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I have a super-insulated house (18" thick walls, whole-house air infiltration barrier, insulated floor system, insulated foundation, conditioned crawlspace). I have a sump pit, but no pump (wasn't sure at build time whether it would be needed; it is).

I need to install a sump pump and an outlet to deal with high groundwater in spring thaw.

Most sump pumps have a 1.5" threaded outlet. Running 1.5" rigid PVC through the insulated floor system and walls will be painful; it will mean a lot of cutting, loose insulation dropping out of the floor, trying to glue a right angle inside an insulated floor system, and a large new puncture in the air infiltration barrier that helps keep the house warm.

I've been thinking of drilling a single diagonal hole (probably 24" or so) through the floor system in the crawlspace out to the exterior wall, and fishing some Pex through it. But I think this would require reducing the 1.5" sump outlet to a 1" or 3/4" Pex pipe.

A couple questions:

  1. I'm fairly confident that this diameter will adequately drain the water -- right now a garden hose through a window is doing the job. But I don't know if there are other reasons not to reduce a sump pump. Will it wear out the motor faster? Increase the chance of outlet pipe bursting?
  2. Is there any reason not to extend the Pex 10-15' from the outlet outside the house, to route it far away from foundation? It's a cold climate, so freezing is possible in e.g. May; but if things are freezing, then there probably isn't as much water to pump.
  3. Any other advice welcome. I've never sumped before.
  • On the next house, when you put in the sump pit "in case" put in the sump discharge pipe "in case" if it will be difficult to do that later... A diagram would help here. Drilling through the foundation wall (which would have been as easy as putting a pipe sleeve in the forms thinking ahead) might be the most straightforward approach, and is easily done with the right tools. – Ecnerwal May 13 at 11:44
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I have a sump pump that I use with a garden hose. It works great but has a slightly reduced flow capacity. If that size works for you, use it. My greatest concern would be exposure of the pipe to the cold climate. If it freezes, it will not flow water. The smaller pipe (hose) will put more "head" on the pump but should not cause any harm to your pump. As far as cutting a hole to the outside, why not hire someone or rent a drill that can "core drill" an exact size hole to the outside through almost any wall thickness. It will make for a neat and clean hole with little cleanup needed.

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Regarding pipe reduction: I set up a cheap sump pump reduced from 1.5" to a 3/4" garden hose and sent it out a window to deal with the acute water issue. It works fine, even with a 12' rise. Next, I reached out to one reputable pump manufacturer, Zoeller, to ask about the setup, and their product support rep recommended not doing it:

Reducing the pipe size connected to the pump could cause issues. When you reduce the pipe you can put back pressure on the motor because it will be working harder moving through a smaller size pipe.

My impression after talking to several people about it is, even though it works, it might shorten the life of the pump and/or void the warranty.

So, instead of reducing to Pex tubing diameters for the permanent outlet, I've ordered a roll of 1.5" flexible PVC tubing. It works with the schedule 40 PVC fittings that are standard for this application and will hopefully be flexible enough (after sitting in the sun for a bit) to fish through a closed floor system.

Regarding outlet freezing: On the exterior I will terminate the outlet tubing fairly close to the wall, then continue the drainage run after an air gap (described in defect #5 here). Might even spring for an IceGuard, but will probably do something more low tech to start.

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What do you mean a garden hose currently works? Is that what you have connected to a sump pump right now?

Aside from a smaller pipe being more likely to freeze up (that's my guess), you could damage the pump. The pump is designed for a specific discharge pipe size. A smaller pipe can cause back pressure because it can't handle the volume the pump is trying to shove through it. The pump will also run longer each cycle as it will take longer to push the water out. This all adds up to a shorter lifespan of the pump.

My pump uses a 1.5" discharge and runs to the top of my basement ceiling then about 20' out to the back wall then another 10' outside. I'm in Ohio and didn't have any problems last winter with that 10' run outside freezing.

  1. I would not recommend using a 3/4" discharge pipe.

  2. Shouldn't be an issue running it farther away, but be sure to have the pipe outside the house drop about 1" for every 8-10' I'd say. Also, just because it's 20 degrees outside and the water you see is frozen, the ground water (which is what comes in your basement) stays a constant temperature and won't freeze. I believe the frost line for my area is 3' down.

  • Pump lifetime is not reduced by long runtimes, indeed, "short cycling" of pumps kills them considerably faster than long runs do, and back pressure is not a problem for centrifugal pumps - it actually reduces the load on the pump, as a watt-meter will demonstrate. You are correct that groundwater flows when the air is freezing. – Ecnerwal May 13 at 11:52
  • Your recommendation about discharge was supported by a pump manufacturer. Regarding groundwater, I agree, it's still liquid in winter, but it's much deeper so not hitting the system. – hoosteeno May 15 at 20:14

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