Energy-Efficient HVAC
What to Look for When You’re Inspecting

For home buyers, energy efficiency is an important factor when searching for a new house. It’s not easy to visually inspect a home to determine if it is energy-efficient without doing some additional testing.
We can look at the previous utility bills, but energy usage may not match from one occupant to the next. For example, what if a family of five moves in after a single person moves out? The energy consumption patterns will likely be considerably different from one to the next.
There are also some behavioral patterns that can cause energy consumption to differ between occupants, such as the amount of time occupants spend at home, how energy conscious the occupants are, and how the occupants set and adjust the thermostat. The HVAC system is usually the largest energy-consuming appliance in the home. Space heating and cooling makes up approximately half of the energy consumed in the typical American household, making energy-efficient HVAC a high priority for homeowners and renters.
The House as a System
The homes we live in behave similarly to the human body: Everything is connected. If one item is changed, it can affect another item somewhere else. This is where the “House As A System” ideology comes into play.
Within our homes we have insulation in our walls, glazing in our windows and doors, the HVAC system, one or more forms of ventilation, heat generated by occupants, appliances and lighting, and air infiltration and exfiltration occurring through the building envelope. All of these items, among others, contribute to the energy efficiency of the home. Each component must work together with the rest as a system to allow the home to operate at its most optimum efficiency.

The relationship between the HVAC system and the building envelope is one function of the home that is often overlooked. This outer skin of the building is what defines inside from outside and is where both the air barrier and thermal barrier (insulation) are located. This creates the container that houses the air conditioned by the HVAC system. If the building envelope is leaky or has poorly installed insulation, the HVAC system will have to work harder to keep the occupants comfortable, thus using more energy.
HVAC Equipment Sizing
Each house generates its own heating and cooling load. This load is the amount of energy that must be offset by the HVAC system to keep the home at a desirable temperature and humidity level. Mechanical contractors and HVAC system designers calculate the load by doing a Manual J load calculation. This is calculated with software approved by the Air Conditioning Contractors of America (ACCA). The contractor inputs specific information about the home—including its location, orientation, windows, doors, insulation, air infiltration, duct location, leakage, R-values, shading from surrounding objects, porches and overhangs, and any internal heat gains from occupants and appliances.

One of the most common energy consumption issues we see in existing homes is HVAC equipment that is oversized. This typically happens when a contractor neglects to do a load calculation or does it incorrectly.
Oversized equipment causes short cycling, where the indoor temperature reaches the thermostat setpoint too fast. This causes the system to start and stop more frequently, which uses more energy and shortens the lifespan of the equipment. Short cycling also prevents the air conditioning equipment from removing moisture from the air. In more humid climates during warmer months, this leads to comfort issues. When this happens the air feels warmer, meaning occupants will set the thermostat to a lower temperature, thus using more energy.
One of the most overlooked components in the HVAC load calculation process is air infiltration through the building envelope. For the calculation to be as accurate as possible, a blower door test should be conducted to measure air infiltration.
When a technician runs a blower door test, a pressure of 50 Pascals (Pa) is induced inside the home using a calibrated fan. A high-precision manometer calculates the leakage at that pressure as the blower door fan is running. It is typically reported as a flow rate of cubic feet per minute at 50 Pa (CFM50) or air changes per hour at 50 Pa (ACH 50). If a blower door test is not conducted, the contractor or designer guesses what the infiltration will be, which can often cause the load calculation to be inaccurate. This inaccuracy can be anywhere between 20 to 70%.
Duct Design and Installation
In many cases, the same software used to calculate the load of a home can also be used to design the ductwork. This duct design should be based on ACCA’s Manual D.
A duct system that is poorly designed or installed can lead to an array of issues that can increase energy consumption. Inefficient duct systems restrict the airflow and increase the static pressure inside the system. A duct system’s static pressure works similarly to blood pressure: We don’t want it too high, or it can cause problems.
High static pressure can place additional strain on the blower as it works to move conditioned air to each part of the home. Some examples of system designs that restrict and reduce airflow lack a trunkline on the supply side of the air handler. Instead, some systems have supply ducts branching from a short plenum. This can reduce the velocity of the air immediately after it leaves the air handler. The use of junction boxes to split duct runs also restricts air flow. Instead, we like to see a wye used to maintain laminar flow.
The quality of the duct system installation is equally important to having an efficient design. Ideally, we want duct systems to be as airtight as possible. Leaky ducts can lead to high energy bills, especially when all or some of the ductwork is outside of the conditioned space. These areas include vented attics, basements, and crawl spaces. The best method for sealing ductwork is with a paintable mastic or with automated duct sealing devices like Aeroseal. Tapes are not the best option, as their sealing capabilities are limited and tend to fail as temperature and humidity levels fluctuate over time.

