optimizing vav system
TRANSCRIPT
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