I actually need to add a check valve to my condensate drip line/sewer drain connection since it had sewer backup recently (the 3/4" PVC condensate pipe was piped down directly into a 2" black iron sewer pipe). And, I found this that seems easy to install in my setup - however, I would need to install it upside down since in my application the flow drips down, not a pump that pushes up. It says it can install vertically but can it install upside down?

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and a link to the instructions: https://images.thdstatic.com/catalog/pdfImages/a0/a034e9df-be8e-40c9-9e58-c800eef33b5e.pdf

and the item at home depot: https://www.homedepot.com/p/2-in-Sewage-Check-Valve-with-Flexible-PVC-Fittings-THD1058/205616018#overlay

and this is where I need to install it:

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Can I instead install a drain check valve in the pipe itself? The kind with a float stopper? Should I get a shorter iron pipe and add an air gap?

  • This install is not (IIRC) to code regardless. And will not be with a check valve. You need an air gap (the incoming pipe ending above the sewer pipe by at least one (incoming) pipe diameter - with or without a fancy fitting for that purpose) to prevent siphoning.
    – Ecnerwal
    Sep 18, 2022 at 13:20

2 Answers 2


You should cut the PVC condensate pipes shorter, and install a longer drain (more distance backflow has to come up before overflowing) leading into the P-trap, with an airgap for the condensate connections.

If you want to prevent backflow from the drain pipe, you need a sewer backflow preventer made for that purpose, not an inverted sump check valve. The most common ones are made to fit shower drains and floor drains, so you may want to park such a drain on the top of your drain pipe as the easiest way to find a common model that fits. You may be able to find one that goes inline in the pipe or end of the pipe (not needing a shower drain or floor drain tacked on) with a bit more searching.

Of course, if you stop it from backflowing here, you should also consider where then becomes the low (open) point that backflows.

  • so if it backflows in the basement it kinda goes everywhere at this point - I put those drain backflow preventers in the floor drains and then it caused it to come out the laundry sink and into the HVAC condensate - at this point I want to seal the HVAC as top priority since that's expensive/hard to clean and let it bubble over into the laundry sink as necessary. I'm assuming that if I block the laundry sink that it will then go up to the first-floor tub? Or is that an incorrect assumption since it will just go to my neighbors house instead since water finds its level? Sep 19, 2022 at 12:28

Yes you can install this as you want but, I am not sure it will work as you expect.

Your condensate lines are not under pressure and rely on gravity fall for any water to exit them. Check valves usually have a flapper which is spring tensioned to keep the valve closed. Your non pressurized water trying to exit the condensate lines may not be able to open the valve.

  • Good answer! Would it be possible to install the valve where the connection with the 2 clamps are? That way you could fit it right side up, but as Kyle said, if it's spring loaded, it may not work right. You'd have to get a check valve that simply has a flapper that's gravity operated. But that would only work if oriented in the right way. Sep 17, 2022 at 18:14
  • Do you mean at the p or s junction? The only spots that are easy for me the diy person would be on the down pipes I think. What if I install a drain check valve in the pipe the kind with the floating stopper? Sep 18, 2022 at 11:39
  • Should I also add an air gap? Sep 18, 2022 at 11:40

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