I have this sticky black liquid slowly leaking from my gas furnace / ac unit. I called an HVAC repairman and he wasn’t sure what it was. He said he thinks it’s an old drip pan made of tar (not sure that makes sense.) He said he cleaned it up a bit and to call if it started leaking again.

It’s one week later and it’s started again. I’m not sure what it is or if it’s a fire risk. He said the AC unit sits above the furnace and that it’s coming from the AC. It’s winter here so the AC unit is never running, just the furnace below it. I included a picture, does anyone have any idea what this is and what’s causing it?

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  • Gas oil or electric furnace?
    – Kris
    Commented Feb 10, 2020 at 3:37
  • Sorry I forgot to include that, it’s a gas furnace.
    – Mmak407
    Commented Feb 10, 2020 at 3:42
  • 1
    If the ac coil is leaking Freon and oil it would make this type of mess. when the heat is on the goo get warm and runs down into the condensate tray which may be a mess with sludge Have a coil inspection and cleaning done. Have the Freon checked to determine if it has any left. Does the sticky goo have any odor?
    – Kris
    Commented Feb 10, 2020 at 4:07
  • 1
    Are there smokers in the house now or in the past? The pan could be tar sealed but that went out of fashion 20+ years ago. Your hvac guy should have suggested cleaning the coil, it could be leaking but if the oil is black the system is ready to fail. I would think a dirty coil set may be the root issue and it may need a good cleaning with a non acid coil cleaner usually in heating mode moisture is not generated in the coil set but excessive dirt or a leak that is mixing with a dirty coil could be the issue.
    – Ed Beal
    Commented Feb 10, 2020 at 5:17

1 Answer 1


Has the combustion been checked on this furnace recently and is the heat exchanger in good shape. Are there any odors in the house, people getting headaches, nausea, etc. If he said that it looks like it is coming from an old drip pan made of tar get someone else to check your system. You may need to gain access to the duct work to find the source of the black sticky stuff.

  • The repairman said that he thinks the system needs to be replaced in the near future so the wife and I may just bite the bullet and get it replaced. He said the furnace looks good but the AC unit is going. We don’t have any headaches, nausea, etc but there is a smell in the foyer. It almost smells like the odor that’s given off when the furnace is first turned on when the colder months come. He checked the ducts but didn’t see anything. Does it just make sense to replace it? How much does that usually cost? Thanks everyone for all your help and advice.
    – Mmak407
    Commented Feb 13, 2020 at 1:34
  • @Mmak407 "How much does that cost" is unanswerable based on the info we have, which is "none". Your location, the size of your hose, the heating load, and many other factors go into determining cost. You'll have to contact several HVAC contractors in your area and get quotes.
    – FreeMan
    Commented Jul 9, 2020 at 13:16
  • If this has been going on for a while and you still get cold air it could not be the freon system, it only holds a few pounds of freon. If nothing was spilled into the furnace it could possibly be mold or tar. When the AC is operating water is condensed on the coils which could possibly wash out mold or other organic growth.
    – Gil
    Commented Jul 4, 2021 at 19:39

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