Adding a Fan to a Passive Radon System

Walk through a closet, garage or attic in a lot of newer Huntsville-area homes and you may find a 3- or 4-inch PVC pipe labeled "radon" running up toward the roof. That's a passive radon system, sometimes sold as "radon-ready" construction. It's a good head start, but it isn't the same as a working mitigation system. This page explains how passive systems work, how well they work, what it takes to activate one with a fan, and what to ask your builder. We're a free referral service, not a contractor. We connect you with independent, certified professionals who can test and activate a system.

What a passive system is

The EPA's radon-resistant new construction (RRNC) approach uses a few parts built in during construction: a gas-permeable layer, usually gravel, under the slab; plastic sheeting over it; sealing of slab openings and joints; and a vent pipe from below the slab up through the roof. In a passive system, the pipe relies on natural draft, warm air rising up the stack, to pull soil gas out from under the house. Many builders also rough in an electrical box in the attic near the pipe so a fan can be added later.

How well do passive systems work?

The EPA says passive RRNC systems can reduce radon levels by about 50%, and that adding a fan to activate the system reduces radon further. Cutting radon in half is a big improvement in a house that would otherwise read 6 pCi/L. It may not be enough in a house that would read 12. That's why the EPA recommends testing every new home, with or without a passive pipe. Huntsville summers matter here too. Natural draft depends on the stack being warmer than the outside air, so a passive stack tends to pull less in hot weather than on a cold winter night.

How to tell what you have

Look for the pipe. It's usually 3- or 4-inch PVC in an interior wall, closet, pantry or garage, often labeled, and it continues through the attic and out the roof.
Look for a fan. An active system has an in-line fan in the attic, garage or outside the house, plus a manometer (a U-shaped tube with liquid) on the pipe. No fan and no manometer usually means passive.
Look for an attic outlet. A junction box or outlet near the pipe in the attic is the builder's rough-in for a future fan.
Check your paperwork. Closing documents, the builder's feature sheet or the warranty packet may describe the radon system.

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Test first

A passive pipe is no reason to skip a test. Test the lowest lived-in level under closed-house conditions. If you have a continuous monitor, look at the long-run average, not a single day. Radon swings with the weather: a Huntsville homeowner on r/HuntsvilleAlabama in February 2026 saw readings jump during the first hard freeze of the year, and another reported spikes to 5.8 pCi/L with a two-year average of 2.2. The EPA recommends fixing a home at 4 pCi/L or higher and suggests considering a fix between 2 and 4.

What activating the system involves

Inspection. The mitigator confirms the pipe really connects to the layer under the slab, checks for cracks and open sump pits, and looks at where a fan can go. The fan has to sit outside the living space, in the attic, a garage attic or outside the house. It can't go in a basement or living area.
Diagnostics. Some installers measure suction under the slab with the fan running to make sure it reaches the whole footprint.
Fan and wiring. The fan is spliced into the existing pipe with rubber couplings. If the builder left an attic outlet or junction box, wiring is usually simple. If not, a circuit or outlet is added, which may need an electrical permit. See our permit guide.
Manometer and label. A manometer shows the system is drawing suction, and the system should be labeled.
Sealing. Accessible slab cracks and openings are sealed so the fan pulls from under the slab, not from the house.
Post-mitigation test. Per EPA guidance, this starts no sooner than 24 hours after the fan is turned on and within 30 days.

Because the pipe and roof penetration are already in place, activating a passive system usually costs less than building a system from scratch. Prices vary by installer and house, so ask for a written quote and a reduction guarantee.

Will my builder do it?

Sometimes, but don't count on it. In September 2026, a DR Horton owner asked the r/HuntsvilleAlabama community whether the builder would activate a passive system after readings climbed. Other homeowners replied that builders generally don't do it unless code requires it or the buyer negotiated it, that verbal promises rarely hold up, and that a written warranty request with documentation is the place to start. Practical steps:

Check the purchase contract and warranty for anything about radon.
If the home is still under warranty, submit a written warranty request with your test results.
Expect the builder to want its own test before acting on yours.
Ask which subcontractor installed the passive pipe. They may be able to activate it.
If the builder declines, a certified mitigator can activate it for you.

Mistakes to avoid

Putting a fan in a closet or basement. The fan and any pressurized pipe have to be outside the living space, so a leak can't push radon indoors.
Using a bathroom exhaust fan or duct fan. Radon fans are sealed and rated for continuous use in an attic or outdoors.
Skipping the post-mitigation test. It's the only proof the system works.
Covering the attic outlet with insulation and forgetting it's there.

Frequently asked questions

Does a passive radon system work without a fan?

Partly. The EPA says passive systems can reduce radon by about 50%, and adding a fan reduces it further. Test your home to see whether the passive system is enough.

Can I add a radon fan to my builder's passive pipe?

Usually, yes. A certified mitigator splices a radon fan into the existing pipe in the attic or outside, adds a manometer and label, seals openings and tests afterward. It usually costs less than a new system.

Will DR Horton or my builder install a radon fan for free?

In most cases, only if your contract or warranty says so. Homeowners in a 2026 Huntsville discussion reported that builders rarely do it otherwise. If you're still covered, submit a written warranty request with your test results.

Where can a radon fan be installed?

Outside the living space: in the attic, a garage attic or on an exterior wall. It shouldn't go in a basement or living area, because a leak in the pressurized pipe could push radon indoors.

Do I need an electrician to add a radon fan?

If there's no outlet or junction box near the pipe, new wiring is needed. Depending on your jurisdiction, that may require a licensed electrician and a permit. Many builders rough in an attic box for this reason.

How do I know if the activated system is working?

The manometer on the pipe should show a pressure difference, and a post-mitigation test, started at least 24 hours after the fan is turned on, should come back below 4 pCi/L.