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solution for the chlorides leaching out of the venting systems

Jean-David Beyer
Jean-David Beyer Member Posts: 2,666
This was inspired by another very long thread. It seems more appropriate to make this a separate thread.

My mod-con has a PVC vent pipe. The installation instructions specifically say the vent pipe should be angled down towards the boiler, so any leaching of chlorides will dump into the boiler.

Now the way the boiler, heat exchanger, etc., are designed, these chlorides will not enter the heat exchanger, but will drain right out the same way the boiler-produced condensate goes, into my mostly-plastic condensate pump.

Now the previous thread is about the suitablility of PVC pipe for venting boilers, especially mod-con boilers, so clearly the way to solve the chloride problem, if it is a problem, is to use stainless steel exhaust pipe instead of PVC.

But I wonder about two things.

1.) In a system such as mine, what is the problem of chlorides in the vent pipe coming back to the boiler, if they go right out the condensate drain without going through the heat exchanger?

2.) Why does the manufacturer say the vent pipe must slope back to the boiler, rather than sloping toward the discharge where the exhaust and "steam" go? Clearly there should be a slope so it does not collect in the pipe and freeze, but why does it matter which way the slope goes?


  • Charlie from wmass
    Charlie from wmass Member Posts: 4,281
    For question # 2

    they want the waste to be controlled and disposed of in compliance with codes. As in neutralized and into a waste disposal system. If sloped away it would be a poisoned piece of ground along your foundation wall as far as plants were concerned and could pose a slip hazard in the colder weather for people walking past.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    OK. That is funny.

    It is funny because the condensate from my boiler goes into a condensate pump where it goes through the wall and dribbles on the ground. None of the inspectors even asked where it went. Two years now, and the flowers do not seem to mind. No one would slip in either place when it turns to ice. I do not have a neutralizer. I guess I should clean a bucket and catch some condensate and measure its pH. I cannot do it at the height of the heating season because it turns to ice.

    If there were a drain anywhere near my boiler, I guess I would neutralize it nid run it into the drain. I would have to rip out some plaster walls to run a condensate line through three rooms to get to the nearest practical drain.
  • icesailor
    icesailor Member Posts: 7,265
    Acid neutralizers:


    They sell an acid neutralizer that connects to the condensate drain as it comes out of the boiler. It neutralizes that acids, raises the PH, and you then drain it into the condensate pump. You can then pump it where you wish ( providing that codes allow it of course) but the waste water is neutralized.  Trust me, if you dump it into your garden, and you have acid loving plants, they will be happy. For all others, it is death. Unless you heavily spread lime to neutralize the PH or the contaminated soil. In fact, that isn't a bad idea. With that non-existent acid rain and the3 non existent global warming, the plants will love you for it.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Unless you heavily spread lime

    I do not spread any lime. I have never measured the pH of the soil around here, so I do not even know if the soil is too acid or too basic. I have been dumping the condensate outside my garage wall for two years now. I burned 955 therms of gas since this boiler was installed, so assuming I condense all the water out of the exhaust (upper limit of what I do), we could estimate the amount of water dumped. Another way to estimate was that I was getting a 2 gallon pail full of water every day during the part of the season where the outside temperature was near, but above freezing; i.e., quite mild. The plants there grow slightly better than those nearby. Mostly daffodils. Whether this is the result of the acidity or just the increased amount of water I cannot say.

    I have not measured things, but right now the condensate drain from the boiler is so low it barely clears the input hole on my condensate pump. I might have to cut the pipe to even get the pump out of there unless the top comes off the reservoir. So it looks as though it makes more sense to put the neutralizer after the pump. If I do that, I would have to mount the neutralizer vertical with the water coming in the bottom and going out the top. I guess that is not a problem. There is a check valve on the pump output connection. This is all in my unheated garage, and I would not want things to freeze. But in two heating seasons so far, the reservoir in the condensate pump has not frozen.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    OK, I'll share my secret....

    JD, your boilers condensate comes back to an aluminum receiver that is subject to the corrosive tendencies of the acidic condensate, and there have been some cases of failure of that piece of aluminum failing early.

