Air conditioning its normally the largest electrical load in homes and commercial buildings, and the associated energy expenses are especially high during summer.
Energy efficiency measures for air conditioning equipment offer an attractive return on investment, and they are frequently suggested by energy consultants for this reason.
Two main factors influence the performance of an air conditioning system: the nameplate efficiency of the equipment itself, and how effectively the building envelope keeps heat outside. Even the most efficient air conditioner in the market will waste energy if used in a room with poor insulation and plenty of air leaks.
Before proceeding with an air conditioning upgrade, a professional energy audit is recommended to detect building envelope issues. If insulation is improved and air leaks are sealed, the new equipment can have both a higher efficiency and a reduced workload.
Improving the Building Envelope to Save on Air Conditioning
The building envelope is critical for energy efficiency, being the barrier that separates indoor and outdoor air. An efficient building envelope yields energy savings all year long, minimizing heat gain during summer and heat loss during winter. This reduces both air conditioning and space heating costs.
The thermal envelope of a building can be improved with adequate insulation, air sealing, and energy efficient windows. New constructions provide an excellent chance to achieve a high level of insulation, by using construction materials such as insulated concrete forms, which become part of the structure. Insulation options are more limited in existing buildings, since modifying construction elements is expensive and disruptive. However, insulation can be installed as a layer on existing walls and roofs, or sprayed as a foam into cavities.
Poorly insulated spots and air leaks are invisible, which makes them very difficult to detect. However, energy consultants can pinpoint their locations with thermal imaging cameras, and can then use energy modeling to estimate the savings from a building envelope upgrade.
A window upgrade also contributes to air conditioning efficiency. Double-pane windows reduce heat transfer by over 50% when replacing single-pane units, and triple-pane windows can reduce it by over 70%. The performance is even better if windows have a low emissivity coating.
Finding the Right Air Conditioning System for a Building
Air conditioning systems are characterized by their variety. Conventional window-type air conditioners are easy to install and affordable, but they are also very inefficient. Ductless mini-split units offer a much higher performance, and the most efficient units consume over 80% less energy than window-type AC units of equivalent capacity.
Packaged rooftop units and chillers are common in larger installations. A packaged unit cools the air directly, while a chiller circulates cold water through hydronic piping and air handlers to deliver space cooling. Chillers are an excellent air conditioning option for high-rise constructions, since cold water can be easily pumped to upper floors from a centralized chiller plant.
Variable refrigerant flow (VRF) systems have emerged as an efficient alternative. Since they use refrigerant lines instead of hydronic piping, they allow a much more compact installation than chillers. VRF systems also offer a reversible operation, and can be used for space heating during winter.
HVAC engineers can suggest the optimal configuration for each building, based on a detailed assessment of the workload and operating conditions. Even a high-efficiency air conditioning unit will suffer from reduced performance if used in the wrong application.
Understanding the Efficiency Ratings of Air Conditioning Equipment
Ductless mini-split units use an efficiency metric called the Seasonal Energy Efficiency Ratio (SEER), which indicates the BTUs of cooling delivered for each watt-hour of electricity consumed. The concept is similar to the gas mileage value of a car, where a higher number results in a lower running cost. The first generation of mini-split units had a SEER of around 10, and there are now units that reach SEER 30 and above.
Larger equipment uses the Integrated Energy Efficiency Ratio (IEER), which follows a similar principle. The difference is that the IEER considers full-load and part-load conditions, which represent the typical workload of large air conditioning systems.
Depending on their size, heat pumps can use both efficiency metrics. Mini-split heat pumps use the SEER in cooling mode, but they have a separate metric called the Heating Seasonal Performance Factor (HSPF) for heating mode. Larger heat pumps like those in VRF systems use the IEER, just like chillers and packaged rooftop units.
An air conditioning equipment upgrade can yield significant energy savings, but better results are possible if the building envelope is optimized first. Instead of approaching air conditioning efficiency as an equipment upgrade project, energy consultants recommend an assessment of the overall building performance.
Author Michael Tobias, PE, LEED AP, CEM.
Michael Tobias is the founder and principal of Chicago Engineers, an Inc 5000 Fastest Growing Company in America. He leads a team of 30+ mechanical, electrical, plumbing, and fire protection engineers from the company headquarters in New York City; and has led over 1,000 projects in Chicago, New York, New Jersey, Pennsylvania, Connecticut, Florida, Maryland and California, as well as Singapore and Malaysia.