Heating, ventilation and air conditioning represent over 50% of energy expenses in many buildings, and this applies for both residential and commercial properties. Therefore, energy efficiency measures that target HVAC can achieve high energy savings.
When building owners want to improve HVAC efficiency, a common mistake is focusing only on equipment. There are many other factors that affect energy consumption, and a good design considers all of them. For example, even if a building uses high-efficiency chillers and boilers, there will be energy waste if the insulation is deficient. Thermostat settings and other usage habits also have a strong effect on energy expenses.
Heating and air conditioning systems work optimally when the following conditions are met:
- The building envelope is airtight and well insulated.
- Heating and cooling equipment is sized correctly, based on building requirements.
- The HVAC configuration is suitable for the type of building.
- Thermostat settings are kept under control.
Importance of the Building Envelope
Heating and cooling efficiency starts with the building envelope. The two most important elements are airtightness and insulation, since they reduce the workload on HVAC equipment.
There are many types of insulation, and they vary in performance and cost. The best opportunity to achieve a high R-value is the initial construction process, since insulation can be embedded in walls and other building elements. Insulation improvements are also possible in existing buildings, but they must be applied as a layer or sprayed into hollow spaces.
Airtightness is also important, since air leaks waste cooling power during summer and heating power during winter. Ideally a building should have an airtight construction from the start, but air leakage can also be detected and fixed in existing buildings.
Insulation issues and air leaks leave no visual signs, but they are detected with several methods during an energy audit. For example, areas with poor insulation and air leaks can both be visualized with a thermal imaging camera, due to the temperature differences they produce.
Specifying the Right HVAC Capacity and Layout
Once the building envelope has minimized heating and cooling loads, the next step is designing a suitable HVAC system. There are two very important elements: the equipment capacity, and the delivery method for heating and cooling. HVAC engineers emphasize using the right capacity for each building, since this ensures a high performance at an optimal cost.
Finding the right equipment capacity is important, since undersizing and oversizing both bring issues. There is a common misconception that extra capacity is a good thing, but there are many disadvantages.
- The HVAC installation becomes more expensive.
- Maintenance costs are also increased, since replacement parts are more expensive.
- Humidity control may be deficient with oversized equipment.
- The equipment will tend to run in shorter cycles due to its high capacity, which wears it down faster.
Another important aspect is using an HVAC configuration that suits the needs of each building. For example, if a building is split into areas with different usage schedules, centralized HVAC equipment is not the best option. If unused areas are heated or cooled all the time, there is a waste of energy even if the equipment has a high efficiency. High-rise buildings provide another example, since they are better suited for hydronic systems; air ducts are impractical and inefficient for vertical distances.
How Thermostat Settings Influence HVAC Efficiency
With an efficient building envelope and a well-designed HVAC system, the final element is controlling the thermostat. Consider that more energy is required to establish a higher temperature difference between indoor and outdoor air. For this reason, setting the thermostat at the lowest or highest temperature available wastes energy.
Setting back the thermostat is an effective way to reduce HVAC costs, and energy savings of up to 3% are possible for every 1°F of setback. Consider that “setback” has a different meaning depending on the season: it means setting the thermostat higher during summer, and lower during winter.
When small HVAC equipment is used, a smart thermostat can optimize the temperature setting in real time. These thermostats learn usage habits and program themselves, saving energy without requiring frequent adjustments. In larger installations with centralized equipment, a Building Automation System (BAS) can perform the same role.
HVAC costs are minimized when the building has an efficient envelope, a suitable HVAC design, and controls to optimize thermostat settings. In many buildings, HVAC systems consume more energy than all other loads combined. As a result, the energy saving opportunity is considerable.