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This pipe emerges from the concrete slab floor in a corner of my house's full basement. I believe it's a 4" polyethylene sewer pipe (narrow walls, black interior, white exterior). I can't tell if it's perforated or not below the surface.

Polyethylene sewer pipe emerging from basement floor, capped Polyethylene sewer pipe emerging from basement floor, uncapped

As you can see from the picture it goes down 4 feet. From that point it bends and appears to penetrate horizontally through the adjacent frost wall. (I know this from fiddling with a tape measure and a flashlight.)

Polyethylene sewer pipe emerging from basement floor, with tape measure

When I uncapped it, I noticed that air seemed to be blowing out of the pipe into the house, and I wondered if it was connected to either of the mystery pipes that emerge from the ground at opposite corners on the outside of my house.

Polyethylene sewer pipe emerging from ground exterior, adjacent to foundation Another polyethylene sewer pipe emerging from ground exterior, adjacent to foundation

I eventually convinced myself that the pipe is connected to the nearer of the two exterior pipes. (I did this by putting a vacuum down the pipe inside the house and noticing that the nearer exterior pipe was sucking air when I turned on the vacuum.)

Polyethylene sewer pipe emerging from basement floor, with vacuum

This is the layout: Diagram of house footprint and pipe locations

Various contractors at the house have speculated about the pipe's purpose. Theories include:

  1. It functioned temporarily during construction, after the house's electricity was connected, to run electrical wires from the house to a trailer outside that needed electricity. (The people who built the house lived in a temporary trailer outside while the house was under construction.)
  2. It's for an illegal basement sink that would drain directly into the ground instead of running out to the septic system.
  3. It's for radon reduction. A fan needs to be added above the interior pipe to pull air from beneath the basement floor, inside the frost wall. (But if so, why does it vent outside? Also, it seems to go through the frost wall immediately. I don't believe there is any perforated pipe beneath the slab inside the frost wall.)
  4. It's for passive radon reduction. The air from beneath the basement floor, inside the frost wall, is intended to vent outside. (But if so, why would the pipe come up through the floor into the basement at all? And again, there doesn't seem to be much perforated pipe inside the frost wall.)
  5. The exterior corner pipes were intended for roof gutters to drain into. (But if so, why plumb one through the frost wall and up into the basement?)

I am about to finish the basement room, and I intend to wall off access to the pipe. But I would like to know what it's for before I lose access to it forever.

  • Got a 6 foot section of Romex cable? Shove it down there and see if it makes it past the first bend without too much fight. If so, they installed a gentle curve (wide radius) and it's for electrical. Honestly, I'm amazed it's not full of water. – Harper - Reinstate Monica Mar 26 '17 at 18:02
  • @Harper, the tape measure did navigate the curve. I was able to run it about 7 feet before it kinked. I live on a rocky hillside in New Hampshire, and the natural drainage around the house seems really good--there's no sump pump, and the basement has been bone dry for years. On the other hand, houses in NH are notorious for radon, which led my thinking in that direction. – Michael H Mar 26 '17 at 18:30
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    It's the right size pipe. But a radon system would blow out, not in. And I'd expect there to be a lot more apparatus, festooned with special markings. Bless your drainage, around here we have to lay stone and drains if we want that. Since it's laid out like a trap (inverted siphon), I would have expected that pipe to be full of water (which is fine for electrical conduit, that's why the wire's rated THWN.) Are you sure they're connected? When you run the vacuum and plug the outside hole, does the motor spool up? Maybe all 3 go to the same destination downhill. – Harper - Reinstate Monica Mar 26 '17 at 18:45
  • You've run a radon test? It was OK? If so, even if it IS a radon (if needed) setup you don't need it... were it active, there would fans attached to it. Otherwise you can hire someone with an in-pipe camera to develop more information about it/them. I would agree with plugging the one you know is connected and checking the far one. I'd also consider (cheaper than the pipe camera) dumping some non-toxic septic dye and water in them and see if you find it downhill - could just be drains (with the stub pipe in the basement not turned into a floor drain.) – Ecnerwal Mar 26 '17 at 19:09
  • It could be a passive radon mitigation system, or it could simply be intended for that purpose, as the question suggests. The fact that air moves into the home doesn't tell us anything other than just that. – isherwood Mar 26 '17 at 21:56
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Yes, your area is known for radon and it It looks like pipe is the rough-in for a radon system.

When we install radon piping, we install a series of perforated pipes embedded into gravel in the crawl space or under a slab-on-grade floor. We also provide a series of pipes in the ground around the exterior of the building too.

Usually the two systems are not connected, but work in tandem. The exterior pipe and gravel provide an inlet for air, so the interior pipe and gravel can exhaust the air under the building. However, the exterior gravel needs to be "connected" to the interior gravel (gravel under the building,) so air can be drawn through the gravel under building to purge the area.

