There is a pvc pipe as part of the installation of my furnace. The red cap (circled) was off and water was overflowing. I pushed the cap in, but I'm concerned that, maybe, that's the wrong thing to do. Any advice?

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    That should be a condensate drain line. I think the red cap was meant to be inplace. I'm not very familiar with furnaces, (In Fla. it all heatpumps or AC units.) I'll let the HVAC experts give the definitive answer.
    – RMDman
    Commented Jan 14 at 23:45
  • I think a condensate drain line makes sense. Running the heater today due to the storm. I'm still wondering why it is overflowing, though. Maybe a block upstream? And I hope it doesn't explode because I put the cap on, lol. (I'm not really lol'ing.) Commented Jan 14 at 23:48
  • Pop the cap off. Condensate is supposed to get out, whether through the drain or overflow. Your drain is blocked. (Deal with that later.) Put a bucket under for overflow. Commented Jan 14 at 23:54
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    Ok, thanks. I will. Commented Jan 14 at 23:54
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    the inflow side of a p-trap should be higher than the outflow side. Commented Jan 15 at 0:44

1 Answer 1


If I’m reading the pic correctly, the cap is off the lower side of the U tube trap. Water will flow out of the lower side as a rule, so replacing the cap may be all you need to do. I would also pop the cap off the horizontal line attached to the left side of the U tube and poke a brush or rod as far into it as you can, to poke a hole if it is clogged. If it’s clogged, clean it out. All this you can do yourself.

  • Good catch that the cap was on the low side, which makes my comment wrong. Just be sure that the outflow is draining -- it's pretty common for it to clog. Commented Jan 15 at 1:03
  • Hi. I took a far shot to help see the full installation. I numbered the pipes to help communication. So, it's overflowing #1 pretty rapidly now. I used the brush to try to clean out from #1 and took the cap off #2 and used the brush. Aside from calling the furnace guys, I don't know what else I should do. (BTW, thanks very much for the responses!) Commented Jan 15 at 18:52

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