A new home won’t perform as designed until all of the equipment has been commissioned by someone who understands the ‘house as a system’ approach
Building a new home usually requires work by several subcontractors, including electricians, plumbers, and HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. installers. At the end of the job, someone — usually the general contractor — has to verify that all of the specified work has been completed.
Has the water heater been installed? Check.
Air conditioner? Check.
Ventilation system? Check.
At this point, if everything looks good (including the drywall and kitchen cabinets, of course), some GCs might call it a day and say, “The punch list is complete.” But the job isn’t really finished until the house has been commissioned.
What is commissioning?
Commissioning is the process of testing and adjusting installed equipment to be sure that it performs in accordance with the manufacturer’s specs and the designer’s intent. Commissioning also requires someone to verify that all of the building’s equipment and components interact well, so that one piece of equipment doesn’t cause unintended consequences.
One article on commissioning describes the process this way: “A good commissioning process will first test individual components for defects and performance, then test each system, and finally look at system interactions and the performance of the house as a whole under all anticipated operating conditions.”
Here’s a simpler definition: commissioning is all about looking for things that aren’t right and fixing them. The aim is for the house to perform as intended.
Some writers use the phrase “commissioning, testing, and balancing” to describe this process, but they are being unnecessarily wordy. The word “commissioning” covers all three acts.
What’s the problem?
Unfortunately, most new homes are never commissioned — because the need for commissioning is not intuitive. After all, once all of the equipment is hooked up and plugged in, it should work — right?
And if something didn’t work, the homeowners would notice the problem and give the builder a call — right? It’s not as if a problem would go unnoticed for months or years — right?
I wish. In many new homes, the duct system leaks; the airflow from the forced-air registers is less than specified; the ventilation system is blowing too much or too little air; the air conditioner’s refrigerant charge is too low; and the heat pump’s backup “strip heat” comes on unnecessarily.
In other homes, exhaust ducts can be entirely blocked by a piece of cardboard intended to protect equipment during shipping, or an attic duct can be completely disconnected from the register boot.
Do the homeowners notice these types of problems? Maybe — or maybe not.
Don’t forget the building envelope
Even builders who have heard about commissioning often forget that, just like a duct system, a building’s thermal envelope needs to be commissioned. How do you commission a thermal envelope? You inspect the insulation; you conduct a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. to check for air leakage; and you seal all of the leaks you can find.
In 2002, four researchers from Lawrence Berkeley National Laboratory (Craig Wray, Iain Walker, Jeff Siegel, and Max Sherman) published a paper called “Practical Diagnostics for Evaluating Residential Commissioning Metrics.” In that paper, the authors discussed some major problems with the residential construction process: “For at least 20 years the building industry has recognized the substantial impact of envelope airtightness on thermal loads, energy use, comfort, and indoor air quality. However, Walker et al. found 50% variances in airtightness for houses with the same design and construction crews, within the same subdivision. A substantial reason for these problems is that few houses are now built or retrofitted using formal design procedures, most are field assembled from a large number of components, and there is no consistent process to identify related energy and non-energy problems or to correct them.”
In addition to the problems listed by the LBNL researchers — the lack of consistent design procedures and the fact that residential systems are field assembled from a variety of components — there are a few other problems they didn’t mention:
- Trade contractors who assemble and install residential equipment are often poorly trained and poorly supervised.
- The materials used to assemble residential duct systems (unlike the materials used for plumbing systems) don’t come with leakproof joints.
- There is a longstanding tradition in some areas of the U.S. to route ductwork through unconditioned spaces.
- Many HVAC contractors are unfamiliar with residential ventilation systems.
- HVAC systems sometimes employ controls that aren’t integrated with installed equipment until programmed by the contractor or homeowner.
- Even contractors who understand their trade may not have been trained in “house as a system” thinking.
- Many building code requirements are unenforced. According to Glenn Hourahan, a vice president for the Air Conditioning Contractors of America, “Jobs aren’t being inspected to be sure they are done to code, because code inspection officers see their job as ensuring health and safety, not preventing poor installations of AC equipment. They are just looking to see that no one will be electrocuted.”
Develop a checklist
What items need to be checked as part of the commissioning process? Not all experts agree, but most lists include the following items:
The building envelope. Visible insulation should be inspected to make sure the insulation is thick enough and that there are no insulation voids. The home’s air leakage rate should be measured with a blower door. If necessary, leaks should be sealed, using the “blower-door-directed air sealing” approach.
The ventilation system. Required verification steps will vary depending on the type of ventilation system that has been installed. In all cases, however, it’s necessary to measure airflow (for example, with a device like a flow hood) to determine the ventilation rate. If the ventilation system has controls — for example, a timer for an exhaust-only system, or a FanCycler control for a supply-only system — then control settings have to be verified. If a central-fan-integrated supply ventilation system isn’t properly adjusted, excess air flow through the fresh air duct can result in low return-air temperatures, a possible problem for condensing furnaces. (For more information on commissioning ventilation systems, see Commissioning Our Heat-Recovery Ventilator.)
Controls. In addition to verifying any ventilation controls, other HVAC controls need to be checked. These may include thermostat setback settings and (in some homes) solar thermal system controls or outdoor temperature reset controls for boilers.
The water heater. Commissioning a water heater may be simple or complicated. If the home has a heat-pump water heaterAn appliance that uses an air-source heat pump to heat domestic hot water. Most heat-pump water heaters include an insulated tank equipped with an electric resistance element to provide backup heat whenever hot water demand exceeds the capacity of the heat pump. Since heat-pump water heaters extract heat from the air, they lower the temperature and humidity of the room in which they are installed. , it might be important to verify (a) that the room where the water heater is located meets minimum volume requirements; (b) that the appliance controls are at the appropriate settings (hybrid mode, heat-pump-only mode, or electric-resistance-only mode); (c) that the aquastat is set to the right temperature; and (d) that the condensate drain has been hooked up. Compared to these steps, commissioning an electric-resistance water heater will be simpler, while commissioning a solar hot water system will be much more complicated.
