During summer, air conditioning is normally the highest energy expense in homes and commercial buildings. To reduce air conditioning costs, the first step is understanding the factors that influence consumption: the specific cooling needs of the building, the efficiency of AC equipment, and usage habits.
Building cooling needs depend on the size of the property, the activities carried out, and the performance of the thermal envelope.
If two properties with equal floor area are used for the same activities, the one with the best insulation and airtightness will have lower air conditioning costs.
The same applies for equally-sized properties that are used for different activities. For example, an office filled with computers produces much more heat than a classroom.
However, even a high-efficiency air conditioning system can consume more energy than necessary with deficient usage habits. If the thermostat is always set at the lowest temperature and the fan always runs at full speed, any air conditioning system will waste energy regardless of efficiency.
Using the Thermostat Correctly
To maximize air conditioning efficiency, the US Department of Energy recommends setting the thermostat at 78°F during summer. Cooling costs increase for every degree below this value, since the air conditioner must sustain a higher temperature difference between indoor and outdoor air. The increased temperature difference also causes a greater heat gain through the building envelope, which is added to the cooling load.
Smart thermostats can learn usage habits to program themselves accordingly. This way, there is no need for constant manual adjustment. It is important to note that not all programmable thermostats can be considered smart, only those with self-programming capacity. A promising application of smart thermostats is setting back the temperature when occupants are away, and returning to a comfortable temperature before they arrive. According to the US Dept. of Energy, this strategy can yield savings of around 10%.
Importance of Having the Right Air Conditioning Capacity
Air conditioning equipment is often selected empirically based on rules of thumb, but this practice is not recommended by mechanical engineering professionals. An oversized air conditioner is more expensive not only in terms of the upfront cost, but also operation and maintenance.
Oversized air conditioners tend to run in short cycles where they produce a blast of cold air, without leaving enough time for dehumidification. As a result, they create an indoor environment that feels cold and moist like a refrigerator. Such an environment can be uncomfortable and detrimental for human health, while stimulating the growth of mold and bacteria.
Improving Insulation and Airtightness to Reduce Cooling Loads
A property with deficient insulation and plenty of air leaks will have high cooling costs, even if the air conditioning system has a high nameplate efficiency. Single-pane windows also reduce the performance of the building envelope, since they allow plenty of heat through.
There are now high-efficiency windows with triple-pane glass and a low emissivity coating, but their installation can be complex in existing properties. The upgrade can be very expensive due to the cost of high-performance windows, and the labor cost of disrupting walls to remove the existing windows.
Insulation deficiencies and air leaks are more difficult to detect, since they are invisible. However, energy consultants can pinpoint their locations with a thermal imaging camera, and air leaks can also be detected with smoke-based pressurization tests.
Building envelope improvements reduce the load on air conditioning equipment, and this is reflected in electricity bills. Property owners who are considering an air conditioner upgrade can improve insulation and airtightness first, and then select a new AC unit that is both smaller and more efficient.
Author Bio: Michael Tobias PE is a visionary in the construction industry. His passion resonates as the Founding Principal of New York Engineers, an Inc 5000 fastest growing company. New York Engineers is the most innovative construction engineering firm focusing on Mechanical, Electrical and Plumbing (MEP) engineering designs in Chicago and New York. Michael has overseen the design of over 1000 construction projects in all market sectors, including LEED certified and Passive House certified projects. He leads a global team of 50 top performers.