    As for your other question, rolling the condensate out towards the vent terminus will result in LARGE stalagtites and stalagmite of ice. If one of these deadly acid flavored popcicles should happen to dislodge itself, it could result in immediate and devastating death to the head of an unfortunate recipient below. Hence, why the condensate is not allowed to roll towards the vent terminus. In some commercial cases, the vent terminus is 10 to 20 feet above the ground.

    Buderus' aluminum boiler has a tee on the exhaust immediately after it leaves the boiler. All vent produced condensation is directed to the boilers external condensate drain and does not go through the aluminum boiler. I did something similar with my Knight Lochinvar up in the mountains.

    Interestingly, I have a catch bucket directly below the boilers heat exchanger drain, and it doesn't see nearly the condensate that the venting system see's. I've not clocked it, but I've had to pump the vent receiver bucket probably 10 times,, and the boilers condensate hasn't even gotten to the half way point yet, and my boiler doesn't operate at temperatures of above 150 degrees F.

    Hope that helps.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Oh! Poo!

    "JD, your boiler's condensate comes back to an aluminum receiver that is

    subject to the corrosive tendencies of the acidic condensate, and there

    have been some cases of failure of that piece of aluminum failing early."

    Grumble. And I suppose that is all part of the heat exchanger, so if that quits, the whole thing has to be replaced. 8-( Wishful thinking: if I get more condensing in the heat exchanger, would that flush out the stuff (chlorides) running back from the vent pipe more than if the boiler were not condensing? Should they not put a hard anodize, or maybe an epoxy coating on the inside of the reciever to protect the aluminum from acids? Of course, bases will eat aluminum too.

    If I end up replacing my PVC with polypropylene, I suppose I could put a T in there too. Is it enough to use a 3x3x1/2 Tee in there (if there is such a thing), or is a trap necessary there to not screw up the pressures?

    I notice I get stalegmites at the exhaust end when it is very cold out. I even measured the vent pipe with a level to be sure it was sloped toward the boiler and it is. No doubt it would be worse if it were sloped the other way.

    Of course, I had hoped that all the water vapor would condense in the heat exchanger (dreaming, I know). But the fact you get more condensing in your vent pipe than from your heat exchanger is disappointing. I am not sure if my average return temperature to my boiler is less than yours or not. I did what I could to keep the supply temperature below 135F, and that is reached only when it is 4F outside. Since design day temperature is 14F, I have not seen it go below 9F here ever (that means in the last 2 years). The zone that takes the most heat is the radiant slab zone that gets at most 120F supply, and at least 75F. So I should get lots of condensing there. The other zone goes from 110F to 135F, but it is a very small zone that does not run more than a couple of hours a day. The indirect runs at 175F nominal, but it often recovers before the boiler gets all the way up to that. I have noticed it condenses some when heating hot water, probably on the ramp up. I guess if I had the patience, I would clock the gas meter and measure the condensate for 24 hours and see what % of the theoretical maximum condensate I actually get.

    Based on John Siegenthaler's graph of efficiency vs return water temperature, I would not expect much condensing at return water temperatures much over 130F. I recognize that that graph is typical and not guaranteed for every combination of boiler design and exact composition of the fuel. I guess condensing boilers do best in snow melting applications, but I do not do that.
  • Tim McElwain
    Tim McElwain Member Posts: 4,593
    JDB just had a chance to read

    this posting. The issue is not chlorides in a liquid form getting back into the combustion chamber it is the chlorides in the vapor (products of combustion) in the flue that once the blower shuts down and there is still enough temperature to keep the vapors in a vapor state that they get back into the chamber especially as we have sometimes more Delta T inside the boiler and furnace than we do in the vent. One manufacturer even went to the trouble of adding a post purge circulator to allow some running time to cool the chamber to prevent this from happening.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    Buderus I&O manual

    See page 22 for their drain configuration for the vent drain tee, and yes, they do make a 3 x 3 x 1/2" tee, although I'd recommend a 3 x 3 x 3/4" tee to avoid possible issues with condensate scum.

    W/Mc supposedly increased the thickness of the casting to avoid issues with the flue gas receiver/condensate drain burnout.


