How to Use an Anemometer to Check Your Radon Fan Size

Ensuring your radon mitigation system uses a properly sized fan is crucial for effectively lowering hazardous gas levels in your home. Radon fans rated in CFM (cubic feet per minute) air flow need to match your home’s size and vent pipe system design. Using an anemometer to measure actual air velocity serves as an empirical way to validate if your installed radon fan aligns with recommended sizing guidelines.

Why Proper Fan Sizing Matters

Radon fans work continuously to vent gasses from beneath the home’s foundation through a piping system. Correctly sized fans generate sufficient air suction and negative pressure differentials to prevent radon entry through slab and wall cracks. Oversized fans waste energy while undersized units strain excessively and fail to reduce indoor radon adequately.

Signs like elevated post-mitigation radon levels, straining fan noise, and condensation buildup indicate a fan may be undersized. Measuring air velocity through the vent pipe helps determine if poor performance stems from a radon fan that is too small for your home’s specifications.

Using an Anemometer to Measure Air Velocity

An anemometer specifically designed for air duct and vent pipe velocity testing allows you to quantify the actual air movement your radon fan produces. Compare this empirical CFM reading against the rated airflow capacity on your fan model’s spec sheet to gauge adequacy.

  1. Turn on the radon mitigation system fan and let it run for 10-15 minutes before testing. This allows airflow to stabilize.
  2. Insert the anemometer probe into the vent pipe, positioning it in the exact center to get a representative reading.
  3. Note the air velocity reading on the anemometer display, shown in FPM (feet per minute) or MPS (meters per second).
  4. Repeat velocity measurements at multiple points across the diameter of the pipe and average the results.
  5. Convert air velocity into volumetric CFM flow using the pipe’s cross sectional interior area (see calculations below).
  6. Compare the anemometer’s CFM to your fan’s rated CFM airflow capacity to determine if it aligns. Significant deviation indicates improper sizing.

Convert Anemometer Air Velocity into CFM

To compare anemometer air velocity readings to your fan’s CFM rating, you need to account for the vent pipe’s interior area like this:

  1. Calculate the interior cross sectional area of the vent pipe:

Area = πr2

(where r is the interior pipe radius)

  1. Multiply the air velocity (FPM) by area (sq ft) to calculate CFM airflow

For example, a 4″ pipe has a 2″ interior radius. The area would be:

π x (2″)2 = 3.14 x 4 = 0.1 sq ft

If the anemometer reads an air velocity of 600 FPM, then the equivalent CFM is:

600 FPM x 0.1 sq ft = 60 CFM

The anemometer measurements validate whether your installed radon fan produces the rated CFM airflow needed for your home based on system design. If CFM is significantly lower, resizing may be required.

Tips for Accurate Velocity Testing

Follow these tips when using an anemometer to measure vent pipe air velocity:

  • Take measurements as close to the fan inlet as possible for most accurate readings.
  • Repeat further along vent pipe to check for airflow restrictions.
  • Record data with fan on and off to quantify CFM generated by the fan itself.
  • Ensure probe is positioned centrally and is not obstructed.
  • Take safety precautions when inserting equipment into radon vent pipe.
  • Allow fan to run 10+ minutes before testing to let airflow stabilize after startup.
  • Take multiple velocity readings and average the results.
  • Compare CFM to manufacturer’s sizing recommendations for your home.

An anemometer allows you to empirically validate radon fan sizing based on real airflow data. This provides greater certainty your mitigation system uses a properly matched fan to effectively reduce hazardous gas levels in your home’s air.

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