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Proper air distribution is essential for comfort, efficiency, and equipment longevity. An HVAC system with perfect refrigeration components will still perform poorly if the duct system restricts airflow or distributes air incorrectly.
Airflow is measured in CFM (cubic feet per minute). Standard rules of thumb for residential HVAC:
A 3-ton AC system needs approximately 1,200 CFM of airflow. Too little airflow causes evaporator coil freeze-up and reduced efficiency. Too much airflow reduces dehumidification performance in cooling mode.
Static pressure is the pressure exerted by air in a duct system, measured in inches of water column (in. WC or in. WG). Think of it as the resistance the fan must overcome to move air through the system.
| Measurement | Location | Normal Range |
|---|---|---|
| Total external static pressure | Measured across entire air handler | 0.5 in. WC or less (per equipment specs) |
| Supply duct static pressure | In supply plenum | Varies by system design |
| Return duct static pressure | In return plenum | Negative (below atmospheric) |
High static pressure (above equipment rating) restricts airflow and causes: reduced equipment capacity, evaporator coil freeze-up (cooling), heat exchanger overheating (heating), blower motor overload, and premature equipment failure. Common causes: dirty filters, undersized ducts, too many elbows, closed dampers.
| Duct Type | Material | Advantages | Disadvantages |
|---|---|---|---|
| Sheet metal (galvanized steel) | Steel | Durable, low leakage when sealed, cleanable | Higher cost, conducts heat, louder |
| Fiberglass duct board | Rigid fiberglass | Insulated, quieter than metal | Less durable, harder to clean, must be sealed |
| Flexible duct (flex duct) | Wire helix with insulated jacket | Easy to install, inexpensive | High friction loss, prone to sagging, hard to seal |
Use mastic sealant (brushable compound) for best results on metal ducts. Mastic is far more durable than duct tape, which dries out and fails over time. UL-181 approved aluminum foil tape is acceptable for metal ducts. Standard gray "duct tape" is NOT appropriate for duct sealing - it fails quickly in temperature extremes.
Filter efficiency is rated by MERV (Minimum Efficiency Reporting Value) on a scale of 1-16 (higher = more filtration):
| MERV Rating | Particle Size Captured | Common Application |
|---|---|---|
| 1-4 | Large particles (pollen, dust bunnies) | Pre-filter, minimal protection |
| 5-8 | Mold spores, dust mites, pet dander | Residential standard filter |
| 9-12 | Fine dust, auto emissions, lead dust | Better residential, commercial |
| 13-16 | Bacteria, smoke, virus carriers | Hospital, clean rooms |
Higher MERV filters are not always better for residential HVAC. MERV 13-16 filters can restrict airflow in systems not designed for them, causing all the problems associated with high static pressure. Always verify the system can handle a high-MERV filter before installing one.
Relative humidity (RH) affects both comfort and equipment performance. The ideal indoor RH is typically 40-60%:
In cooling mode, air conditioners dehumidify as a byproduct - moist air passes over the cold evaporator coil, moisture condenses, and drains away. Proper airflow is critical for dehumidification - too much airflow reduces the coil's ability to dehumidify because air passes too quickly for moisture to condense effectively.
Remember: 400 CFM per ton for cooling, MERV measures filter efficiency (higher = better but can restrict airflow), and static pressure is measured in inches of water column. High static pressure reduces airflow. Mastic is the proper sealant for ductwork - not standard duct tape.