Natalia Camblor, CEMProject EngineerSustainable Engineering Group LLC

Considerations for Building Controls

Does Nighttime Setback really save energy?

Learning Objectives

1. Recognize the operational challenges with night setback

2. Understand the energy impact of typical solutions to the operational challenges

AIA Quality Assurance

Night-time Building Setback - Definition

Widely used control strategy in which the system is cycled off during unoccupied hours, and the temperature in each zone is allowed to drift away from the occupied setpoint.

Night-time Building Setback - Definition

The Reason for Night-time Building Setback

Allowing the indoor temperature to drift saves energy by reducing the temperature difference between the ambient and the indoors.

Typically Good Setback Candidates

•K-12 Schools

•Office buildings

•Places of worship

•Courthouses

•Government

buildings

•Strip malls

•Most higher education buildings

Typically Poor Setback Candidates

•Hospitals

•Nursing Homes

•Dormitories

•Art Museums

•Prisons & Jails

•Transportation Terminals

•Public Safety Buildings

Energy Model – Building Description

• Location: Madison, WI

• Size: 60,000 ft2

• Construction Year: 1997

• Facility Type: Middle School

• Systems: Natural Gas HW VAV with DX cooling

• Hours: 7:00 am – 6:00 pm

• DDC system

Energy Model – 3-D Rendering

Correctly Programmed Setbacks Save $

8ºF Setback No Setback (Baseline)

9% Savings

Consequences of Setbacks

• Building envelope is cooled down during setback period

• Building air temperature may be recovered in most spaces within 1 or 2 hours

• Envelope and furniture temperature takes longer to recover

Air Temperature vs. Radiant Temperature

Perceived Temperature

Perceived temperature is roughly the average of the mean radiant (surface) temperature and the air temperature.

The Untold Reality of Setbacks…

• Some spaces do not reach temperature setpoint when occupants start using the space

• Most spaces have low radiant temperature causing the perceived temperature to be outside the comfort zone

• This causes COMPLAINTS!!!

The NUMBER 1 Solution…

Put Air Handling Units (AHUs) into Occupied Mode one to ten hours earlier, never mind the added ventilation…

Complaints typically go away.

Everyone is happy!

Three Hours Earlier Start Time – Breakeven Point

3 Additional Hours Occupied Mode

No Setback (Baseline)

All savings from setbacks disappear

The NUMBER 2 Solution…

When everything else fails, put AHUs into Occupied Mode 24/7

Dampers… What Dampers?

The NUMBER 2 Solution…

Complaints ALWAYS go away.

But what about the BILL???

24 Hour Occupied Mode Operation

24 Hour Occupied Mode

No Setback (Baseline)

30% Cost Increase

• Increase boiler supply water temperature

• Increase speed of boiler VFD

• Portable electric heaters

Other Observed Solutions

Recommended Solutions

• Improve warm-up sequenceUtilize DDC to close outdoor air dampers

• Reduce night setback temperatureIf there is no DDC, reduce or eliminate setback

Nighttime Setback Requirements

• Night setback can save energy IF:o Warm-up sequence does NOT ventilate (no open OA

dampers!)

o All occupied spaces are able to recover during warm-up period

o All occupants are comfortable even though the perceived temperatures may be below setpoint

In practice,

this is very difficult to accomplish

Conclusions

• Night setback is perhaps even more

complicated than economizer sequencing

• Only 3 hours extra operation in Wisconsin climate for a school wipes out ALL energy

savings

• Night setback is NOT recommended unless the building will be closely monitored by a skilled professional, as it often increases the

energy usage of the building

AIA Quality Assurance

Portland Energy Conservation, Inc is a registered provider with The

American Institute of Architects Continuing Education Systems. Credit

earned on completion of this program will be reported to CES Records

for AIA members. Certificates of Completion for non-AIA members are

available on request.

This program is registered with the AIA/CES for continuing

professional education. As such, it does not include content that may

be deemed or construed to be an approval or endorsement by the AIA

of any material of construction or any method or manner of handling,

using, distributing, or dealing in any material or product. Questions

related to specific materials, methods, and services will be addressed

at the conclusion of this presentation.

Thank you!

Natalia Camblor, CEM