequences for meeting the Standard 90.1 reset and DDC

requirements.

Setpoint Reset LogicFor simplicity, the discussion that follows is based on

resetting static pressure setpoint for VAV systems based

on zone damper position. But the same basic logic can

be used to reset variable speed pump pressure setpoint

based on valve position, reset supply air temperature * Recent Addendum ak to Standard 90.1-2013 allows either pump differential pressure setpoint reset or chilled water supply temperature reset, or both, at the designer’s option. On systems with high head and constant speed chillers, the former tends to minimize energy use; systems with variable speed chillers tend to be more efficient using the latter.

This article was published in ASHRAE Journal, November 20145. Copyright 2015 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.

N O V E M B E R 2 0 1 5 a s h r a e . o r g A S H R A E J O U R N A L 5 3

setpoint based on zone cooling or heating control loop

output, reset hot water supply temperature setpoint

based on valve position, etc.

The basic logic behind setpoint reset is straightfor-

ward: simply adjust the pressure setpoint until one

VAV damper is nearly wide open. But achieving this in

a stable manner is quite difficult because damper posi-

tion and pressure are highly interactive. Approaches for

implementing the reset logic include:

• PID Loop on VAV damper position. This approach

uses a standard proportional-integral-derivative (PID)

control loop to adjust pressure setpoint to maintain the

most-open VAV damper position at 90% open. (90% is

used rather than 100% since a PID loop requires that

there be error from setpoint on both sides of the set-

point.)

• “Trim & Respond” logic based on zone pressure

requests. With this approach, pressure setpoint is

trimmed slowly and regularly until a zone indicates

through a pressure “request” that more pressure is re-

quired, in which case the controller responds by bump-

ing the setpoint up a small amount.

Trim & Respond (T&R) logic has been shown to be

more effective3,4 than PID logic and was selected for

1455-RP sequences for the following reasons:

• It is easier to tune. The T&R logic includes several

fairly intuitive tuning parameters (see detailed sequenc-

es in “Detailed 1455-RP T&R Logic” on Page 54), while

the PID logic includes only nonintuitive proportional,

integral, and derivative gains.

• It can “respond” more quickly than “trim,” i.e.,

the setpoint “trim” rate can be adjusted to be much

slower than the “respond” rate. A very slow trim rate

improves stability, but a comparatively fast respond

rate ensures that zone demands for increased airflow

can be quickly addressed. PID loops typically have

only one speed: if the loop is slow when decreasing

the setpoint, it is slow when increasing it. Hence, if

the loop is made slow enough to avoid instability, it may

be so slow that zones are starved of airflow when zone

demand rises.

• It reduces control system network traffic. The

requests for setpoint changes are generated at the zone

level and sent up to the controller controlling the fan

only as needed. PID logic requires constant polling of

damper position of each zone served, resulting in sig-

nificantly higher network traffic.

• It is easier to ignore “rogue” zones. With T&R logic,

it is simple to ignore one or more zones (see detailed

sequences in the sidebar on Page 54), diminishing the

impact of rogue zones.

• It allows zones to be treated differently based on their

importance. With T&R logic, critical zones can be weight-

ed more heavily than noncritical zones in how the reset

logic responds. This is not easily done with PID logic.

1455-RP Trim & Respond LogicT&R logic resets setpoints for a system based on the

demand of zones it serves. Demand is measured using

“requests” generated by the zone controller based on

damper position. When the damper nears full open,

one or more requests is generated; when the damper

closes to a given position, no request is generated.

Non-zero requests are sent up to the air handler con-

troller. This controller constantly reduces the pres-

sure setpoint at a fixed rate if there are no requests

from zones.

When a sufficient number of zone requests are pres-

ent, the pressure setpoint is increased in response. The

importance of each zone’s requests can be adjusted to

ensure that critical zones are always satisfied. A running

total of the requests generated by each zone is retained

to identify the zones that are driving the reset logic.

The sidebar on Page 54 shows the detailed 1455-RP

T&R logic (almost verbatim). The sidebar on Page 56

shows an example of T&R implementation. Again, the

focus is on VAV system static pressure setpoint reset, but the

same logic can be used for resetting other central system

setpoints.

Figure 1 is an example of the T&R logic based on the

example in the “T&R Implementation” sidebar.

The static pressure setpoint slowly decreases toward

the minimum setpoint (thus saving energy), but

responds rapidly to increasing demand from the VAV

boxes. This cyclic pattern is characteristic of Trim &

Respond logic (as well as PID logic). The period of the

oscillations, which should be about 30 to 60 minutes (a

balance of saving energy vs. instability), can be tuned by

adjusting T&R parameters.