The reason the two pipes seem to be connected is that the two gravel systems are connected.

However, the interior solid pipe (not perforated) needs to be extended through the attic and connected to an exhaust fan. (Often the interior pipe is extended through the attic/roof without an exhaust fan. If tests show radon has decreased, then we never install the exhaust fan.)

  • This sounds right to me. The basement is shallow at this end of the house--nearly at ground level, so the footings should sit a full 4 feet below the floor. But I'm struggling to picture how it works. A radon fan would draw air up through the basement floor, but that air is coming through a non-perforated pipe which goes down 4 feet and immediately elbows out through the frost wall. It seems to me like it would be creating an underground vacuum outside of the house while leaving the area directly under the basement floor, where the radon seeps up, unaffected. – Michael H Mar 27 '17 at 1:02
  • The technique is designed to move air under your house (purge the air under your house) up and away from your house. (If it's a slab-on-grade, then there is a layer of gravel between the bottom of the slab and the ground.) You can't get rid of what's causing the radon, but you can remove the gas that has accumulated. I think the idea is to move the radon gas away from living spaces. So, you need an "inlet" (pipe outside) so there can be an "outlet" (pipe inside, except it was not extended through the roof.) (It's like a straw.) However, the pipes do not need to be connected. – Lee Sam Mar 27 '17 at 1:37
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I'm sure that you've walled that in by now...

Would the general location of that pipe be a good spot for a wood or pellet stove?

My speculation is that pipe layout may be for a 'cold air intake' for a wood stove. When installed in mobile homes wood stoves must be fed by outside air because the mobile home is supposedly 'so tight' relative to air leaking in. The benefit of having a cold air intake for a wood stove in any home is that the air that goes out the chimney has to get into the house from somewhere (through electrical receptacles, around windows and doors, etc.); if piped directly from the outside the air can pass through the wood stove without creating negative pressure in the house and pulling cold air directly into the living space.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Jul 31 at 15:40
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First, I must compliment your diligence and documentation. You'd most likely make an excellent engineer.

Given the location of the exterior pipes, one would be led to believe it was for connecting downspouts to. It appears in the pictures to be "thinwall" sewer and drain pipe, as opposed to the more durable Schedule 40 pipe normally used in residential plumbing. However, I am unfamiliar with that ABS-looking black interior of the pipe, and the "rings" that appear on the exterior, as if it's some sort of hybrid.

With that being said, it looks as though all 3 of the stubouts are of the same pipe, and were placed during construction. It is extremely unlikely they were intended for radon mitigation...especially if the pipe isn't perforated.

The thing that doesn't make a great deal of sense to me is that the connected pipe in the basement goes 4 feet deeper for whatever reason. If the terrain outside is level, this would put the lateral depth of that inside pipe around 10-12' lower than the exterior ones? Why? Even if it were a cleanout to theoretically auger a clogged downspout line, why make it 4' deep?

Is there some really strange terrain around the outside of the house that would require this?

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There's no reason to perform radon "mitigation" in a basement with a poured concrete floor and walls. Radon is a gas produced by the radioactive decay of uranium and radium in granitic rocks and their sediments. If you had one of those 100 year old exposed stone basements with a dirt floor, you'd have a radon problem (maybe) worth considering. As it is, your basement is as close to being a fortified bunker against radon intrusion as you could hope to build.

Maybe it's just a dryer vent.

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    I live in Mass. Yes we have poured concrete basements, and you better believe that we test for radon, and radon mitigation is a real thing. The basement construction is not completely air tight, and it finds ways to get in. – DaveM Mar 27 '17 at 0:53
  • Of course it's a real thing. And someone has to feed this $200 million a year industry. But don't let me, a retired industrial hygienist from Boston, try and convince you that $30 worth of caulk is better spent than a $20,000 soil gas extraction system. Nor will I try and convince you that somehow, radon exposure is not a problem in the hard-rock mining industry, where workers are continuously exposed to much higher level of radon daughters than you suburban punters. Now, why do you suppose that is? – Knob Scratcher Mar 27 '17 at 1:27
  • @KnobScratcher - I'm now curious about the things you're talking about, but I can't seem to penetrate the sarcasm. – William - Rem Mar 27 '17 at 4:05
  • @KnobScratcher - you just need your sister to have radon problems like mine and then you'll understand why that $200 million a year industry exists. – Lee Sam Mar 27 '17 at 22:46
  • @KnobScratcher I don't know what you are going on about. I don't like expensive radon mitigation systems either. I don't have one and would do anything to avoid them. But they are a real thing. – DaveM Apr 4 '17 at 2:37

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