optimizing vav system

Optimizing VAV Systems

Steven T. Taylor, PETaylor Engineering, LLC

Agenda

Three ways to improve energy performance of VAV systems

“Dual Maximum” VAV box control for reheat terminalsStatic pressure setpoint resetSupply air temperature setpoint reset

All are required by Standard 90.1-2010!

Resources

EDR Advanced VAV System Design Guide ASHRAE Journal 7/2007: Increasing Efficiency with VAV System Static Pressure ResetASHRAE Journal 5/2006: CO2-Based DCV Using 62.1-2004All are available at no charge from http://www.taylor-engineering.com/publications/articles.shtml

VAV Reheat Box

Q: Why do some buildings use 3 -5 times as much energy as others?

A: Reheat

Conventional VAV Control

Reheat Valve Position

Maximum Airflow Setpoint

Minimum Airflow Setpoint

Heating Loop Cooling Loop

Airflow Setpoint

DeadBand

(shaded area = reheat)

Conventional VAV Box Minimum Setpoint

No less than larger of:Minimum ventilation rate

Per Standard 62.1 or local codeController minimum

Typically ~10%Do NOT use VAV box manufacturer limits – too high

Limit “dumping” Probably not a real issue

Limit stratification (per 90.1-2010)No more than 20°F SAT above space temperature (~90°F)

No more than larger of (per 90.1-2010):30% of cooling maximumMinimum ventilation rate

Dual Maximum Control (typical available with “configurable” controllers)

(shaded area = reheat)

Reheat Valve Position

Heating Loop Cooling Loop

Airflow Setpoint

DeadBand

Maximum Heating Airflow Setpoint

Max Cooling Airflow Setpoint

Minimum Airflow Setpoint

Sort of Dual Maximum Control (Found in a few configurable controllers)

(shaded area = reheat)

Reheat Valve Position

Heating Loop Cooling Loop

Airflow Setpoint

DeadBand

Maximum Heating Airflow Setpoint

Max Cooling Airflow Setpoint

Minimum Airflow Setpoint

This logic does NOT comply with Standard 90.1 Dual Maximum Requirements

Supply Air Temperature Setpoint(requires discharge temp. sensor)

Maximum Heating Airflow Setpoint

Max Cooling Airflow Setpoint

Minimum Airflow Setpoint

Heating Loop Cooling Loop

Maximum Supply Air

Temperature

Airflow Setpoint

DeadBand

(shaded area = reheat)

Dual Maximum Control with 2-stage heat(typically only available with fully programmable controllers)

•Use SAT not valve directly

•Sequence airflow and temperature

VAV Box Dual Maximum Control Setpoints per Standard 90.1-2010

Minimum: No less than larger of:

Minimum ventilation rateController minimum

No more than 20% of cooling maximumHeating Maximum

No less than larger of:Minimum ventilation rateLimit stratification - ≤20°F SAT above space temperature (~90°F)

No more than 50% of cooling maximum

Dual Maximum Controls in Action

2-stage Dual Maximum Control

BenefitsLower fan energyLower reheat energyLower heating energy by not pushing zone into heatingReduces two-positioning of HW valve and overshootLimits supply air temperature to reduce stratificationProvides self-balancing of HW flow with 2-way valves

IssuesRequires discharge temperature sensor

This is nice for diagnostics anywayUsually requires programmable zone controller

Configurable controller programming should soon catch upMaintaining ventilation

Use 62MZCalc spreadsheet to verify minimum setpoints Air diffuser performance at low airflows

Not likely a comfort issue at low flow rates

Supply Static Pressure Control

Fan Energy at Varying SP Setpoints

Surge Region

Real Fan SystemsMany Hours At Low Loads

Static Pressure Reset ControlsPIER Research

~20% (type of fan)

~66% (SP reset by zone demand)

Zone Pressure Demand Options

Analog actuators Damper signal

Floating actuators Control loop outputCalculated damper positionDamper position feedbackDamper full-open end-switch Airflow below setpoint

Static Pressure Setpoint ResetControl Logic Options

Option 1: Using a PID Loop on VAV damper position to maintain one near wide open

Option 2: Using “Trim & Respond” logic tied to VAV zone “requests” based on damper position or airflow demand