GraphicsTo help meet the DDC functionality requirements of

Standard 90.1, two key DDC system graphics should be

specified:

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Detailed 1455-RP T&R Logic1. Trim & Respond (T&R) Setpoint Reset Logic

a. A “Request” is a call to reset a static pressure or temperature setpoint, generated by downstream zones or air handling systems. These Requests are sent upstream to the plant or system that serves the zone or air handler that generated the Request. Here is an example of a VAV static pressure request:

1) VAV Zone Static Pressure Reset Requests:a) If the measured airflow is less than 50% of

setpoint while setpoint is greater than zero for 1 minute, send 3 Requests.

b) Else if the measured airflow is less than 70% of setpoint while setpoint is greater than zero for 1 minute, send 2 Requests.

c) Else if the Damper Loop is greater than 95%, send 1 Request until the Damper Loop is less than 85%.

d) Else if the Damper Loop is less than 95%, send 0 Requests.

2) For each downstream zone or system, and for each type of setpoint reset Request listed for the zone/system, provide the following software points:

Importance Multiplier is used to scale the number of requests the zone/system is generating. A value of zero causes the requests from that zone or system to be ignored. A value greater than one can be used to effectively increase the number of requests from the zone/system based on the critical nature of the spaces served.

a) Importance Multiplier (default = 1).

Request-Hours accumulates the integral of requests (prior to adjustment of Importance Multiplier) to help identify zones/systems that are driving the reset logic. Rogue zone identification is particularly critical in this context, since a single rogue zone can keep the Trim & Response loop at maximum, and prevent it from saving any energy.

b) Request-Hours. Every x minutes (default 5 minutes), add x/60 times the current number of Requests to this request-hours accumulator point. The request-hours point is reset to zero upon a global command from the system/plant serving the zone/sys-tem – this global point simultaneously resets the request-hours point for all zones/systems served by this system/plant.

c) Cumulative%-Request-Hours. This is the zone/system Request Hours divided by the zone/system run-hours (the hours in any Mode other than Unoccupied Mode) since the last reset, expressed as a percentage.

d) A Level 4 alarm is generated if the zone Im-portance Multiplier is greater than zero, the zone/system Cumulative%-Request-Hours

exceeds 70%, and the total number of zone/system run-hours exceeds 40.

Level 4 alarms are noncritical alarms indicating a fault that affects energy efficiency.

3) See zone and air handling system control se-quences for logic to generate Requests.

4) Multiply the number of Requests determined from zone/system logic times the Importance Multiplier and send to the system/plant that serves the zone/system. See system/plant logic to see how Requests are used in Trim & Respond logic.

b. For each upstream system or plant setpoint being controlled by a T&R loop, define the following variables. All variables below shall be adjustable from a reset graphic accessible from a hyperlink on the associated system/plant graphic. Initial values are defined in system/plant sequenc-es below. Values for trim, respond, time step, etc., shall be tuned to provide stable control.

Variable DefinitionDevice Associated device (e.g., fan, pump)SP0 Initial setpointSPmin Minimum setpointSPmax Maximum setpointTd Delay timerT Time step I Number of ignored Requests R Number of Requests from zones/

systemsSPtrim Trim amountSPres Respond amount (must be opposite in

sign to SPtrim)SPres-max Maximum response per time interval

(must be same sign as SPres)

Note that it is recommended that | SPres | > | SPtrim | so that the reset logic does not get stuck at a value, as can happen if SPres and SPtrim are equal in absolute value.

The number of Ignored Request (I) should be set to zero for criti-cal applications.

c. Trim & Respond logic shall reset setpoint within the range SPmin to SPmax. When the associated device is off, the setpoint shall be SP0. The reset logic shall be active while the associated device is proven on, starting Td after initial device start command. When active, every time step T, trim the setpoint by SPtrim. If there are more than I Requests, respond by changing the setpoint by SPres × (R – I), (i.e., the number of Requests minus the number of Ignored Re-quests), but no more than SPres-max. In other words, every time step T:

Change setpoint by SPtrim.

If R > I, also change setpoint by (R – I) × SPres but no larger than SPres-max.

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A S H R A E J O U R N A L a s h r a e . o r g N O V E M B E R 2 0 1 55 6

1. VAV Box Summary Graphic. This is a table show-

ing current values of all of the key zone control points,

including T&R parameters: current number of Requests,

Cumulative-%-Request-Hours, and Importance Multi-

plier (see “Detailed 1455-RP T&R Logic” for definitions).

See Figure 2 for example. The zones that are driving the

reset can be identified by the Cumulative-%-Request-

Hours column. Zones with Cumulative-%-Request-

Hours exceeding 70% are considered “rogue” zones

and generate an alarm. From this graphic, the operator

can easily adjust the Importance Multiplier to zero, for

instance, to ignore a rogue zone.

