I am looking at adding a water softener and water filter to my house. I've drawn up what I'm thinking but looking for advice on some of what I show in case there is a better/different way I should be doing this. Located in central USA.

I'm on municipal water. The main thing I want to do is soften the water; currently hardness is at ~6-8 GPG. The filter comes with the water softener, so thought I'd just put it in to add any sort of filter before the water goes to the softener but I probably don't even need the filter to be honest.

sketch of plan

  • I would think you’d put the filter after the softener to prevent mineral buildup in the filter.
    – DoxyLover
    Commented Oct 23, 2023 at 16:33
  • If the water has sand/silt/particulates, you want those out before a softener meets them. Nothing a softener takes out should precipitate in a mechanical filter. Step zero is the water testing to determine what you need to filter, if anything.
    – Ecnerwal
    Commented Oct 23, 2023 at 16:59
  • 1
    Is this municipal water or well water? You should have different approaches for each. For well water, test for radon before you do anything. Putting a charcoal filter in a radon-laden water supply creates a radiation hazard from the filter itself, which must then be disposed of as hazardous waste. Please edit your question to state your water source and say what all of your water testing results have revealed so far. If you haven't tested yet, stop everything until you've run a full battery of tests. No point building a system only to fix it later after you learn what's in your water.
    – MTA
    Commented Oct 23, 2023 at 17:10
  • Thanks! I've edited my comment above, municipal water just trying to bring hardness down so honestly probably don't even need the filter. Any concerns with adding that sanitary tee to the drain? Or with how the piping is set up?
    – Andrew
    Commented Oct 23, 2023 at 19:09

1 Answer 1


From here it looks like this is in a basement, possibly below grade. The low placement of the air gap makes me uncomfortable. I don't know your neighborhood, but if flooding is ever a problem, or -- say -- if there's a restaurant around the corner that creates fatbergs in the sewer system, you could end up with raw sewage shooting out of the air gap.

If this air gap is not lower than the lowest existing drain in the house and you've been there a number of years with no sewage backup issues, you'll probably get away with it. But if this were mine, I'd plumb the backwash drain all the way up to the ceiling to be above the outdoor grade and if applicable, above the 100-year flood level to be safe.

  • Thanks! There is a floor drain below this point that I've not seen backup, wondered if I should just shoot the backwash drain into that but I'm afraid that will shoot all over the floor being that it's under pressure. I could definitely move the connection to the 3" sanitary higher though just to be on the safe side, maybe even to the ceiling there. Any thoughts on teeing into that 3" stack though (putting aside the risk of blockages)?
    – Andrew
    Commented Oct 23, 2023 at 19:56
  • @Andrew I'm not qualified to answer on teeing the stack. Anyone else?
    – MTA
    Commented Oct 23, 2023 at 19:58

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