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Community and Q&A

Where should I drain condensate from a high-efficiency furnace?

BobAllison | Posted in Mechanicals on

I have a high-efficiency furnace, installed in 1995 and I now do not think the condensate drain was installed correctly. It has now corroded and is leaking inside my basement.

The condensate line was run from the pump through 39 feet of 1/2′ clear flexible “fish tank” tubing up into the exposed joist area in the utility room and over to the laundry drain pipe. The run was secured to pipes and things by looping it around objects and with tape. The drain end of the tubing was attached to a 6″ piece of copper tubing (3/8″ I think but I can’t tell now) and that was stuck into the laundry drain.
The copper tubing has now corroded away and the drain is now hanging free dripping on the floor behind the washing machine.

Here are what I think are my issues. 1) I have a septic tank so I do not think they should have ever drained the acidic condensate into it. 2) it is a 1966 house and has galvanized drain pipes and cast iron sewer pipes, not more modern plastic, so I do not think they should have drained into those pipes where the acid could corrode them. 3) From what I have gathered online there may be an issue with there being enough of an air gap with the washer and condensate line both into the laundry drain. 4 )I live in Massachusetts where I am guessing they didn’t drain it outside because it could freeze in the winter.

But I have noticed when walking my dog that there are other houses in the neighborhood with small pipes going outside near the plastic flue pipes in some homes. So I am guessing they must be other high-efficiency furnaces and I am guessing the small pipes are the condensate drains. In a couple cases, it looks like there is a separate 1/2 or 3/4 PVC pipe coming out of the wall. In another, it looks like there is a small drain pipe inside the flue pipe leading out. Maybe this was done somehow to keep it warm in the winter to keep it from freezing.

Can I redo the drain to go outside? How? I can’t find instructions online. Maybe someone can point to them if they are there.

I am concerned now that the contractor may have messed up bad enough that I worry about the galvanized laundry drain in the wall and about the cast iron sewer below the concrete floor.
Do I need to open the plaster wall and check the laundry drain? Do I need to hire someone to run a camera to inspect my main line? If I do and there is a problem how do I get the installer to repair it after this length of time?

Thanks for any help or advice anyone can provide.
Bob Allison

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  1. GBA Editor
    Martin Holladay | | #1

    There is no simple answer. You may need a system to neutralize the liquid condensate before you send the liquid to your drainage system. One brand of condensate neutralizers is NeutraSafe.

    For more information on this issue, see Furnace Condensate Proves Tricky to Manage.

  2. Expert Member
    Dana Dorsett | | #2

    Don't sweat the acidity of the condensate going into the septic tank. The volume is low (relative to daily drain flows of grey & black water) and of things go down the drain that are at least as acidic as natural gas or propane condensate. It's strong enough to react over time with copper drains, but with iron, not so much.

    The lower the pH, the more acid the solution is. Even the most-acid gas-burner condensate has a pH no lower than 3, most are between 3-5. Dry wine has a pH of about 3.3 (about the same as orange juice), even sweet wine still under 4, typical soft drinks are between 2.5-3.5, cranberrry juice is between 2.0-2.5.

    So the only real mistake in the original installation was the use of copper, which is far more reactive to acids than iron/steel. Brass/bronze and other alloys of copper will have the same issues when chronically exposed to condensate. With drains down stream of a dilution point such as the laundry sink hardly matter what the material is, since in a typical applications there is many times greater flow of the washer outflow as there would be from a condenting natural gas or propane burner mixing in with it.

  3. Jon_R | | #3

    Condensate eventually destroyed a steel floor drain cover I had.

  4. Expert Member
    Dana Dorsett | | #4

    Undiluted, I imagine.

  5. Jon_R | | #5

    Yes. I'd be careful counting on dilution - sporadic laundry tub use could leave a lot of hours of acid contact near the tub.

    With plastic lines, I wasn't worried about corrosion - but I put a few limestones in the pump reservoir anyway.

    1. Deleted | | #15


  6. AlanB4 | | #6

    This is very interesting, i had a high efficiency natural gas furnace installed 4 years ago, its condensate is going into a laundry tub that has copper drain pipes (and no stack so it drains slowly). How concerned should i be, being near Toronto Ontario with a 99% design temp load of about 25K?

  7. Expert Member
    Dana Dorsett | | #7

    Alan: With copper drains playing it safe with a neutralizer/buffer cartridge is probably the best, (unless you run a couple loads of laundry daily during cold weather.)

  8. AlanB4 | | #8

    What is a neutralizer/buffer cartridge?

  9. walta100 | | #9

    What is a neutralizer/buffer cartridge?

    It is a section of clear pipe filled with crushed limestone. The any acid will attack the limestone and be neutralizes so any pipe after the neutralizer will not be damaged. The pipe is clear so you can inspect it and replace it before all the limestone is dissolved.

    For a photo click on the link in Martin’s reply


  10. BobAllison | | #10

    Thanks, everyone. Great resources Martin, thanks.

    The thing is Dana I am very careful with my septic system. I don't dump any of those things down my drain in any except tiny amounts from an empty glass, nor raw food, grease or oil, or other chemicals, and we are also careful about not putting too much water through each day too -- and there is a lot of condensates.
    When the kids were around there was laundry almost daily so the laundry drain was likely not to too bad off then, but for the past 15 years there has only been laundry each week, sometimes over a week, so I do worry about the galvanized drain where the acid could sit. And there is a reason why nowadays so many jurisdictions prohibit dumping un-neutralized condensate down a drain.

    I am looking into installing a neutralizer, maybe a homemade one. I am also looking into a way to pipe it so it dumps outside but has a bypass (automatic with no moving parts) to the drain if the outside drain freezes.
    If I come up with something I'll come back and post it for everyone's info.

