Using Vent Caps on Floor Vents During Duct Leakage Testing (Positive & Negative Pressure)

Floor supply registers behave differently depending on whether you're depressurizing or pressurizing.

Vent Caps™ work under both positive and negative pressure testing — but the setup changes slightly.

Here’s how to handle each scenario correctly.

Negative Pressure Testing (Depressurizing at 25 Pa)

When pulling the house or duct system negative (typical duct leakage testing at 25 Pa), the pressure differential works in your favor.

Hardwood Floors

1. Drop the Vent Cap directly over the floor register.
2. Turn the fan on.

The suction pulls the cap tight against the floor.

In most hardwood situations, no tape, hooks, or weight are required during depressurization.

The negative pressure holds it in place.

Carpeted Floors

If the cap does not immediately seal, place a light object on top:

• A roll of quarters
• A small book
• Your gauge case
• Any modest weight

At 25 Pa, it doesn’t take much.

The downward force compresses the carpet pile, often creating a better seal than taping over the register and attempting to adhere tape to carpet fibers.

Important: The hardwood “drop and go” method applies to depressurization only.

Positive Pressure Testing (Pressurizing at 25 Pa)

When pressurizing the house or duct system, airflow pushes outward.

The cap must be mechanically secured or weighted.

Hardwood or Tile Floors (Pressurizing)

• Place weight on top of the Vent Cap, or
• Use the hook-and-drawstring method below

Positive pressure will lift the cap if it is not restrained.

Even at 25 Pa, it must be secured.

Carpeted Floors (Pressurizing)

Carpet friction helps, but do not rely on friction alone under positive pressure.

Add weight or use hooks for consistent, repeatable results.

Maximum Control: Remove the Register and Hook Tight

Most floor registers are not screwed down and lift out easily.

This method provides the most secure seal for both positive and negative pressure testing.

Hook + Drawstring Method

1. Lift out the floor register.
2. Attach the malleable vinyl-coated metal hooks to:
   • A subfloor screw
   • The boot collar
   • Any solid anchoring point
3. Place the Vent Cap over the opening.
4. Pull the drawstring tight.

This creates a mechanical seal that performs under:

• Negative pressure
• Positive pressure
• High-flow duct testing

It removes uncertainty.