2. Trim & Respond Reset Graphic. Next to the display

of the setpoint that is being reset, a link should be

included to a page showing all of the T&R parameters,

plus the current number of requests and current

T&R ImplementationThe following is an example of a sequence that uses Trim

& Respond to control the static pressure setpoint of a VAV AHU serving multiple downstream zones.

This sequence defines the T&R variables as follows:

Variable DefinitionDevice Supply Fan Status

SP0 0.5

SPmin 0.15

SPmax 1.50

Td 5

T 2

I 2

SPtrim – 0.04

SPres 0.06

SPres-max 0.15

Description of general operation: Starting 5 minutes after the fan status indicates the supply fan is on, the sequence will slowly reduce the AHU’s static pressure setpoint by 0.04 in. w.g. every 2 minutes. As static pressure drops, downstream VAV box dampers will open further for a given load. When the combination of reduced static pressure and changes in load drives more than two VAV boxes fully open, the system will respond by increasing static pressure setpoint by 0.06 in. w.g. for every Request, but no more than a maximum of 0.15 in. w.g. regardless of the number of Requests. The setpoint will continue to increase every 2 minutes until all but 2 VAV boxes (for ignore value of 2) are satisfied (damper position < 85%). Subsequently, the setpoint will continue to decrease by 0.04 in. w.g. every 2 minutes.

Example • System starts at 12 p.m. Initial setpoint is 0.5 in. w.g.

At 12:05 p.m. (Td after start time), the reset begins.

• At 12:07 p.m. (i.e., 1 × T after the reset logic starts), there is one Request (i.e., R = 1). Setpoint is reduced by SPtrim, which is 0.04 in w.g.; since R – I < 0, there is no re-sponse. Net result: Setpoint is 0.46 in. w.g.

• At 12:09 p.m., there are two Requests (i.e., R = 2): Set-point is reduced by 0.04 in. w.g.; since R – I = 0, there is no response. Net result: Setpoint is 0.42 in. w.g.

• At 12:11 p.m., there are three Requests (i.e., R = 3): Setpoint is reduced by 0.04 in. w.g.; since R – I = 1, response increases Setpoint by 0.06 in. w.g. (i.e., 1 × SPres). Net result: Setpoint is 0.44 in. w.g. (i.e., +0.02 in. w.g. net change).

• At 12:13 p.m., there are four Requests (i.e., R = 4): Setpoint is reduced by 0.04 in. w.g.; since R – I = 2, response increases Setpoint by 0.12 in. w.g. (i.e., 2 × SPres). Net result: Setpoint is 0.52 in. w.g. (i.e., +0.08 in. w.g. net change).

• At 12:15 p.m., there are six Requests (i.e., R = 6): Setpoint is reduced by 0.04 in. w.g.; since R – I = 4, but SPres max = 0.15 in. w.g., response increases Setpoint by the maximum of 0.15 in. w.g. (i.e., not 4 × SPres = 0.24 in. w.g.). Net result: Setpoint is 0.63 in. w.g. (i.e., +0.11 in. w.g. net change).

• At 12:17 p.m., there are three Requests (i.e., R = 3): Setpoint is reduced by 0.04 in. w.g.; since R – I = 1, response increases Setpoint by 0.06 in. w.g. (i.e., 1 × SPres). Net result: Setpoint is 0.65 in. w.g.

• At 12:19 p.m., there are zero Requests (i.e., R = 0): Set-point is reduced by 0.04 in. w.g.; since R – I < 0, there is no response. Net result: Setpoint is 0.61 in. w.g.

Figure 1 is an example of the above logic, continued for a period of an hour.

setpoint. Figure 3 is an example. Trim & Respond points

should be adjustable from this graphic. The page could

also include a graph of the setpoint trend for the last 24

hours.

FIGURE 1 Example trim and respond trend data.

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ConclusionsStandard 90.1 has required setpoint reset for systems

with DDC for many years. Trim & Respond logic has

been found to be the most effective approach (so far) to

demand-based reset and was included in 1455-RP and

proposed Guideline 36P. Their implementation also

includes a means to automatically identify rogue zones

(Cumulative-%-Request-Hours > 70%) and easily elimi-

nate them from the logic (Importance Multiplier = 0),

FIGURE 3 Example Trim & Respond Block.FIGURE 2 Example VAV box summary.

as required by the 2013 version of Standard 90.1.

References1. ANSI/ASHRAE/IES Standard 90.1-2013, Energy Conservation in New

Buildings Except Low-Rise Residential Buildings.

2. Hydeman, M., B. Eubanks. 2015. “Advanced Control Sequences for HVAC Systems, Phase I.” ASHRAE Research Project 1455-RP, Final Report.

3. Taylor, S. 2007. “Increasing efficiency with VAV system VAV sys-tem static pressure static pressure setpoint reset.” ASHRAE Journal (6).

4. Seidl, R. 2008. “Using Demand Based Reset Strategies.” National Conference on Building Commissioning.

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