Efficient HVAC equipment can drastically reduce the electricity and gas bills of a building. However, the best performance is only achieved if thermostats are set correctly. Even the most efficient boilers and chillers in the market can underperform with a poorly configured thermostat.

A common practice that wastes plenty of energy is setting the thermostat at extreme values - using the lowest temperature in the scale during summer, and the highest temperature for winter. HVAC engineers do not recommend this practice, since even the most efficient equipment performs poorly with extreme temperature settings.

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Thermostat Settings and Energy Efficiency

Sustaining a temperature difference between a building interior and exterior requires energy, and the energy demand becomes higher as the temperature difference increases. For example, keeping a room 10°F colder than the outdoor air uses less energy than keeping it 15°F colder, and the same logic applies for space heating during winter.

Extreme thermostat settings not only increase the load on HVAC systems, they also reduce the overall efficiency of a building. Consider that heat transfer increases when a temperature difference is higher, and this has two negative effects:

• Summer heat gain is higher when the building interior is kept colder.
• Winter heat loss is higher when the building interior is kept warmer.

In a few words, inadequate thermostat settings increase the workload on HVAC systems while reducing overall efficiency. Consider that heating and cooling are among the highest energy expenses in a building, and incorrect use of HVAC equipment can increase them notably.

The US Department of Energy recommends a thermostat setting of 68°F during winter and 78°F during summer. When there are no occupants in the area, these values can be set back by 7-10°F, considering that “set back” has a different meaning for each mode of operation:

• In summer, the thermostat should be set 7-10°F higher for zero occupancy conditions.
• In winter, it should be set 7-10°F lower for zero occupancy.

The average US household can expect energy savings of around 10% when following the recommendations above. With the average household consumption of $900/year reported by the ENERGY STAR program, this translates into $90/year in savings. If a smart thermostat can be purchased for $250, the payback period is less than three years.

In New York City, the business case for smart thermostats improves with the Con Edison incentive program. The rebates vary depending on the client’s location and project conditions, but in some cases they are high enough to cover the full cost of a smart thermostat.

Difference Between Programmable and Smart Thermostats

The terms “programmable” and “smart” are often used without distinction when referring to thermostats, but there are important differences. Both thermostat types can control HVAC equipment based on schedules, but smart thermostats can program themselves based on usage habits, while programmable thermostats require an initial configuration.

Compatibility is a very important aspect when purchasing programmable or smart thermostats.

• Compatibility issues with space cooling systems are rare, since most air conditioners are based on an electrically-powered refrigeration cycle.
• On the other hand, some space heating systems can be challenging to integrate with smart thermostats. Many models are designed for gas-fired heating systems, and do not integrate well with resistance heaters and heat pumps.
• Radiant heating and steam heating systems can also be difficult to integrate with thermostats, due to their slow thermal response.

If you are considering a smart thermostat, HVAC engineers strongly recommend that you look for a product with the ENERGY STAR label. This guarantees high performance, and eligibility for the rebate programs available at your location.

Final Recommendations

Smart thermostats are very useful in residential and light commercial applications. However, larger properties require a building automation system (BAS) to control the temperature settings of HVAC equipment. The complexity and scale of large commercial and industrial installations are beyond the capabilities of residential smart thermostats.

Professional advice is strongly recommended before proceeding with any HVAC project, and this includes control systems for existing installations. When HVAC controls are poorly matched with the equipment or configured incorrectly, they can actually reduce energy efficiency.