Forced-air ductwork. Commissioning forced air ductwork requires:
- a visual inspection to look for disconnected joints and to verify that joints are properly sealed;
- a visual inspection of the duct insulation;
- a Duct BlasterCalibrated air-flow measurement system developed to test the airtightness of forced-air duct systems. All outlets for the duct system, except for the one attached to the duct blaster, are sealed off and the system is either pressurized or depressurized; the work needed by the fan to maintain a given pressure difference provides a measure of duct leakage. test for leakage;
- sealing of any leaks revealed by the Duct Blaster test;
- measuring the airflow at each register and grille to verify that the system meets design specifications; and
- verification that there is a return-air pathway (via a return-air duct, a jumper duct, or a transfer grille) from every room in the house that has a supply register.
Combustion appliances. It’s important to perform combustion safety checks on all combustion appliances like furnaces and water heaters. All venting systems need to be inspected. If there are any atmospherically vented appliances, all of the exhaust fans in the house need to be turned on for a worst-case depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. test.
The air conditioner. If the house a central air conditioning, it’s important to verify the equipment’s refrigerant charge and the airflow rate across the cooling coil. According to the Quality Installation (QI) specification developed by the Air Conditioning Contractors of America (ACCA), airflow across the cooling coil should be within the range recommended by the equipment manufacturer and within 15% of the airflow specified by the system design.
Energy Star Version 3 requires some commissioning steps
An increasing number of “above code” programs, including the Energy Star program, recognize the need for residential commissioning.
Version 3 of the Energy Star Homes program requires:
- Duct systems must be tested at 25 pascals to verify that the ducts leak no more than 6 cfm per 100 square feet of the home’s conditioned floor area, and that duct leakage to the outdoors is no more than 4 cfm per 100 square feet of the home’s conditioned floor area.
- The air flow of the supply duct system must be measured to verify that it is within 15% of the design air flow.
- The air flow at each supply register must be measured to verify that it is within 20% of the design air flow.
- The pressure difference between each bedroom and the common areas of the house (measured with the forced-air system operating and bedroom doors closed) must be measured to verify that it is no more than 3 pascals.
- If the home has central air conditioning, then the installer will need to verify that the subcooling deviation is no greater than ±3°F and the superheat deviation is no greater than ±5°F.
Ground-source heat pumps are tricky to commission
By now, alert readers will notice a pattern: the systems that need careful commissioning tend to be systems that are field-assembled by local contractors using parts purchased from a variety of manufacturers. No one ever needs to commission a refrigerator, because it arrives from the factory as a packaged system.
There’s a corollary to the above observation: the residential systems with the greatest opportunities for commissioning problems are ground-source heat pumps (GSHPs). These systems often require five or six different contractors to have a finger in the pie: the engineer who designed the system, the well driller or backhoe operator, the GSHP contractor, the plumber, the duct installer, and the manufacturer’s rep. Considering the opportunities for screw-ups, it’s remarkable that any of these systems work at all.
Vernon McKown is a builder who owns Ideal Homes in Norman, Oklahoma. In 2005, McKown built a zero-energy home that included a ground-source heat pump. One of the consultants involved in the project was Mark Sevier, who at the time was working for the Building Science Corporation. When I interviewed Sevier in 2005 about the project, he told me, “A ground-source heat pump installation is not a straightforward deal unless the installer has a lot of experience with different types of installations. We had to commission the ground loops, to make sure there was no debris in the loops. We had debris in our case. I would recommend to anyone installing such a system that they get someone who knows how to commission the system. The overall energy transfer of the system is heavily dependent on what the ground loop does. You have to select a water temperature from the manufacturer’s chart. But how am I supposed to know what the water temperature will be in this hole in the wintertime? As it turned out, the loops hadn’t been properly flushed and pressurized. This was a split system. The supplier didn’t have an air handler to match their outdoor unit. A significant problem that we experienced was matching the indoor unit with the outdoor unit. The coil size is substantially different for ground-source systems than air-source systems.”
I also talked to Vernon McKown about the same project. He told me, “When Mark Sevier was here, he tested the temperature of the fluid in the lines coming in and going out, and we were way off of what the manufacturer’s specs targeted. So we asked why. They started digging into that. Checking these temperatures needs to be part of the commissioning process. I heard the contractor say, ‘We are changing the coil and maybe the fan.’ And I’m like, ‘What the hell is going on?’ Hell, they were replacing the whole son of a bitch. Here is the sad part about it: We build these test houses, and we’re looking at emerging technology. When we were building the models on paper, it looked like the big winner was going to be the ground-source heat pump. The models looked great. But now the performance piece is not working, and that is disappointing.”
The end of commissioning?
The moral of story told by Sevier and McKown is clear: if you want to reduce commissioning headaches, specify factory-packaged systems instead of field-assembled systems.
As energy consultant Marc Rosenbaum — a big fan of ductless minisplits — said at this year's BuildingEnergy conference in Boston, “One of the most frustrating technologies I have ever had to work with are ground-source heat pumps.”
In a GBAGreenBuildingAdvisor.com blog published this year, Rosenbaum wrote, "One thing I really like about the minisplits is how they are packaged systems from a single supplier, and are highly engineered as a system and therefore very reliable.”
These days, smart designers are specifying HVAC systems that don't require much commissioning. As Rosenbaum points out, “Really, commissioning shouldn’t exist.”
Martin Holladay’s previous blog: “Deep-Energy Retrofits Are Often Misguided.”
- Building America
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