The Stack Effect: In winter, as the air is heated, it becomes less dense and rises to the top of the home. This action creates a positive pressure zone in the upper portions of the home. The pressure forces air out through leaks in the attic or upper walls while drawing in cold air from lower levels or basements. These pressure shifts can make some rooms feel significantly colder in winter, leading to excessive thermostat adjustments and higher energy costs.

The Reverse Stack Effect: In summer, the process reverses. Cool air, being denser, settles at the lower portions of the home, increasing pressure in those areas and forcing conditioned air out through leaks. This results in unintentional infiltration of humid, unconditioned air from the upper portions of the structure, which can contribute to mold growth and excessive wear on cooling systems.

Both effects illustrate why sealing a building envelope is crucial. Without proper control, the HVAC system is fighting against constant air leakage, reducing efficiency, and increasing energy costs.

Sealing leaks, improving insulation, and ensuring balanced airflow between supply and return vents are crucial to mitigating these issues.

Measuring System Performance: Essential Tools and Techniques

HVAC contractors should embrace testing and measurement techniques to quantify performance and diagnose issues. Several tools can provide valuable insights:

1. Manometers for Conducting Pressure Diagnostics

• A micro-manometer instrument measures room pressures, identifying imbalances that may lead to inefficient operation.
• Testing involves placing a hose under a closed door and recording pressure differentials while the system runs.
• If pressures exceed three Pascals, pressure relief strategies may be needed, such as jumper ducts or transfer grilles. Adjusting duct layouts and balancing dampers can also help correct excessive room pressures.

2. Blower Door Testing for Infiltration Control

• This test quantifies how much air leaks through the home’s envelope.
• A blower door depressurizes the home, revealing unintended pathways where air enters or escapes
• Sealing leaks can significantly improve efficiency and IAQ, reducing the load on HVAC equipment and lowering energy bills.

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