Duct leakage can also induce a positive or negative pressure on the house when the duct system is outside of the conditioned space. If the return side of the system is leaky, it will pressurize the house. If the supply side is leaky, it will depressurize the house, which amplifies leakage through cracks and holes in the building envelope.
Duct tightness can be tested with a duct tester kit. This kit includes a calibrated fan and high-precision manometer that connects to the duct system. During the test, a pressure of 25 Pa is induced inside the duct system and the leakage is measured at that pressure. Leakage is recorded as a percentage of the floor area served by the HVAC system.
When flex ducts are used, it is best to keep them as straight as possible without any dips when suspended from trusses, rafters, or floor joints. U-turns should also be avoided to maintain airflow. Ducts shouldn’t be crimped or compressed when installed in confined spaces.
Balanced Air Flow and Room Pressures
The load calculation software can also determine the amount of air each room in a home should be supplied with. We check this in the field by using a flow hood and high precision manometer. The flow hood reads how much CFM of air is being supplied to the room or returned to the air handler. We want this to be as close as possible to the flows calculated by the software.

We use the high-precision manometer to see how positive or negative a room is with respect to the main body of the house. Not only does this help diagnose comfort issues, but it also helps determine energy losses. For example, if we have a home with a bedroom with a significant difference in pressure when compared to the rest of the house, it can push conditioned air outside through leaks in the building envelope if it is positive, or pull outside air into the room if it is negative.
HVAC Equipment Types
After ACCA Manual J and D are conducted to determine the load and design the duct system, Manual S is used by the HVAC technician to select the correct system to install.
There are several types of air conditioners, heat pumps, and furnaces used to condition the air in residential buildings. For air conditioners and heat pumps in cooling mode, the Seasonal Energy Efficiency Ratio (SEER) is used to determine how efficient the equipment is. For heat pumps in heating mode, the Heating Seasonal Performance Factor (HSPF) is used. For furnaces, the Annualized Fuel Utilization Efficiency (AFUE) factor is used.
Less efficient furnaces are atmospherically vented (natural draft) while high-efficiency systems are power-vented. For each type of rating, the higher these numbers are, the more efficient the equipment will be.
Air conditioners and heat pumps can be single-stage, two-stage, or multi-stage. This means the blower fan runs at one speed, two speeds, or multiple speeds. Most systems we see in the field are single-stage. One common issue we see with these systems is fan speed. There are several speeds the HVAC technician can select. The proper speed will allow the system to operate at a healthy static pressure and provide proper flow to each room. Testing the total system airflow of the blower fan along with the static pressure can help determine the proper fan speed. In many cases, the fan speed is left at the factory setting, which is often incorrect for the house the system is serving, especially if the system is oversized.

Two-stage and multi-stage systems are more efficient since they can automatically adjust the amount of energy they consume as the load changes throughout the year. These are not as common as single-stage systems because they are more expensive. There are also inverter heat pumps, which are the most efficient. They can run at any speed to reach and maintain a desired temperature.
As the load changes throughout the year, these systems can adjust at a higher resolution as needed. Inverter systems can also be ductless, which uses even less energy since there is no load from ductwork. However, some inverter systems tend to struggle to remove moisture from the air. In more humid climates, these systems often work best when paired with a whole-house dehumidifier for optimum comfort.

There are many factors to consider when determining the energy efficiency of a heating and cooling system. Just because energy-efficient equipment is installed doesn’t always mean the occupant will receive a low utility bill. There must be a proper duct design and installation, and the home’s building envelope must also do its part to keep the system running efficiently. Fortunately, there are methods available to test the HVAC system efficiency with a few diagnostic tools, and these testing processes have become much faster and easier as technology has advanced over the years.
The opinions expressed in this article are those of the author only and do not necessarily reflect the opinions or views of ASHI. The information contained in the article is general and readers should always independently verify for accuracy, completeness and reliability.
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