How to Add a Fan to a Passive Radon System (RRNC Activation)

What Radon-Resistant New Construction (RRNC) Looks Like

Since the 1990s, many builders in high-radon areas have installed passive radon systems as part of construction — sometimes required by code, sometimes voluntarily. A passive RRNC system includes:

1. A layer of gas-permeable aggregate (gravel) beneath the slab
2. A polyethylene vapor barrier over the aggregate
3. A 3" or 4" PVC pipe running from the aggregate layer through the slab, up through the home, and terminating above the roofline with an airtight cap
4. Electrical rough-in near the pipe for a future fan (in some but not all installations)

The pipe is ready for a fan. The gravel and vapor barrier are already in place. You just need to add the mechanical component.

Finding the Passive Pipe

In the basement/crawl space: Look for a capped PVC pipe coming through the slab. It's often in the utility area, a closet, or near the sump pit. May be labeled "Radon Vent."

In the attic: Look for the pipe terminating in the attic with a cap — this is where the fan would mount for an interior-routed system.

On the exterior: Some passive systems terminate through an exterior wall with a cap. The fan would mount on the exterior here.

In the garage: Many contractors route through the garage wall to avoid interior penetrations. Check the garage ceiling or walls for a capped PVC pipe.

Step-by-Step: Fan Addition

Step 1: Test first

Before adding a fan, test your current radon level with a short-term kit. This establishes your baseline. If levels are already below 2.0 pCi/L, the passive system may be working well and fan addition may have minimal impact. If levels are above 4.0 pCi/L, fan addition is clearly indicated.

Step 2: Select the fan

Match the fan to the pipe size (3" or 4") and your sub-slab conditions:

• Good gravel, radon under 8 pCi/L: RadonAway XP201
• Clay or sand sub-slab, or radon 8–15 pCi/L: RadonAway GX3 Pro
• High radon (15+ pCi/L) or very tight sub-slab: RadonAway GX5 Pro

Step 3: Remove the cap

The capped end of the passive pipe is where the fan mounts. Remove the cap (usually friction-fit or screwed on). The pipe will have a standard 3" or 4" coupler end.

Step 4: Mount the fan inline

The fan installs inline in the pipe — inlet connected to the pipe coming from the slab, outlet connected to the continuation pipe going to the exterior exhaust. If the passive system doesn't have continuation pipe above the cap location (if the cap is at the top of the pipe), you'll add a short section of pipe from the fan outlet to the exterior.

Use PVC primer and cement for glued connections, or union fittings for serviceable connections (recommended for the fan so future replacement is easier).

Step 5: Connect electrical

If electrical rough-in is present, the fan connects directly. If there's no outlet near the fan location, run an extension cord temporarily for testing — a permanent installation needs a properly wired outlet or hardwired circuit (consult an electrician).

Step 6: Verify and test

Turn the fan on. Check the manometer — it should show differential pressure immediately. Wait 24–48 hours, then check again. Post-mitigation test with a 30-day short-term kit to confirm levels.

What If the Passive System Isn't Working After Fan Addition?

If you add a fan but levels remain elevated:

• Check for gaps in the vapor barrier: The sub-slab depressurization only works if the barrier is intact — air bypassing through barrier gaps defeats the system
• Check the pipe connection at the slab penetration: The pipe must be airtight at the concrete — any gaps allow air to bypass the sub-slab
• Consider adding a second suction point: If the sub-slab aggregate doesn't extend pressure field coverage across the full slab, a second point may be needed

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