    Thanks all. Bob

  11. AlanB4 | | #11

    Interesting, i wish someone had mentioned this to me before...
    At what pH is water going to start breaking down copper pipe?

  12. walta100 | | #12

    If you burn gas in a condensing furnace and have metal drain pipes you need a neutralizer.

    What the PH number you have as this moment does not matter as the gas suppliers are constantly changing the mix they supply so your, PH will change with the mix.

    I had a copper drain in my furnace it lasted for 10 years before leaking.


  13. AlanB4 | | #13

    @Walt thanks for that

    I am curious what the range might be and and what pH will start harming the pipe. Unfortunately i can't afford to make major alterations and the nature of my setup makes retrofitting very difficult so i have limited facilities to prevent this problem. I wish someone had mentioned this to me 4 years ago but i can't undo the past.

  14. BobAllison | | #14

    In a manner of speaking any degree of non-dilute acidity will cause corrosion in copper, it is just a matter of how long it takes. A month or 10,000 years. Copper is usually fairly non-reactive.
    It depends on the thickness of the metal, the strength of the acid, and the length of contact time before it is washed ways by the water from the washer or the water in the summer from the AC condensate which usually drains in the same system if there is an AC.
    Bendable tubing, like I had, is thinner. Copper piping comes in two thicknesses, "regular" for water piping and "heavy wall" used for heating piping.

    Reactivity is in this order: platinum, gold, silver, mercury, copper, lead, tin, iron, zinc, aluminum, magnesium, ... Anything more reactive than gold will react with oxygen (silver tarnish, copper patina) and anything more reactive than copper will react with dilute acids and even water.

    for why copper reacts with nitric and sulfuric acids.

    As a point of reference, it took 23 years for the 5" piece of copper tubing that connected to the vinyl tube led into my drain to completely be eaten away. It probably started leaking 3-5 years ago but I always just thought it was the washer leaking.
    - It does leak occasionally because we never use the bleach dispenser and it gets filled with lint thus causing the mechanism to overflow if we don't get it cleaned often enough.

  15. AlanB4 | | #16

    I've been giving this thread some thought recently, Limestone was mentioned above, can i buy something like the link below and add stones to the condensate pump reservoir to solve the problem (refilling as needed)?

  16. walta100 | | #17

    I think adding rocks to the condensate pump is a bad idea because

    1 Putting rocks in the pump makes the tank hold less water so without the rocks the pump comes on and empties the tank. Just guessing but about 40 ounces, if it is full of rocks 15 or 20 ounces. So now the pump runs twice as often and wears out sooner.

    2 The acid keep dissolving the rocks at some point a rock will be small enough to enter the pump and get jammed and stop the pump.

    3 The rocks could get under the float so the pump may not run when needed.

    You can buy the condensate acid neutralizer for $50.00

    In a lot of places the crushed stone bed under every road, drive way or basement, is limestone especially if it is snow white.


    1. AlanB4 | | #18

      1. I don't care much about the life, its old anyways so if i reduce its life a few months its not a huge deal

      2. I thought of that but the weight of the rock will probably not make this a problem

      3. This could be a concern though would it not be the other way around, the pump keep running dry?

      I am in Canada so i don't know where to get a neutralizer but $50 is not a bad price and if i can grab and retrofit it easily i don't mind spending the money.

      If i were to go with limestone in the pump reservoir $6 instead of hunting for it is not a big outlay.

  17. AlanB4 | | #19

    I have been having trouble finding a condensate neutralizer in Canada thats under $100 from amazon. I'm not sure if i am googling the wrong thing or they are just incredibly uncommon. If anyone knows of any from a big box store or somewhere at a half decent price please post it.

  18. AlanB4 | | #20

    Is cast iron as susceptible to condensate as copper?
    In the interim i can perhaps rerun the line to an upstairs sink which i think is cast iron till i can find a neutralizer. I'm still considering the limestone in the condensate pump if i have no luck finding one easily in Canada.

    1. joshdurston | | #21

      I had the same problem (also in Canada), all the online outlets wanted $100 or more for what is essentially a plastic tube of gravel. I made my own for about $15. I got about 25lbs or limestone chips for free from a local landscaping depot. (It was such a small amount they said don't worry about paying.) I used some 2" abs pipes installed a union so I could open it up to inspect and fill, and put a hose barb on each end. Only downside is that it isn't clear, so I just open it up annually and top up the limestone chips.

      1. AlanB4 | | #22

        Now thats clever :)
        Can you link what you used to make yours?
        I don't have any chips handy so i was just going to buy a bag of stones which should be a lifetime supply

  19. HouseRepair632 | | #23

    39' of 1/2" id. tube will hold a fair amount of water, and depending on how high the pump has to lift that water and the slope(s) of the tube run, your pump may be running a lot longer than needed, especially if there's no check valve at the pump outlet. I've seen installations like this where the water in the tube keeps draining back into the tank after the float switch in the pump tank shuts off the pump which causes the whole cycle to start up again and continue repeating until enough water has been removed, a bit at a time. I'd suggest watching the pump work, pour water into it's tank if you have to, and if what I described is happening, install a check valve if there isn't one and a new, smaller diameter tube that starts sloping down asap from a high point as close to the pump as possible so gravity can help you out. Also, don't loop it around other utilities etc. as every bend increases resistance to flow.

  20. AlanB4 | | #24

    In my case the condensate is going from the furnace to the laundry sink (with copper drain pipe) so i'm thinking of having the condensate pumped to an old protein powder container filled with limestone and a few holes drilled near the bottom. Is Limestone such as the link below the correct medium for neutralization?

    This links has the following MSDS which has calcium carbonate and magnesium oxide and quartz

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