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Old 04-25-2015, 08:47 PM  
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Default Furnace drain corrosion - VERY strange

My 10-year-old gas furnace has a small drain pipe to carry away condensation, which I plumbed into the house system using an arrangement whereby the drain drips into an open trap. On average the furnace puts out almost a quart of water per day.

Recently there was some strange seepage on the floor around the false cabinet where I had enclosed the drip drain. When I removed the top of the cabinet and took a look I found that the trap, an ordinary 2-inch black PVC p-trap, was in two pieces with a jagged break, as if if it had been hit by a giant hammer. So the furnace drain was just dripping on the floor.

What could have made a piece of PVC break like that?

- It didn't freeze; it is well inside the house, and our heating system has never failed.
- There's no stress on the trap itself, nothing even touching it except the outgoing pipe it is glued to.
- The space it's in has been nailed shut for 10 years (and no, there are no nails anywhere near it).
- We've never had rats or any other pests, not even so much as a cockroach.

Groping into the realm of the bizarre for an explanation, could the furnace condensate contain tiny amounts of burn products from the natural gas, which very slowly corroded the PVC? I know that's probably ridiculous but I'm completely mystified.

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Old 04-26-2015, 05:16 AM  
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Condensate does contain some nasty byproducts. But on the other hand PVC is good with most of those kind of byproducts.

Do you have a photo? You could have a sample tested or google what is in it and you might find a answer.

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Old 04-27-2015, 03:25 PM  
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Photo link

The white speckles on the outside of the pipe are wall texturing overspray, and you can see rust stains from the dripping water. At the edges of the break there are places with little flaps of surface material sort of peeled outward.

One forum I read says the pH of gas furnace condensate is about 4, similar to that of coca-cola. So I guess that could be the explanation, given what happens when you leave objects in a glass of coke for a while. Interesting phenomenon

The fittings in this area are so close together that replacing the trap would required replacing six or seven other fittings, working in a very confined space next to the toilet. So to solve the problem I rerouted the drip line through the wall into the laundry room, using a dishwasher fitting to connect it above the sink trap. Essentially the same idea, but the sink trap is regularly flushed with a lot of other water.

Last edited by DougLeary; 04-27-2015 at 03:32 PM. Reason: fix photo link, add new info
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Old 04-27-2015, 05:45 PM  
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I would think if was corrosion from acidic water that the pipe would have worn much thinner before breaking. Any workers in that area in the past few months? Maybe routine maintenance on the furnace? And what accounts for the 'rust' (you did say it was PVC, right?). On the left side of the pipe in your photo, right at the break line, it looks like there was a small tee. Was anything there?
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