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Community and Q&A

Effects of 2″ to 3″ PVC dry drain pipes through crawl space

ylekyote | Posted in Mechanicals on

Good evening. I have a 2,600 SF home that has 900 SF crawl space under half of it. The builder placed all the overflow type drains (next to water heater/washer, next to my mechanicals, etc, to run to daylight into my far yard. These don’t have water traps because they run to daylight I suppose and rarely if ever have drain water in them.

My house does not use A/C and in winter we currently use a large indoor wood burning stove and supplement cold rooms with electric oil radiant heaters, as-needed.

What are the effects of 4 to 5 two-inch diameter PVC drain pipes leading to larger 3″ pipes that find daylight, without water traps?

Should I leave it this way?

Can or should I install a large check valve in the two 3″ pipes they feed into the yard, to block outdoor air from sucking into house? The 3″ open drain closest to house foundation (2′ exact) seems like air is going into it, toward the house.

Not sure if the fresh air is good or bad, especially since I burn wood in a modern EPA stove. We also have a gas water heater chimney stack and another small gas stack for a wall heater we never use. And two roof vented bathroom exhaust fans. Man, all this seems to be a lot of holes to fill. Good or bad?


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  1. GBA Editor
    Martin Holladay | | #1

    I'm trying to visualize these drains. Does your crawl space have a concrete floor? Do you have to crawl into the crawl space on your hands and knees to access your washing machine?

    Some people do, indeed, create holes in the wall to introduce outdoor combustion air into their house (to supply the needs of a wood stove). This method works -- and in fact, in a very tight house, this may even be necessary. However, these combustion air ducts should be carefully sized -- the holes are usually smaller than the drains you describe -- and (ideally) these holes shouldn't be open all the time. It's a good idea to have a ball valve on these combustion air ducts so that they can be sealed if necessary.

    In short, I think that your house has too many holes in it. As a result, you are incurring an energy penalty -- it takes more heat to keep your house warm than it should, because your house is leaky. One option is just to cap the drains -- after all, if your water heater or washing machine ever sprang a leak, how much damage would really occur (especially if these appliances are located in a crawl space)?

    Another option is to install check valves at the outlets. These check valves introduce small risks -- they need to be monitored as part of a routine maintenance procedure, because check valves can become clogged or can seize up -- but the risks are probably acceptable.

  2. ylekyote | | #2

    The drain pipes are in the floor, next to the appliances, over or besides the crawl space. House is rural and has septic, but instead of running the pipes to septic, he ran them to countryside.

    Even though the drains aren't frequently used, they do sometimes get water in them from my cistern when I overfill it (done so manually with wellhead pump switch).

    So I have about 4 small drain holes going to daylight, that at some point consolidate into two 3" PVC drains in the yard, and the usual roof vents for toilets air an 3 combustion chimneys (1 large for wood stove, and 2 small ones for hot water, unused gas boiler, and wall heater.

    Are there gravity type flapper valves that will let water out but seal (at least mostly) to keep air from sucking in them when unused?

    I can seal up my 2 unused combustion chimneys, but can't do anything about the breather roof vents for toilets.

  3. GBA Editor
    Martin Holladay | | #3

    The valve you want is a 3-inch backwater check valve. Here is a link to a page that shows what these valves look like, and typical prices:

    The plumbing vents near your toilets -- the ones that penetrate your roof -- aren't air leaks, and don't need to be sealed. In fact, it is important that these vents not be sealed.

    Your plan to seal up your two unused chimneys is a good idea.

  4. ylekyote | | #4

    Thanks. I don't think those valves prevent backflow of air do they? None of the descriptions say that anyways. Is it only water it prevents? I don't have a water back flow concern. Just the air.

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