Radon mitigation in a DC rowhouse cellar with fire sprinklers: the suction pit, the pipe run, the sealed sump, and the riser in the way

A radon test comes back high, usually during a sale or right after a basement unit is finished, and a mitigation contractor quotes a system in an afternoon. In a rowhouse that already has fire sprinklers, that afternoon touches the slab the sprinkler supply comes through, the sump the main drain discharges into, and the chase the sprinkler pipe shares with everything else going up. None of it is hard. All of it goes better when the two contractors have talked, and this article is what they need to talk about.

What a mitigation system is, in one paragraph

The common design is sub slab depressurization. The installer cores a hole through the cellar slab, digs out a small pit underneath it, and runs a plastic pipe from that pit up and out of the house, with an inline fan pulling air from under the slab and pushing it out above the roof. Cracks, the slab edge, and the sump pit get sealed so the fan draws from the soil rather than from the room. Where the pipe runs depends on the house. In a rowhouse it is either up through the interior, usually in a closet or the old chimney chase, or out the rear wall and up the back of the house with the fan outside.

The suction pit and the underground supply

The sprinkler supply enters a rowhouse cellar under the slab and turns up at the riser. In a standalone system that is a dedicated line from the front wall, and in a multipurpose house it is the domestic service. Either way there is a pipe under the slab whose location is known only from the sprinkler drawings and a vague memory of where the slab was patched. A suction pit is cored where the slab is most open and farthest from the walls, which in a narrow cellar is often the same stretch the supply crosses.

Mark the supply route on the slab before the core drill arrives. The sprinkler drawings show it, and so does the slab patch if the retrofit was recent. The dig out article describes the same risk at a larger scale. A pit cored three feet or more from the marked line is safe. A pit cored into the line floods the cellar and opens a sprinkler permit for the repair.

The sealed sump and the main drain

Many rowhouse sprinkler systems discharge the main drain into the sump, because it is the nearest place that can take a few gallons a minute during the annual test. The waterproofing article covers why. Radon mitigation seals the sump with an airtight cover, because an open pit is the largest leak in the slab. A main drain pipe that was simply aimed at the pit now has to pass through the cover, and a hole cut around it with a gap defeats the seal.

There are three clean answers. A gasketed pipe fitting through the lid, which the radon installer can supply if told the pipe size. Rerouting the main drain to a floor drain. Or running it outside to the areaway, which is what we prefer when the front wall is close. Pick one before the lid is ordered. The radon installer will also want the sump pump discharge and the sprinkler main drain kept separate from each other, since a check valve on the pump line should never be in the sprinkler drain path.

The pipe run and the chase

An interior radon pipe is usually three or four inches across and it wants a straight run from the cellar to the roof. In a rowhouse the candidate routes are the same ones the sprinkler installer used, because there are only so many. The closet stack, the chase beside the chimney, the corner of the rear addition. A radon installer who finds sprinkler pipe in the chase they planned on will either move over or, in the worst case, move the sprinkler pipe, which is not theirs to move. The other trades article has the three sentences to say to any trade that opens a wall, and they apply here.

Where both pipes end up in one chase, the sprinkler pipe keeps its hangers and its clearance and the radon pipe is strapped independently. Firestopping at each floor penetration applies to both. If the chase passes through an unheated attic on the way to the roof, the sprinkler pipe there is wrapped or otherwise protected, and the freeze protection article explains why that cover has to go back exactly as it was after the radon installer leaves.

The fan and the power

The radon fan is a small motor on a dedicated circuit, placed in the attic or outside so that any leak on the pressure side is outside the living space. It draws little and makes no heat worth mentioning. The one place it meets the sprinkler system is the electrical panel, where its circuit joins the flow switch circuit and whatever else the renovation added. A full panel is a reason some owners end up doing a heavy up, and the heavy up article covers what the new panel does to the riser wall.

Sealing the slab around the riser

The riser comes through the slab inside a sleeve or a patched hole, and that opening is one of the leaks the radon installer will want to caulk. Sealing around the pipe is fine. Filling the sleeve solid with a rigid material is not, because the riser needs to move a little with temperature and the pipe below should never be locked to the slab. A flexible sealant at the surface does the job. Ask the installer to use the same approach they use around the domestic water service, since the sprinkler supply is the same kind of pipe in the same kind of hole.

The test after both systems are in

The radon installer runs a post mitigation test a few days after the fan starts. That is a good week to also run the sprinkler main drain test, because the sump cover, the main drain route, and the slab seals are all new and any mistake shows up in the first flow. The main drain article explains what you are watching for. If the drain backs up, the fitting through the sump lid is the first suspect.

Before the radon contractor is hired

1. Mark the sprinkler supply route on the slab, with the riser location and the three foot zone around the line.
2. Decide where the main drain will discharge once the sump is sealed, and give the pipe size to the radon installer.
3. Walk the planned pipe route together and confirm it does not share hangers or displace sprinkler pipe.
4. If the route crosses an unheated attic, photograph the sprinkler pipe insulation before and after.
5. Confirm the fan circuit has a home on the panel without touching the flow switch breaker.

Quick answers