Because forced-air heating and cooling systems are assembled on site from a great many parts, there are many ways for installers to make mistakes. Researchers have repeatedly shown that a high percentage of residential forced-air systems have major problems, including duct systems that are poorly designed, poorly located, and leaky. Other problems include incorrect refrigerant charge and too much or too little airflow over the cooling coil.
The classic solution to these problems — in addition to the obvious step of better duct system design — is to insist on a more rigorous commissioning process. “Commissioning” refers to 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. Although commissioning is rare for residential HVAC systems, it is a routine step for commercial and institutional buildings.
Energy Star makes commissioning mandatory
As I noted in an earlier blog, The Energy Star Homes Program Raises the Bar with Version 3, the Energy Star program has decided to include requirements for the commissioning of forced-air HVAC systems, beginning January 1, 2012. The following commissioning steps will be mandated:
- Duct systems will need to 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 will need to be measured to verify that it is within 15% of the design air flow.
- The air flow at each supply register will need to 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.
For years, energy experts have been advising HVAC installers to implement all of these time-consuming and tricky commissioning steps; yet few installers actually perform them — or even have the specialized equipment needed to do the required testing.
Until recently, I accepted the logic behind this package of commissioning recommendations, and advised builders to insist that these steps were followed. However, a recent conversation with Marc Rosenbaum, an engineer at South Mountain Company in Massachusetts, has led me to rethink the wisdom of residential HVAC commissioning.
“Really, commissioning shouldn’t exist”
Instead of forcing HVAC installers need to do a better job of commissioning their equipment, it might make more sense to conclude that conventional forced-air systems are so problematic that they shouldn’t be used at all. Perhaps we should be choosing different equipment — equipment that doesn’t need a complicated commissioning protocol. This light bulb was illuminated when Rosenbaum told me, “Really, commissioning shouldn’t exist.”
For an increasing number of residential designers, the “different equipment” is a ductless minisplit air-source heat pump from Asia. (Minisplit units are available in both ducted and ductless versions; in general, the ductless systems have fewer opportunities for installer error than the ducted systems.) According to Rosenbaum, the ductless minisplit systems he has specified in recent years have worked flawlessly.
They’re packaged systems
When I asked Rosenbaum a general question about residential equipment commissioning, he said, “I can tell you what we haven’t been doing: we haven’t been commissioning our buildings with ductless minsiplits. If the building has a system with a lot of zones and a central controller, there may be some training from the rep — but no commissioning. These are self-contained products, so you are buying the control system with the unit. It is all packaged.”
After all, we don’t commission refrigerators. The main reasons that we need to commission forced-air heating systems are:
- The systems are site-assembled by tradespeople of varying skills;
- The systems sometimes employ controls that aren’t integrated with the heating or cooling units until programmed by the installer;
- Unlike plumbing systems, the materials used for residential duct systems do not come with leakproof joints; and
- There is a long-standing tradition — at least in the southern regions of the U.S. — to route our duct systems through unconditioned spaces.
A ductless minisplit has no ductwork and the controls are installed at the factory. That makes installation a relative snap — and neatly cuts the Gordian Knot of post-installation commissioning.
Last week’s blog: “How to Install Cellulose Insulation.”