Installing a radon mitigation system in a sprinklered house: the suction pit, the fan, and the pipe it has to miss

Radon and fire sprinklers meet in the same corner of the basement. Fairfax and Loudoun sit in EPA Zone 1, the highest radon potential the agency maps, and a radon test is on nearly every home inspection in the region. When the number comes back over 4 picocuries per liter, a mitigation contractor shows up within a week of closing and cores a hole in the slab, runs a pipe up through the house, and hangs a fan. Most of that happens within arm's reach of the sprinkler riser and the CPVC feeding the first floor. This article covers the three places the two systems cross, what the radon contractor needs to know before the first hole, and the one sealant question nobody asks.

What a mitigation system is

Almost every system in a Northern Virginia basement is sub slab depressurization. The contractor cuts a hole in the slab, digs out a small pit, drops a three or four inch PVC pipe into it, seals the slab around the pipe, and runs the pipe to a fan that pulls air from under the slab and vents it above the roof. The pipe goes up through the house in a closet or chase, or out through the band joist and up the outside wall. The fan goes in the attic, in the garage, or outside. In a house with a crawlspace, the same idea works under a sealed membrane, and the crawlspace article covers what that membrane does around sprinkler pipe. None of this touches the sprinkler system on purpose. All three crossings below are accidents, and all three are avoidable with the plan in hand.

Crossing one: the suction pit and the water service

The riser is in the utility corner because the water service enters there. On public water in Fairfax, Loudoun, Prince William, Arlington, and Alexandria, the service comes through the slab or the foundation wall a short distance from the meter and the riser. On a well property the line from the storage tank does the same. The radon contractor wants the suction pit in a corner too, out of the finished area, close to a wall for the pipe run. It is often the same corner. A core bit through the service line is the worst version of this job, and it has happened. Before the pit location is marked, find the service entry on the sprinkler plan or on the slab itself, and keep the pit several feet away on the other side of the riser. The riser article shows what you are looking at. On a multipurpose system the sprinkler and domestic water share that one entry, so the stakes are the whole house, not one system.

Crossing two: the pipe route through the house

An interior route takes the radon pipe up through the first and second floors to the attic. The usual path is a closet stacked over the utility room, or the chase on the party wall in a townhouse. That chase is where the sprinkler main often runs. Cutting a four inch hole through a floor system and a top plate in a bay that carries orange CPVC is the second crossing, and it is the one the hit the pipe article is about. The fix is a marked up plan and a tape mark on the bay before the hole saw comes out. In a three story townhouse with a slab, the first floor heads are fed from above, so the floor system between levels carries pipe nearly everywhere, and an exterior route up the back of the house is often the simpler answer.

Once the pipe is in, it cannot be used as a support. Radon pipe is rigid PVC on its own hangers, and sprinkler pipe stays on its own hangers. Strapping one to the other is the kind of shortcut that looks fine for ten years and then shows up as a cracked fitting when the fan is replaced.

Crossing three: the fan in the attic

An attic fan sits on the radon pipe near where it exits the roof, with a circuit run to it. The attic in a two story Northern Virginia house also carries the sprinkler feed for the second floor heads, and in an unheated attic that feed is wrapped or tented for the freeze protection detail the designer specified. The fan needs its own bracket, clear of the sprinkler pipe and clear of the insulation tent. An electrician drilling the top plate for the fan circuit is drilling the same plates the sprinkler drops pass through. Mark them. And the roof penetration for the vent goes above the fan, not above a head drop, because a slow roof leak at a boot lands on the sprinkler pipe and on the ceiling below.

The sealant question

Radon contractors seal. They caulk the slab crack, the pit, the cold joint at the wall, and the gap around every pipe that comes through the slab, including the sprinkler supply. That is good practice. The question is what goes in the tube. CPVC has a published list of materials it should not touch, and some polyurethane caulks and expanding foams are on it. The chemical compatibility article explains why a sealant that is fine on PVC can craze CPVC over a few years. Ask the contractor what sealant is going against the orange pipe and check it against the pipe maker's list. A silicone that is on the list is a two minute swap. A foam that is not on the list is a cut out and repipe later.

After the install

Walk it the day the crew leaves, the same way the HVAC article says to walk an HVAC job. Look for radon pipe resting on sprinkler pipe, straps shared between them, sealant on the CPVC, and orange shavings under any new hole. Read the riser gauge and compare it with what it read before. Then listen to the fan. A radon fan hums all day, and a sprinkler pipe touching it will carry that hum through the house, which is annoying but harmless. The pipe noise article covers the sounds that are not harmless. The county inspection for the radon work, where one happens, does not look at the sprinkler system. The person who checks the pipe is you, or us on an inspection visit.

Permits and who does the work

Virginia has no state license for radon mitigators, and the contractor you hire should be able to show a current national certification. The county side varies. The fan circuit is electrical work and the slab and roof penetrations are building work, and each county decides what it wants to see. Ask the building office directly. The sprinkler system is not part of a radon permit, and nothing about a radon install needs the sprinkler valve closed. If the pipe route truly cannot miss a head or a run of pipe, that becomes a sprinkler job with its own permit, and the head relocation guide covers it. Pricing for that is on the published table.

The lines to hand the radon contractor

• The house has a residential fire sprinkler system. The plan is attached and the water service entry is marked on the slab.
• The suction pit stays several feet from the service entry and the riser.
• Orange CPVC in the floor system, the chase, and the attic is sprinkler pipe. No hole saw through a marked bay.
• Radon pipe and fan hang on their own supports. Nothing straps to the sprinkler pipe.
• Sealant against the CPVC comes from the pipe maker's compatibility list.
• The sprinkler control valve stays open for the whole job.