Figure #12: Continuous Building Pressurization Control

ASHRAE Standard 62-1999, Ventilation for Acceptable Indoor Air Quality only peripherally address the need to maintain humidity control. In Section 5.10 & 11 of Standard 62, habitable spaces are required to have between 30% and 60% relative humidity. Section 5.12 & 13 require occupied spaces, ducts, and plenums not to exceed 70% relative humidity. Nothing is said regarding air and moisture infiltration related to mold generation.²⁶ It is left to the designer's imagination to protect the owner.

CONVENTIONAL CONTROL STRATEGIES
HVAC control strategies are strongly influenced by the initial acquisition cost of equipment. The pressure to cut costs on HVAC systems has helped to amplify the IAQ problem in many buildings. Several methods are used on the majority of designs as pressure control strategies, either directly or indirectly. They often result in negative pressure buildings and are detailed below.

Control Scheme #1:
Fixed Outside Air Damper (Supply Fan Only Systems)

Active pressurization control is often ignored on systems without return or relief fans. However, if more air is mechanically exhausted from the building than is brought in through the air handler, the building will be negative to its environment. From direct observation and anecdotal references, we know that many cases exist where external pressures induced on the intake system result in outside air intakes acting as an exhaust.

Systems only having a supply fan appear to be almost exclusively set up by Test and Balance contractors and have no active control for outside air intake flow rates, and therefore no direct control of building pressurization. In most cases, a fixed minimum outside air intake damper is set to allow for the proper amount of outside air. Changes in wind and stack effect will result in significant airflow variations of the outside air intake on both constant volume (CV) and variable air volume (VAV) systems.^{1, 2, 3, 29} VAV systems are far worse due to the modulation of supply airflow rates and the resulting changes in mixed air plenum pressure, which directly impacts the amount of outside air allowed to enter the system.^{1, 2, 3, 29} Building pressure is also affected, if the system does not include a method of reliable pressure relief or exhaust.

Control Scheme #2:
VFD Proportional Drive Slaving (Systems with a Return or Relief Fan)

VFD proportional drive slaving is very common. In fact, many manufacturers of VFD units allow for the programming of a fan performance curve into a VFD controller. Unfortunately, unless the duct system being controlled has a constant system resistance, this technique is not valid for fan tracking. The errors generated are

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