2.4 Inspection of Existing Radon Mitigation Systems

Inspection of Existing Radon Mitigation Systems

When conducting a radon measurement evaluation in a property equipped with an existing radon mitigation system, the measurement professional must perform a visual and diagnostic inspection of the system. Evaluating system operational status, component compliance, and installation safety ensures that the mitigation system is functioning properly and that testing accurately reflects building conditions. This section details the components of Active Soil Depressurization (ASD) systems, diagnostic interpretation of liquid U-tube manometers, fan placement and discharge pipe termination standards under ANSI/AARST SGM-2023 (Soil Gas Mitigation Standards for Existing Homes), and post-mitigation re-testing protocols.

Overview of Active Soil Depressurization (ASD) Systems

Active Soil Depressurization (ASD) is the most effective and widely installed radon reduction technology in North America. ASD systems operate on a straightforward physical principle: using an inline mechanical radon fan to maintain continuous negative air pressure beneath the building foundation slab relative to the interior living space. By reversing the natural building-to-soil pressure gradient, soil gas (including radon, moisture, and VOCs) is drawn into a sub-slab suction pit, pulled through PVC piping, and safely vented to the atmosphere above the roofline.

Key ASD sub-types include:

• Sub-Slab Depressurization (SSD): Suction pipe penetrates directly through a concrete basement or slab-on-grade floor into sub-slab aggregate.
• Sub-Membrane Depressurization (SMD): Suction pipe penetrates a durable, sealed poly membrane (minimum 6-mil or reinforced 20-mil cross-laminated polyethylene) covering an earthen crawlspace floor.
• Drain Tile Depressurization (DTD): Suction pipe connects directly to interior or exterior perforated weeping tile drainage loops.
• Block Wall Depressurization (BWD): Suction pipe penetrates hollow concrete block foundation wall cores to depressurize network cells.

Diagnostic Evaluation of U-Tube Manometers

Every active ASD system must be equipped with a visible, permanently installed system performance indicator—most commonly a liquid U-tube manometer mounted on the vertical PVC suction pipe in an easily accessible location.

Mechanics of the Liquid U-Tube Manometer

• The U-tube manometer is a clear, vertical U-shaped plastic tube partially filled with colored fluid (typically red or blue gauge oil/water).
• One side of the U-tube is open to ambient room air pressure; the opposite side is connected via a flexible vinyl tube to a small hole drilled into the interior of the PVC suction pipe.
• When the radon fan operates, it creates a vacuum (negative pressure) inside the pipe. This vacuum draws fluid up the pipe-side column while atmospheric room pressure pushes fluid down on the ambient-side column.
• The vertical distance between the oil levels in the two column arms represents the fan vacuum suction differential, measured in inches of water column (in. W.C.).

Diagnostic Reading Interpretation

• Normal Operational Reading (0.5 to 2.5 in. W.C.): Indicates active fan operation maintaining effective vacuum pressure. (The exact baseline reading varies by fan model, soil permeability, and suction pit size, and should be recorded on the system label).
• Zero Reading (Fluid levels equal across both arms at 0.0 in. W.C.): Indicates COMPLETE SYSTEM FAILURE or absence of vacuum pressure. Causes include:
  1. Fan electrical power loss (tripped circuit breaker, switch turned OFF, blown fuse).
  2. Complete fan motor burnout or mechanical failure.
  3. Disconnected or kinked vinyl manometer gauge tubing.
  4. Major suction pipe severance or complete blockage.
• Abnormally High Reading (e.g., >3.5 to 4.5 in. W.C.): Indicates severe air flow restriction on the suction side, such as a chocked/collapsed suction pit, standing water accumulation in low-point piping, or completely saturated sub-slab soil.

Radon Fan Placement Standards (ANSI/AARST SGM-2023)

ANSI/AARST SGM-2023 establishes strict safety rules regarding the physical location of inline radon fans:

Approved Fan Locations

Radon fans must be installed exclusively in spaces OUTSIDE the conditioned living envelope of the building:

1. Exterior Building Walls: Mounted on the outside wall of the structure.
2. Unconditioned Attics: Mounted vertically inside an unconditioned attic space above living areas.
3. Unconditioned Garages: Mounted in an unconditioned attached garage, positioned above the lowest residential ceiling level.

Strictly Prohibited Fan Locations

Radon fans must NEVER be installed inside:

• Basements or cellars.
• Conditioned living spaces, bedrooms, or closets.
• Unvented or conditioned crawlspaces.
• Conditioned garages below living spaces.

Safety Rationale

The piping segment located upstream of the radon fan operates under negative pressure (vacuum suction). If a leak occurs in suction piping, room air is drawn into the pipe. However, the piping segment located downstream of the fan operates under positive pressure. If a fan installed inside a basement suffers a cracked housing or loose glue joint downstream of the impeller, high-concentration radon soil gas (often 1,000 to 10,000+ pCi/L) would be continuously pumped under positive pressure directly into the living space air.

Discharge Piping and Termination Requirements

To prevent exhausted soil gas from re-entering the building, discharge piping must satisfy precise geometry and clearance rules:

• Roofline Termination: Discharge pipes must terminate vertically upward above the highest eave or roofline of the structure.
• Height Clearance: Minimum 10 feet above ground grade.
• Opening Clearances: Minimum 10 feet away from any window, door, skylight, chimney, or operable building intake opening that is less than 2 feet above the discharge point.
• Vertical Orientation: Pipe must discharge straight up to propel exhaust gas into atmospheric air currents. Restrictive rain caps or 180-degree return bends that direct gas downward toward the roof or ground are strictly prohibited.

System Labeling, Electrical Disconnects, and System Types

• Electrical Disconnect: A dedicated electrical shutoff switch or plug-in disconnect must be installed within sight of the radon fan for service safety.
• System Labeling: A durable system label must be affixed to the suction pipe near the manometer, displaying system operational details, installer contact info, date of installation, and baseline vacuum reading.
• Passive vs. Active Systems: Passive systems (installed during new construction under RRNC standards) feature continuous PVC piping from sub-slab gravel through the roof, but contain no inline fan and no U-tube manometer. Active systems incorporate an inline fan and manometer.

Post-Mitigation Testing Protocols

Whenever a mitigation system is installed, repaired, or modified, a post-mitigation radon test must be performed no sooner than 24 hours and no later than 30 days following system activation to verify mitigation efficacy.

Mitigation System Visual Inspection & Deficiency Matrix