HVAC and Home Performance are inextricably tied at the hip. Whether you look at it through a building or an HVAC lens the four most important elements of High-Performance HVAC™ are safety, health, comfort, and energy efficiency.

Any time you make changes to one side of the coin, those changes have a direct effect on the other side. Think of the home as the load side of the equation, and the HVAC system as the Btu removal or addition side.

While this may sound simplistic, it’s far from it. A home is not just one big room, and the HVAC distribution system is not just one big duct. The exposure and usage characteristics of each room vary based on a number of factors, and the air distribution system must be designed to condition each room accordingly.

Then you must consider that the individual areas of a home allow air to move between them and the outside at different rates. The HVAC system’s supply and return air systems also need to compensate for the varying loads while keeping even building pressures.

MAD Air

Put it all together and you can have almost infinite combinations of scenarios in the same structure. Variables include envelope tightness, exposures, communicating areas, windows, duct locations, airflows, register types, and more.

The end result can be quite chaotic. A few decades ago John Tooley, one of the pioneers of building science, coined this phenomena, “MAD Air” (Mechanical Air Distribution And Interactive Relationships).

Fortunately, when you get control of the variables that have the most impact, everything typically calms down and falls into place.

While not perfect, it is very possible to create a well-balanced indoor environment with an HVAC system that performs according to intended design and reasonable operating equipment capacities. Many IAQ (Indoor Air Quality) issues often resolve themselves as well.

Building science teaches how numerous things affect how air moves around and through a house. This includes wind, stack effect, duct leakage, interior door closures, clothes dryers and exhaust fans, duct leakage, air exchangers, and so forth.

HVAC system performance can be affected by all those variables plus equipment installation, undersized ducts, register and grill types (and placement), air distribution design and balancing, combustion issues, and improper charging – to name a few.

Connecting The Dots

When you connect the dots between all those variables and how the building affects the HVAC system (and vice-versa), you start to “see it” as a complete system, and begin to understand how each side of the coin impacts the other.

It’s important to keep in mind that when you try to improve the performance of either side of the “system” to not turn it into a time consuming science experiment aiming for a perfect home or a perfect HVAC system.

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