General

The issue of durability may not immediately come to mind with respect to a home’s heating, ventilating, and air conditioning (HVAC) system, but a poorly designed and installed system can create humidity problems that damage the structure as well as home furnishings. Such a system can also create unbalanced pressure zones in a building, which essentially push or pull air (and the water vapor it carries) through gaps in the building envelope that may result in condensation. Moreover, an improperly designed system can shorten the life of the equipment due to frequent short cycling, restricted air flow that causes the blower motor to work harder and less efficiently, or excessive dirt and dust accumulation.

In recent years, the International Residential Code (IRC) and the International Energy Conservation Code (IECC) have incorporated more provisions that pertain to best HVAC practices. Yet, these are not comprehensive enough when it comes to the broad durability implications of HVAC—nor are they consistently adopted, implemented, and enforced. So while some of the recommendations in this section reiterate important basic code minimum requirements, others are beyond-code practices that will help to assure efficient HVAC operation as well as durability of the system and the home itself.

Although recommendations are called out individually, efficient and durability-enhancing HVAC must be implemented as a system. All components of the HVAC system work together as a system so that the equipment, ductwork, registers, and controls must be designed and installed in an integrated approach.

For instance, a new home will often have a properly sized central heating/cooling system (good), along with a duct system without any sizing or design (bad) and an undersized, poorly installed exhaust fan to “ventilate” the master bathroom (common). The result: a phone call to the builder from an unhappy homeowner complaining about high humidity and window condensation in the master bedroom. Besides being a comfort problem, over a longer term, this high humidity and condensation can lead to mold growth, degradation of drywall, and similar durability issues.

So we cannot design a good HVAC system using an a la carte approach. Efficient, durability-enhancing HVAC as a system is the goal. And this “system” goes beyond heating, cooling, and ventilation as HVAC is so interconnected with other building systems (see Durability Web graphic Figure 1–1). Foremost among the connections are HVAC—building envelope interactions, and the implications flow both ways. The envelope’s thermal and air sealing characteristics dictate the HVAC size. At the same time, the HVAC system can drive air/moisture movement into building cavities and cause envelope damage. While these are just two examples, the key message is that enhanced durability of the envelope involves looking at HVAC interactions, and vice versa.