Nothing will kill the efficiency of a system faster than a poor installation. A good mantra to remember is that “You get what you inspect, not what you expect.” This was noted before, but it bears repeating here. There is no way for even a true professional to know if the airflow, total external static pressure or refrigerant charge of an HVAC system is right except by testing and measuring key parameters. If anyone tells you differently, run for the door with your hand on your wallet.
Houses are systems; think of them as being like cars. It is easy to ruin the performance of even a beautifully engineered car by making just a few errors in the fuel/air mix, timing, transmission operation or plug firing sequence. Well, a house is like that. Even if designed well, errors made in the field can easily turn a 19-SEER air conditioner into a 6 SEER or an R-21 wall into one that delivers R-10 so that your house doesn’t deliver the comfort or efficiency you expected.
These errors are not made on purpose. Crews don’t get up in the morning and say, “I feel like messing up someone’s house today, just for grins.” They are proud of what they do on the job, like most of us. But there is a shortage of skilled and well-trained people in every aspect of home construction today, and contractors across the country are finding it more and more difficult to hire knowledgeable, experienced people. That is why we should require independent third-party inspections on everything.
It is the responsibility of the HVAC contractor to provide a design layout for the duct system (according to the Manual D mentioned earlier). Their decisions here include defining whether each register will be ducted independently as a “home run” or if they will use trunk ducts. Then they have to decide which branch runs to make off which main trunk lines, where to place supply and return registers and what types of connectors to use between different duct runs. In many cases, those connectors are job built, so they must decide how to design them for good airflow.
Poor duct layout is a result of a building design that fails to take the space needs of the HVAC and its ducts into account and/or poorly trained installation contractors. It’s not unusual to see duct systems that look like giant dreadlocks, with big ducts twisted and compressed through insufficient openings in floor trusses and holes cut out of structural beams (really!). It’s also commonplace to see ducts run across attics and then have to make 90° or 180° turns to connect to the ceiling register, because the contractor didn’t turn the register around for a straight-in shot. Poorly located chases take duct runs far off their desired path to get air where we need it, and undersized plenums create unnecessary turbulence that results in poor air distribution. These practices significantly reduce the volume and velocity of the air that will make it to its destination. These field installation errors mean that even though the duct was sized according to the Manual D software, as installed it is actually performing at only a fraction of its capacity. Also, if ducts run through areas outside of the home’s thermal envelope (like in unconditioned attics), they will lose some efficiency through air leakage as well as heat gain or loss from the duct’s surface.
The design provided by your architect should provide clear unobstructed duct runs with minimal changes in direction or compressions through structural members such as trusses. Like stepping on a garden hose, these compressions impede airflow, thus reducing efficient operation and comfort. The typical order of construction is for the house design to be completed and then a few months later for the HVAC contractor to walk on to the partially completed house site and figure out what might be possible. This is just plain stupid. The HVAC contractor can just as easily size and design the system based on 80 percent complete house plans and then provide the locations and sizes of the units, registers and ducts to the architect. The architect can then alter the plans to make allowances for the HVAC system components. The truss company can then build the floor and ceiling trusses with engineered spaces left open in them sized to accommodate the system and its ducts.
As was discussed in Building & Materials Category , placing the air handler/furnace close to the center of the house means that the supply ducts to each room or register are shorter, which also lowers the initial cost of the system because fewer materials are required to install it. It also means that the air-conditioned or heated air will have less distance to travel, so less temperature loss (or gain) is experienced through the delivery. This results in colder air being supplied to the rooms in summer and warmer air in winter. These efficiencies result in savings through lower installation and operating costs. The resulting cost savings on the initial investment should be used to offset the cost of high-performance equipment improvements.
Most of all, do not accept poorly sealed equipment, ductwork, and connections. Duct tape is not, in our opinion, an acceptable sealant. The only means of sealing duct connections for the life of the building is with mastic, and lots of it. It is not uncommon to lose 20-30 percent of conditioned air through leaks in poorly sealed ductwork. This means that you are not only losing that much efficiency in the system, but also losing that much conditioned air, meaning your return air is trying to find that much air from outside sources (that you do not want) to make up the airflow volume.
It is also necessary to seal ceiling, floor or wall boots to the structure. So, the supply boots and return boxes must be caulked to the sheetrock and floor. And, finally, check the leakage rate of the equipment itself, especially the air handler. These cabinets should be well-sealed. It is also critical that a system static pressure test is performed to assure the system is performing to the manufacturer’s specifications. In new home construction, the 2012 International Energy Conservation Code (IECC) and International Residential Code (IRC) require that ducts and furnaces be tested and certified as tight and fireplaces be sealed. Homes built to this code and later versions should have only sealed combustion appliances to ensure safe operation. On existing home remodels, it is important to have an independent third party (this can be an HVAC contractor who has been approved to audit system performance by your utility rebate program) perform a combustion backdraft safety test (often called the combustion appliance zone or CAZ test). This assures that there is not a negative pressure situation, where conditions in the house might backdraft air from combustion gas exhaust piping for water heaters or furnaces. This test should be performed with all of the bath and kitchen fan exhausts turned on. Also, make sure that any indoor fireplaces have tight-fitting glass doors so that smoke and combustion gases from fireplace operations are not pulled back into the living space by negative pressure when exhaust vent appliances are operating. Airflow testing on indoor fireplaces with chimneys, but not enclosed with sealed glass doors, show they can exhaust 600-800 cubic feet per minute of conditioned (furnace heated air) out of the living space when a fire is burning.
Performing a duct leakage test should confirm that the system is sealed up enough for total leakage to be less than four CFM per 100 square feet of conditioned floor area (less than 3 CFM/100 square feet if the air handler has not yet been installed). The 2009 and 2012 IECC codes require that all new HVAC duct systems be tested using a duct leakage testing fan and certified to leak less than the code allows. The duct leakage, CFM and total external static pressure tests should all be performed before the drywall is installed. This allows for easy access to leak sites and airflow balancing dampers to adjust for comfortable airflows to each room before everything is hidden forever. Be sure that correcting problems found by these tests is a part of your contract with your HVAC contractor. It is wise to have an independent third party perform all HVAC tests. In most locales, a Residential Energy Services Network (RESNET)- or Building Performance Institute (BPI)-certified home performance technician is available to provide these services. While the total external static pressure test and the measurement of supply and return airflows in CFMs are not yet a code-mandated requirement, these tests are strongly advised.
Other performance testing is also necessary to assure you are getting a good installation. Measure the individual airflows to each room to determine that the CFM volume (cubic feet per minute) is within ± ten percent of the design volume. Remember, the Manual J software determines the exact volume of air needed to provide comfort in each individual room. This is accomplished by delivering the right amount of airflow (based on room volume) to achieve the set point on the thermostat at the same time as other rooms being serviced by the system. So the kitchen airflow is based on appliance heat loads, occupant heat loads and window heat loads, etc., while each bedroom load is based on number of occupants, size, etc. Also confirm that you have adequate pressure relief in each room by testing pressure differential between these spaces and adjacent areas to assure that these mechanisms are functioning to return the full amount of supplied CFM.
Your HVAC contractor should perform the manufacturer’s recommended start-up procedures to make certain the equipment is going to perform properly. This includes: verifying the refrigerant charge by super-heat and/or sub-cooling; setting the burner to fire at nameplate input; and ensuring the air handler and fan speed settings meet the manufacturer’s instructions, the total airflow is within + ten percent of the Manual J design flow and the total external static pressure does not exceed equipment capability at rated airflow. Following these will result in more efficient delivering of the air to the room, improving comfort and saving money.
