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Thou shall not use “rule-of-thumb” or “educated guess” for the design of your HVAC system!!

This is the most critical part of an energy efficient home and deserves the most careful analysis, design, and installation. It is one of the biggest sources of headaches, and maybe heavy drinking, of builders that want to build high performance homes. I have made many calls to HVAC companies to discuss a system for our Habitat for Humanity homes. The typical first question is “how may square feet do you have”, then they proceed to tell me how may tons of air conditioning I need. Nothing is asked about air leakage, window specs and which way they face, how the house is insulated, how many people live in the home, and on and on…. There is no way one can design and install a proper system by rule of thumb! Their sizing is typically at least twice the size the home actually needs and that is your $$’s down the drain…

Systems that are too small (hardly ever the case) can’t maintain the proper temperatures in the home. Systems that are too large, require more energy when running, require larger electrical service, circulate too much air when running and short cycle. In areas with humidity, a proper system needs to run long enough to condense moisture from the air in order to achieve a comfortable environment in the house. If the system is too large, it doesn’t run long enough to control humidity (referred to as short cycling). Also, the larger the system, the more it costs!!

The proper way to design an HVAC system is to use tools that model the home then use that information to determine how much heating and cooling is needed. This is referred to as the Manual-J. Our experience is that this analysis be done by an independent 3rd party. Often this can be done by the energy rater that is essentially doing this anyway with the energy modeling that he/she performs. If not, find a company that does this type of analysis.

We had been doing Manual-J for several years, independently, but one year had the HVAC contractor do one. Interestingly enough, his numbers came out almost twice as high as previous year’s homes (nearly identically construction). Just because the computer does the calculations, you can fudge numbers and parameters to make it say what you want.. We looked at the figures and found where incorrect assumptions were made.

Once a Manual-J is performed and verified, then design the system and the ducting using Manual-S and Manual-D. This provides the information necessary to purchase the proper equipment, and make the ducting runs to each room in the home that will ensure you can get the necessary air volume in each room.

When a system is installed, there are tests that must be performed (typically by the HERS rater) to ensure the system is installed and performing to the design. The first test is typically a duct leakage test and involves pressuring the ducting system and measuring the amount of air that is needed to maintain a specified pressure. To meet IRC 2012 specifications, the leakage must be no more than 4%.

When the air handler is operational, the air flow of the system must be verified. The design flow is determined by the Manual-J. Additionally, the air flow to each room is verified and adjusted if necessary by adjusting a damper installed in the duct at the supply air plenum. Because we do not want homeowners “fiddling” with the duct registers and changing that air balance for the entire home, we either purchase registers without an adjustment or we disable it by removing the lever. Once the system is balanced, is should never be touched again.

Depending on the green building program, there may be other tests and documentation requirements for things like pressures, power draw, etc.

For bedrooms or other larger rooms that can be closed off by closing doors, we provide an alternate return air path. The underside of the door is typically too small and restricts airflow when the door is closed. We have used “through-the-wall” vents and “bottom of the door vents”. The latter is considerably easier and actually looks better than expected.. Now when you close a door, the airflow to that room is unaffected. No more stuffy room at night when the door is closed.