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-   -   Condensing gas furnace and acidity (https://www.johnbridge.com/vbulletin/showthread.php?t=131825)

makethatkerdistick 01-06-2022 10:36 PM

Condensing gas furnace and acidity
I love my condensing gas furnace. 97% AFUE rating, very low operating cost and keeps the house warm quietly with variable speed blower and inducer running on low, but I knew its condensate was acidic. The installer directed it into an old copper drain line that terminated in my cast iron plumbing.
He should not have done that.

Last year I built a new condensate drain line to the exterior, using PVC. And I am glad I did. I recently got a digital pH meter and have been playing a bit with it. My furnace’s condensate has a whopping pH value of 2.9. If unmitigated, this will surely eat away quickly at your metal waste plumbing, especially since there is a lot of condensate produced when the furnace runs, even on the lowest output.

Just thought I would share this.

Boilermaker 98 01-07-2022 10:50 AM

If running it to a PVC drain line or outside is not an option, they make a condensate neutralization kit for this purpose. Or take some 3" PVC pipe and fill it with washed limestone (no fines) rock and then adapt it to your condensate drain line size.


Above are several options.

makethatkerdistick 01-07-2022 04:37 PM

Great ideas! My former contractor should have put in the neutralizer. He probably does not know to this very day what he is doing. Many a replacement install in attics in older homes here. One day, one of his customers will have a soaked ceiling and other water-related damage. Unbelievably ignorant.

jadnashua 01-09-2022 05:00 PM

Running it outside without neutralizing it first can kill your grass and maybe eat out the concrete slab or mortar of the foundation. Running it down the drain without neutralizing can get costly down the road.

I live in a row of townhouses. Over the years, many of the people replaced their furnaces with higher efficiency units that are condensing...nobody put in neutralization. We may need to replace our main cast iron drain line at some significant expense as a result. I've tried to get people to recognize the problem. If I did it to mine, it wouldn't really help, as there are nine others dumping acid into the line that goes through my place (internally, we have PVC, but the main line between us is CI).

The pH meters often come with calibration powders you mix up in distilled water during the calibration procedure. 2.9 is a bit low, it usually is in the area of -4 or so for condensate...still acidic and about what acid rain is on average.

PhilWA 01-10-2022 01:39 PM

I installed a condensing furnace a couple years ago and don't remember any mention of acidity in the furnace installation manual! I've seen a few high efficiency furnaces installed in other homes and never saw a PH neutralizer in those few examples. I used litmus paper on my condensate and it was neutral, no orange or red.

Anyhow, I poured some baking soda down the drain and ordered an expensive plastic tube filled with limestone for installation next week.

Thanks for the posts!

makethatkerdistick 01-11-2022 11:25 PM

Jim, mine is 2.9. Where do you see that it is “usually in the area of 4”?

Boilermaker 98 01-12-2022 08:49 AM

I think the acidity is based upon a few factors.

1. Region of natural gas. If the natural gas is from a deposit with sour oil, the gas will be more acidic.

2. Is the furnace a 93%, 95% or 97% efficient unit. The higher the efficiency, the more concentrated the condensate will be.

3. Was the furnace tuned with a flue gas analyzer to achieve the best ratio of air to gas. Incomplete combustion allows for more of the natural gas to carry over in the flue gas stream.

jadnashua 01-12-2022 03:54 PM

From what I read, 2.9 is the low end of the range which is -2.9 to -4. So, -2.9 is still within the normal range, but on the edge.

Regardless, it's still pretty acidic. A common vinegar is in the order of -2. Drop a seashell in some, and it will be gone in a day or so depending on how thick the thing is. Acid rain is in the order of -4.2 and 4.4, but that's an average.

makethatkerdistick 01-13-2022 10:06 PM

I think Brian's comments make sense. Mine is officially a 98% unit, so the condensate is as concentrated as possible, thus maximum acidity. And no, I never had a combustion analysis done. Might be something to consider at some point. I have done periodic gas valve adjustments with a manometer to set the input BTU properly, though. Based on its low consumption and temperature rise I think it is running within specs.

I collect the condensate in a big bucket outside and pour it in the street every week or so as to not kill the vegetation close to the house. Some installers here sell 90+% as premium options (they're not standard in Texas) but then don't know much about specific installation requirements. Nor does the inspector, but that doesn't surprise me at this point.

Redlands Okie 01-17-2022 08:27 PM

If I understand it correctly here in north central oklahoma they are supposed to set a barrel like container into the ground, fill it with gravel, and run the condensate into it.

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