Our furnace is in an attic, above a two story home. The installed condensate drain runs to an outside wall and drips. The slightly corrosive condensate drips onto a piling with a steel connection holding one of the main glue-lams to a marine piling. The house is on a steep hillside. The piling started to split last year so I ran some extensions to the 3/4" PVC to redirect the drainage under the house, away from our pilings and into the ravine. I insulated the PVC but it's a fairly long run (20+ feet vertical plus another 20+ sloped downhill) and although it is steeply angled down the slope, it froze last winter twice.

The question is: Can I redirect the 3/4" drain into an air vent that runs through the attic? It's the washing machine drain from the laundry room below the attic and the vent rises through the attic to (and beyond) the roof.

2nd question: Do I need to add another trap to the vent line at the condensate entry point?

2 Answers 2


You can connect it. You should make sure everything connected to that vent stack at or below the level where you add your pipe, is connected with a wye and not at 90 degrees. You don't want your condensate running into a fixture downstairs. You should make sure the stack is vertical below the level where you add your connection. Vents that are only vents have some flexibility as to how they are run. Once you add your condensate line, that vent will no longer be just a vent and it has to be run correctly for a main drain. You should be fine if the laundry is directly below, but you should check.

Yes you should add a trap to your condensate line if it doesn't already have one. Read the furnace manualto understand where and how a trap should be located. There should already be one to prevent flue gases from moving down the condensate line. The same trap ought to prevent sewer gases from moving up it. You need to make sure that trap stays full. During the summer you may need to fill it a couple of times. During the summer there are no flue gases to flow through an empty trap but there are still sewer gases if you connect it to the vent.

My advice: If you have a laundry room directly below there is an easier way. Run the condensate line through the laundry room ceiling and into the laundry machine drain. You don't need a trap. The drain, whether it is a standpipe or a sink, will already have a trap and an air gap.

  • 2
    I think the advice is to the point, can you drain the condensation into an indoor drain. 3/4" inches pipe is just too small to run outdoor under frigid environment.
    – r13
    Oct 3, 2021 at 23:18
  • No outdoor drain available but am looking at the washing machine drain one floor below or the vent stack coming from that drain (where I can connect in the attic instead of going through the wall cap and down 8 feet.
    – Silver1210
    Oct 5, 2021 at 0:49

Thank you for the advice. I will look at putting the line directly into the drain. The drain is inside the wall behind the washer so it would mean drilling into the top plate, which I assume is double. I am not sure what is in that wall and don’t want to cut out the Sheetrock if I don’t have to, that’s why I saw the vent as an easy option. It may be simpler though than going to the vent stack so I appreciate the advice.

  • 1
    How does the washer access the drain? Isn't there an opening in the laundry room? The thing you shove the hose into? I was thinking to run the condensate line into that. No drilling or opening anything, just a hole in the laundry room ceiling.
    – jay613
    Oct 5, 2021 at 3:00
  • The drain is in a plastic box in the wall behind the washer so I will have to drill through the top plates and line up with the drain so I can drop a pvc down and connect the pvc drain line in the attic. Not rocket science but I wanted some confirmation before I started.
    – Silver1210
    Oct 6, 2021 at 3:56

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