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    adding a post purge circulator

    My boiler does not have a separate circulator that runs post-pump, but the controller allows circulators to run for a time after the thermostat calling for heat is satisfied. The default post pump time is 30 seconds. For my slab zone I have increased it to 90 seconds and for my indirect it is 120 seconds. The last zone runs only 30 seconds for the boiler circulator, but that does not matter much because the zone circulator shuts off. Since the circulators are Taco 007's, that should empty the three quarts from the heat exchanger pretty quickly. It does not get the heat exchanger temperature al the way down to room temperature, but it gets it down to the return water temperature pretty quickly.
  • VictoriaEnergy
    VictoriaEnergy Member Posts: 126
    edited May 2011
    ...leaching out of the venting..?...?

    I thought the chlorine and chlorides in PVC are stable, and in a functionally inert form in gas vent systems.

    I've only practiced avoidance drawing contaminated combustion air with chlorides occurring from other sources, such as swimming pools, hot tubs, laundry rooms, cloths dryer vents etc.  Chlorine from these sources are chemically available and harmful to practically any combustion chamber......, or so I thought. 

    Anyone aware of significant chloride leaching issue demonstrated in our industry??
    Home Owners Please Note:

    You are receiving advice from some very skilled pros completely free of charge. One of the reasons I participate is to sharpen my own troubleshooting skills. So; did we get it right? I would be grateful if you extend this courtesy back by posting the final outcome of the issue you are inquiring about. Thanks
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840

    Great question.

    I'd heard that it was leaching back in the condensate. I'd NOT heard, as Tim explained, that it was a gaseous issue.

    Based on personal experience, I find it hard to see how a vapor would make it back to the combustion chamber, A great convective loop was established while the burner was on, and would continue until equalibrium air temperatures resulted, stopping any convective currents.

    Most all of the Gianonni HXered boilers I've worked with already have a time delay program for their circulators, and the time frames are adjustable. Not really necessary to run them for a long time due to the lack of fluid content. Differential temperature reaches zero in short order.

    Now, with some of the aluminum or fire tube down fired conditions, I can see how it would be difficult to maintain a shut down differential. Maybe instead of a pump delay, they need a longer post purge run time on the burner blower.

    It would be interesting to see exactly what kind and how many ppm of chlorides there actually were in the resultant condensation from leachate...

    Not questioning the validity of your response Tim, but wondering if you can share your source? Would they be willing to come here and share their experiences?

    Hopefully we didn't chase off your friend on the PVC issue.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Henry
    Henry Member Posts: 996

    A number of years ago, I built an aquatics lab for one of the universities. It was some dozen 1200 aquariums. All the water was filtered and even carbon filtered to remove chlorine etc. Well, the fish stated to die. There were chlorides in the water. The Engineer had us replace the carbon filter by one that was 120 gallon. It still made no difference. But, when we replaced the filter, I noticed that the old PVC sch 40 pipe smelled and the new did not. As the water was so pure, the chlorides were leaching from the pipe. We finaly installed a baking powder feeder to balance the PH, problem solved.

    I replaced my "Complete Heat" several years ago. The PVC venting had some yellow stains and smelled similar to the fish piping that was leaching chlorides. I have never seen any similar problems with ABS DWV pipe. Also note that ABS has a higher softning temperature (217F) compared to PVC or CPVC and a muchlower expansion rate.

    I have done some lab work in Germany with PPE pipe for use on vents and testing under the S636 standard. The pipe is softer but more resistant. Fused joints will not leak.

    We have had much more problems with AL29-4C than any other venting product. The usual ones are pinholes in tee, elbows or caps.

  • icesailor
    icesailor Member Posts: 7,265
    PVC Chlorides:

    Is there any PVC pipe manufacturer that specifically lists their DWV pipe for exhaust gas use? From what I've read from Charlotte, the only approved application they approve is with potable water drains, sewerage (plumbing) waste and plumbing vents. If a boiler manufacturer lists an approval for a product that isn't approved by the product manufacturer, does it make the "listing" legal?

    As far as the pitch of the exhaust, somehow in my brain, I thought an additional reason to pitch it back was to get any other possible heat out of the exhaust gas by condensation. Where the gasses give off additional heat to the condensed water. The colder the exhaust, the higher the efficiency rating.

    I thought.
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