I have a Carrier high efficiency furnace which uses 2-inch PVC pipes to pull in combustion air from outside and exhaust the combustion gasses back outside (through a concentric vent, but that's not really important.)

It seems that the installer drilled a 0.5" hole in the exhaust pipe about 2 feet up from the heater.

picture of heater with hole

When the heater is running, you can feel air coming out of this hole, it is warm and moist, and it smells like you would expect natural gas exhaust to smell. (Before you ask, yes, there IS a carbon monoxide detector right outside the door of the room, and no, it does not detect anything.)

The issue I have is that there is a lot of water vapor present in the exhaust, which condenses on the inside of the pipe and then drips out through the hole, collecting in a large puddle on the floor. (If you look closely at the picture, you can see the trail of water below the hole.)

(I notice that my neighbors always have clouds of water vapor coming out of their exhaust on cold days, but I never do. As far as I can tell, the reason for this is that their exhaust pipe is only a few feet long since their unit is right next to an exterior wall, while my pipe is 30 feet long so the water has space/time to condense.)

In any case, the puddle is annoying and messy, and the room is too narrow to just put a bucket there -- it would obstruct the walkway to the washer/dryer.

Right below the exhaust pipe (not shown in the picture) is the condensate pump for the heater drain and humidifier drain. (For some reason, they piped the AC drain directly to outside but the other two drains to a pump which goes to the sink.)

  1. Am I correct that condensation in such a long pipe is normal?
  2. If not, what is wrong with my furnace?
  3. If I seal up the hole so the condensation has to drain back through the unit and out the actual furnace drain, will that cause any damage?
  4. If I can't seal the hole, is there a simple way to catch the moisture at the hole and direct it into the condensate pump instead of onto the floor?
  • Call the installer and ask him if the hole is intended, and necessary, and if so what's going wrong. Commented Dec 12, 2019 at 18:59
  • 5
    I've seen holes like that for efficiency measurements. They're usually filled with silicone caulk. Perhaps the plug fell out.
    – isherwood
    Commented Dec 12, 2019 at 20:41
  • If that room is used for laundry, it should have a CO alarm in that room. I would be uncomfortable with only one alarm in this situation. Commented Aug 19, 2023 at 14:31

1 Answer 1


The short answer is this: it makes no sense for a sealed combustion appliance to intentionally leak exhaust gases to the building interior. There was probably a legitimate reason for the hole to be drilled -- maybe there was a need to measure the pressure inside that pipe to assure the installation was okay, or as part of the troubleshooting of a "blocked exhaust" error code thrown by the furnace controls -- it's difficult to be sure. In any case it seems reasonable to seal the hole.

I can't speak for your specific appliance, but in general, yes it's normal to have condensate when longer exhaust lengths are involved. The liquid may flow to the exterior or back to the appliance depending on the slope of the exhaust. Some appliances are designed to receive and drain the condensate; others are not (but a condensate catchment accessory may be available). Consult the appliance installation instructions for details.

You mentioned both an air conditioner drain and a heater drain. Does the heater drain originate in the inducer fan housing? If so it sounds like your furnace is equipped to deal with condensate. I'd verify that the drain is working, maybe by running a few ounces of water in through that exhaust hole and verifying that most of it flows out the drain. Then seal up the hole in the exhaust pipe.

Exhaust condensate is somewhat corrosive (acidic), so it should be handled with materials designed to deal with it. A puddle of condensate on the floor is Less Ideal.

  • Yes, the heater drain comes from the inducer housing.I know that it handles the condensate properly, because I can occasionally see the condensate pump running. I suspect that if I close up the hole the pump will just have to run more frequently, but the question then is really just if there is a continuous downslope from the exhaust to the drain. I will see if the manual says anything about that.
    – Moshe Katz
    Commented Dec 12, 2019 at 21:05
  • 6
    I had an HVAC technician here for an unrelated reason. He saw the bucket of water on the floor, saw the hole above it, and immediately said "that is for an exhaust gas analyzer, but there should be a plug in it!" He put in a plug, and the dripping is gone.
    – Moshe Katz
    Commented Jan 22, 2020 at 17:29
  • 1
    @MosheKatz: that looks like an Answer to me!
    – keshlam
    Commented Aug 19, 2023 at 13:24
  • 1
    @keshlam I considered it a confirmation that Greg's answer is correct, that's why I didn't post it as a separate answer.
    – Moshe Katz
    Commented Aug 20, 2023 at 2:05
  • 1
    @Mazura They did not charge me anything extra for using the analyzer when they were replacing the gas valve and were making sure it was adjusted properly.
    – Moshe Katz
    Commented Aug 20, 2023 at 2:07

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