Standpipe used for HRV condensate getting clogged

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I have a standpipe in my basement which has a HRV and 2 minisplits draining condensate water into. So theoretically it should just be basically distilled condensate water going into the standpipe, and just dripping slowly down into it. The condensate tubing lines are only inserted a couple inches into the standpipe, so there is an air gap between the bottom of the tubing lines and where the water level would normally be around the level of the trap.

Recently the standpipe backed up and overflowed out the top.

I used a sink clog tool to pull up what I can best describe as black and pink goo. Kind of the consistency of Jello.

Once I got a few tablespoons of that up, the standpipe started flowing again when I poured water down it.

Does anyone have any idea of what the substance could be that could clog this? It's not like a washing machine standpipe, where there could be lint in the water. It should only be clean water going down.

My best guess is that some insects came in to the HRV condensate line, since the HRV pulls in air from outside and there can occasionally be very small insects that make it past the filters, and they laid eggs in the condensate water and those larvae created some sort of goo. And since water just drips in there with very little force, they never got washed down the drain. But this is just a guess and I could be completely wrong!

And then, what can I do to prevent this from happening in the future? Is there something I should be putting in the standpipe to keep it clear? Or should I just pour a gallon of water down it quickly every once in a while?



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You could have had a backup.

Do you have a drain lower than this standpipe in the basement, such as a laundry tub? If not, you may have have gotten a backup of sewage.

If you have septic, get that tank pumped.

If you have city sewer
  1. determine if there are sewer backups in your area, and if so, deal with that. Other people's sewage backing up is nasty. This event could be your early warning.
  2. If it is not the city sewer backing up, get your sewer lines cleaned out.
If there are lower drains, there still could be a blockage of some line. For example, perhaps the washing machine discharge shares a drain pipe before that pipe joins the rest of your drain pipes.

Incidentally, what you described is not considered an air gap.

It took me a while to search out that HRV stands for heat-recovery ventilator.
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both my furnace condensate pump and dehumidifier condensate line produce a clear slime every once in a while. I assume that the chemical nature of the condensate - which I assumed to be distilled water - promotes some kind of bacterial growth. A bleach solution in both condensate lines have resolved the issue for me.
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