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Community and Q&A

Can I drain my furnace’s condensate line into the sealed sump pit?

robkjm | Posted in GBA Pro Help on

Some background:

– Radon remediation system in place in the basement (usual sub-slab depressurization).
– Sump pit covered with plexiglass and sealed.
– Basement slab perimeter has drainage channel that wasn’t filled in by the radon folks (I gather not optimal).
– We are planning to replace the 80% efficient furnace in our unconditioned attic with a high efficiency unit since we are planning to condition the attic using a “flash and batt” approach.
– Condensate line for our current furnace and A/C unit terminates in the basement in that perimeter channel and drains there. In theory, that would drain into the sump pit via the underground perforated pipe that terminates in the sump pit (in practice I’ve never seen any water in the sump pit and the pump has never gone off to my knowledge).

Q1. The new high efficiency furnace will generate much more condensation. Should I continue to have it drain into that perimeter channel or is it better to seal the channel to improve radon remediation and drain the condensate into the sump pit?

Q2. If the area under the slab and presumably the sealed sump pit are under neg pressure, does that pose a problem if trying to terminate the condensate line there, ie is it bad to pull on the condensate with that pressure, or is it too small? The “manometer” measuring the radon pump seems to read only an inch or so of water.

Q3. If ok to drain the condensate line into the sump, are there covers that are made for this?

Thanks. Rob.

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  1. GBA Editor
    Martin Holladay | | #1

    You should consult the furnace manufacturer's installation instructions for information on draining the furnace condensate. The issue is complicated; you may wish to read this article about problems arising from draining corrosive condensate: Furnace Condensate Proves Tricky to Manage.

    Before you decide whether it makes sense to cap your perimeter French drain with concrete, you should test your indoor air to determine your radon levels. If your indoor radon levels are low, you may not need to cap the drain with concrete. That said, I think it's unusual to install a sub-slab depressurization system without capping these open French drains.

  2. robkjm | | #2

    Thanks for the response. I read the article and it looks like there are some potential solutions to neutralizing the nitric acid that flows out. I'll also talk to the HVAC contractor who we will likely use for the install.

    A few years ago I purchased a continuous radon detector (Safety Siren Pro Series 3). The levels run in the 0.8-1.5 pCi/L range which isn't as low as I'd like but lower than the threshold for putting in a system (levels were in the high 20s when we first had it measured before we bought the house, and that prompted the installation of the system by the former owners). I plan to finish the basement soon and will consult with a radon expert to see what they recommend. I'm not concerned about water so don't think we need the perimeter channel (it appears to have been created intentionally when the slab was poured 10 years ago).

  3. user-2310254 | | #3

    I am constantly amazed by how complicated houses are becoming. An efficient house is a system with many interrelated parts. It never would have occurred to me that furnace condensate might be corrosive or, as I read in another thread, that open cell foam on the underside of sheathing might be problematic.

  4. GBA Editor
    Martin Holladay | | #4

    Skilled builders are often underappreciated. There's a lot to learn to build a quality house.

    Of course, the reputation of builders is marred by builders who don't keep up to date with the latest building science. But it shouldn't come as a surprise to learn that you need to be skilled and knowledgeable to build a house. It's a profession that requires study.

  5. Expert Member
    Dana Dorsett | | #5

    A radon concentration of 0.8 pCi/L is barely above the concentration found in outdoor air in some locations. The fact that your house was in the 20s prior to the sub-slab depressurization may be an indication that your bedrock substrates are granite or some other high radon type of stone.

    In situations such as yours installing a dedicated basement-zone-only balanced ventilation system such as a small HRV may be more effective than changing-up sub-slab depressurization by sealing the sumps/drains. At your radon levels messing with the air/gas flows of the radon system may end up stealing defeat from the jaws of victory, but 50-100cfm of continuous outdoor ventilation that doesn't depressurize the basement would dilute the radon level.

    Typical condensing natural gas furnaces produce roughly a gallon of condensate per therm, and unless the condensate is being neutralized or diluted with graywater etc dumping it into the sump could become an issue over time. If you go through only 100 therms/year (not likely) it's probably going to be fine to let the groundwater/rainwater dilute it out, but if it's burning 1000 therms/year (hopefully it's less than that) it's probably not a good idea to dump it under the slab without chemically buffering it to a more neutral PH. (There are many commercially available cartridge type calcium carbonate media flue condensate neutralizers on the market for <$100.)

  6. robkjm | | #6

    Thanks for weighing in and for bringing up the HRV idea as a way to help dilute the radon. I have read about that approach here and elsewhere and have been thinking about it for a while. I have been waiting until I finish the basement to address this and other HVAC issues. It might also be reasonable since we plan to spray closed cell foam on the concrete block foundation and rim joist/sill plate. When combined with sealing the attic this will tighten the enclosure and so bringing in some fresh air might be a good thing (though I suspect I won't get down to the low levels of air exchange to mandate it). Does closed cell foam slow the diffusion of radon?

    I suppose I could do an experiment and remove the sump cover to see what that did to the radon levels--I wasn't sure if you were implying that that cover isn't doing much. Now that I think about it, I wouldn't expect it to contribute since surface area of open channel all around the basement's perimeter should be much larger than the area of the sump pit.

    Although our HVAC person said he's been installing condensing furnaces for years with no issues (apart from problems with early pumps that used metal impellers) I agree with you and don't see a downside in installing a neutralizing cartridge.

  7. Expert Member
    Dana Dorsett | | #7

    An inch or three of closed cell foam may in fact inhibit radon diffusion slightly, but it's nowhere near as effective as aluminum foil facers, or 10 mil polyethylene vapor barriers.

    If the perimeter of the slab is air permeable to the conditioned space the radon fan will pull fairly evenly through the resistance of the gravel/soil, purging the gases under the slab somewhat evenly. (It is also depressurizing the whole basement, not just the slab, but it's interrupting the radon before it's getting into the room air, sweeping it away with room air under the slab.) If you pop the seal off a sump it may create a short-circuit path where most of the movement of air from the room to where the radon fan is terminated is flowing through the open sump, letting more radon in from elsewhere in the slab.

    It's hard to guess how likely that short circuit possibility would be, but keeping the sumps sealed would be a configuration that you actually works, since it has lowered the radon levels by more than an order of magnitude.

  8. robkjm | | #8

    I hadn't thought about the short circuit effect--interesting. If I understand you correctly, your point is that although the perimeter channel is a lower resistance path than the sump, it actually could help decrease radon levels by virtue of its uniform placement, whereas the open sump will lower the resistance to flow (thus decreasing flow through the perimeter drain) without pulling out much radon. Did I get that right?

    If I recall, the levels after the system was installed (but before I asked them to seal the sump pit) were in the 2's (pCi/L) but they fluctuate and although they run closer to 1 have been as high as 2 on occasion for a few days, so maybe the cover isn't doing much. The scientist in me says I should just do the experiment. But getting back to the perimeter drain, isn't the general rule (Martin mentioned this above) to seal them to help maintain a negative subslab pressure?

    I realize I'm veering off my original question into radon practices, but when we finish the basement, would it make sense to put down a sheet of 10 mil poly (with say an inch of rigid foam on top) before putting in a subloor?

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