Trim & Respond SP Setpoint Reset Logic

Static pressure setpoint shall be reset using trim & respond logic within the range 0.10 inches to 1.5 inches. When fan is initially started, setpoint shall be 0.5 inches. While fan is proven on, every 2 minutes, trim the setpoint by 0.04 inches if there are 2 or fewer pressure requests. If there are more than 2 pressure requests, increase the setpoint by 0.05 inches times the number of pressure requests minus 2 but no more than 0.15 inches. A pressure request is generated:

Where VAV zone damper position is known: when zone VAV damper is 95% open until it falls below 85% openWhere VAV zone damper position is unknown: when the ratio of the VAV zone’s actual supply airflow to supply airflow setpoint ratio is less than 85% until it rises to 100%.

Variations on this Sequence

Damper position >80% counts as one request, while a position >95% counts as two requests.Multiply zone requests times a zone Importance Multiplier

Allows rogue zone to be ignored and critical zones to exceed ignore value (2 in example)

Accumulate request-hours to indicate which zones may be rogues

Trim &Respond vs. PID Loop

AdvantagesEasier to tuneCan “respond” more quickly than “trim”Does not require knowledge of damper positionEasier to ignore “rogue” zones

Tuning – Unstable Control

y = 0.6709x + 0.2656R2 = 0.5971

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Actual Static Pressure

Stat

ic P

ress

ure

Setp

oint

Tuning – Stable Control

Demand Based Reset Control Trim &Respond Example

Supply Air Temperature Reset

SAT Reset is back in 90.1-2010!

Supply Air Temperature Control Optimization

Analyzed for:California climatesChilled water plant at building (not central)Variable speed fans

Probably applicable to more extreme climates as well

Humid climates should use low end of outdoor air temperature reset ranges

Impact of SAT on Energy UsageSacramento Office Building

Recommended SAT Reset Logic

Reheat Economizer SAT Logic

None None Constant SAT at design (~55ºF)

Low(e.g. DFDD,

PIU)Yes

Constant SAT at design. Allow to float up by locking out cooling until zone request indicates need for CHW

Medium to High

(e.g. VAVRH)Yes

Reset by zone cooling demand during cool weather (<55ºF to 60ºF), then ramp down to design SAT in warm weather (>60ºF to 70ºF)

Reheat System SAT Reset Logic

VAV Reheat System SAT Reset Logic

During occupied mode, the setpoint is reset from 53°F when the outdoor air temperature is 70°F and above, proportionally up to T-Max when the outdoor air temperature is 60°F and below. T-Max shall be reset using trim & respond logic within the range 55°F to 65°F. When fan is initially started, setpoint shall be 65°F. While fan is proven on, every 2 minutes, increase the setpoint by 0.3°F if there 2 or fewer reset requests. If there are more than 2 reset requests, reduce the setpoint by 0.4°F times the number of pressure requests minus 2 but no more than 1.0°F. A reset request is generated when VAV zone cooling loop output is 95% until it falls below 85%

Lower to 65/55 if reheat reduced using “dual maximum” logic or central plant always on

Performance – Oversized Zones

Performance – Rogue Zones

Rogue Zone

Rogue Zones

CausesUndersized zone/unexpected loadsThermostat setting too lowDuct design problem – high pressure drop fitting or duct section

Handling Rogue Zones

Exclude them from reset sequenceMay result in temperature problems in these zonesNot advised for boss’ office

Limit thermostat setpoint adjustmentsFix duct restrictions/sizing issuesAdd auxiliary cooling to augment VAV

Summary

VAV box controlDual Maximum logic significantly improves efficiency of VAV ReheatUse 2-stage logic for best performance

Static pressure setpoint resetUse Trim & Respond SP setpoint reset logic based on damper position or airflow demandTuning for stability is criticalEvaluate trend data to identify rogue zones

Supply air temperature setpoint resetUse Trim & Respond SP setpoint reset logic based on zone cooling loop outputTuning for stability is criticalEvaluate trend data to identify rogue zones

All of the above required by current Standard 90.1!

Questions