Introduction
It takes a substantial amount of energy to heat or cool air. And to maintain a constant indoor temperature, the HVAC must replace heat that leaks out of the building through various loss areas. Using appropriate temperature set points seasonally and temporally maximizes occupant comfort while minimizing heating and cooling energy costs. This article will help you calculate potential savings from employing night setbacks on your HVAC system.

The Physics of Heat
The basic heat loss equation is Q = UA (T_inside - T_outside), where Q is total energy, U is the overall heat transfer coefficient, A is building area, and T_inside - T_outside is the temperature difference between inside and outside. Since building area is fixed, either the overall heat transfer coefficient or the temperature difference must be improved (lowered) to decrease energy usage.

Most energy saving measures focus on lowering the overall heat transfer coefficient, U, or reducing the thermal leakiness of the building. Weatherizing doors, adding insulation, and covering windows all lower the value of U and produce varying savings. However, all of these techniques to lower U require an investment and maintenance.

Using seasonally appropriate temperature set points minimizes the temperature difference while maintaining comfort. During the heating season, a temperature set point of 68°F will optimize comfort and energy costs. As a rule of thumb, each degree over 68°F yields a 1% increase in heating energy cost because you increase the temperature difference. In the cooling season, use a temperature set point of 72°F to maintain a comfortable working environment while reducing energy usage; each degree cooler will increase cooling energy usage by 2%.

Temperature setbacks further reduce energy usage by decreasing the temperature difference between inside and outside during times when the building is unoccupied. Night setbacks allow the temperature to be set to normally uncomfortably cool or warm levels during heating and cooling seasons, respectively, without compromising occupant comfort. A 10°F setback is generally recommended (55°F in heating seasons, 82°F in cooling seasons). With the given rules of thumb, a 10°F setback will reduce annual heating and cooling energy costs by 10-15%, and the only equipment needed is a programmable thermostat.

Some critics argue that setback does not reduce energy usage because the heating or cooling system must run higher to return the building to set point conditions before occupants arrive. It is important to realize that most HVAC systems cycle between fully on or fully off. Therefore, during morning warm-up, the system simply cycles on for a longer period of time. When setback periods last more than six hours, this longer warm-up cycle requires less energy than would be consumed by the system cycling to maintain temperature during unoccupied times. Hence, night and weekend setbacks, lasting 12 or more hours per night for commercial buildings, greatly reduce energy consumption. Occasionally, strip heating is necessary to supplement the system during warm-up and will lower available savings. Therefore, setbacks should be carefully planned to allow plenty of time for an unassisted warm-up.

Setback Savings
Night setback makes dollars and cents by lowering energy use, and the savings can be significant. As Figure 1 demonstrates, a 10°F winter night setback saves more than $0.04 per square foot per year. In a 100,000-ft² building using a natural gas boiler, this setback results in approximately $4,250 of savings each heating season!

To calculate your potential savings, you may use Figure 1 to produce an estimate, or you may use the process below to find a more accurate depiction of potential savings. It is helpful to gather some basic site information before completing the form. First, visit www.noaa.gov and find climate information for your area. You will need average monthly temperatures, annual heating degree days (HDD), and annual cooling degree days (CDD). Find the average winter temperature (T_avg,win) by averaging the temperature from January, February, March, April, October, November, and December. Find the average summer temperature (T_avg,sum) by averaging the temperature from May, June, July, August, and September.

Then, use the last 12 months of energy bills to determine your building's overall heat transfer coefficient, U. To find U, create a Howdy graph by plotting the average monthly temperature versus the sum of all energy bills (in mmBtu) for that month. The linear coefficient of a trendline plotted through these points is your building's U value. Finally, determine your heating and cooling systems' average operating efficiency, η.

Now, apply the following to determine your savings:

Winter setback savings = [(U x Area x HDD x Unoccupied hours) / (η x 1000000)] x ( 1 - [(T_{unoccupied,win} - T_avg,win) / (T_occupied,win - T_avg,win)] x {use 293 if electric, 10 if natural gas} x {your energy rate, $/kWh if electric, $/therm if natural gas}

Summer setback savings: [(U x Area x CDD x Unoccupied hours) / (η x 1000000)] x ( 1 - [(T_avg,sum - T_{unoccupied,sum}) / (T_occupied,sum - T_avg,sum)] x {use 293 if electric, 10 if natural gas} x {your energy rate, $/kWh if electric, $/therm if natural gas}

Proper Utilization
Set points and appropriate insulation still only provide so much comfort and energy savings. Regular HVAC system maintenance is core to truly maximize both comfort and savings. With regular maintenance, the system operates at peak efficiency and reduces temperature gradients inside the building. Thwarting hot spots and cold spots within the building through regular maintenance raises comfort levels and prevents the HVAC system from working overtime to heat the cold spots or cool the hot spots.

Each year, have an HVAC professional clean the exchanger coils, charge the refrigerant as needed, and find and repair leaky ducts. With proper maintenance and appropriate set points, HVAC energy consumption is minimized and occupant comfort is maximized.