Insane Drain Idea?
I am planning on running an underground line from the downspout to a drainage spot in the yard.  I want to use schedule 40 PVC, the thick stuff, not DWV.  I know 4" is standard for this application, but I do NOT get much flow out of this downspout and the yard is rather flat in this part, which gives me not much leeway to adjust for slope.  Could I get away with 3" PVC or even 2" PVC?  Each step down of an inch results in a huge savings in cost and makes installation so much easier. (With 2" I could use a trencher, making the job a one day job.)  I have stood outside during heavy and light rain and have watched the amount of water that comes out of this downspout, and it just isn't much at all.  (We have leaf guards that end up moving about half the water of a medium or heavy rain simply over the edge of the gutter.  We have never had water into the home issues.)  The problem is our attic HVAC condensate line runs into this downspout, and with the very hot and humid summers we are having I'm getting a bog in the yard where the current system drains.

Please let me know if I can get away with a 3" drain line or even a 2" drain line.  It's about 60 feet to the drainage spot with a decent slope and a couple 45 degree bends.  Thank you.
Do the 4 inch, better to have excess capacity than less
More cost up front, but better in the long run.

We had a hail, rain storm, over an inch in 20 minutes, glad to have larger diameter drain pipes for the gutters and downspouts
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How many downspouts are you going to tie in.  If only 1 or 2 then 3 inch would be fine.  If like you say that the Leaf Guard gutters 3 inch is plenty maybe even 2 inch.  you could also have 2 runs and split your downspouts
I ran some sch 40  4" pipe and attached an emitter  (the elbow has a small hole in the bottom to allow the pipe to drain) Put a small amount of gravel below it for better drainage. See video;
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They called those water cannons when the western states were hydraulically mined. Big pipe collects the water at a high elevation. The pipe gets narrower as the elevation drops so you basically have a pressure washer at the business end of the pipe.
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You don't say where your location this case it matters, big time.
If your in an area where it freezes, use 4", bury it 1" lower than necessary. This allows for varation in the trench depth. A high spot will hold water and freeze with a small pipe, a large pipe buried a little deep gives you a cushion on your grade and smoothness (or lack of) on the trench bottom.

If depth is critical, a trencher is the last piece of equipment I would want on my job.

2" pipe for foundation drainage is LUNACY

Why are you set on SCHD. 40 ?   It's more expensive and the thicker walls offer little / no advantage in this application.

If you're going to tie in your condensate line , I'd suggest having an open funnel installed so that the lien pours into it and allows for visual confirmation that your line is clear.  Last thing you want is to have your A/C backup into your home.
(08-15-2022, 11:28 AM)Cabinet Monkey Wrote: 2" pipe for foundation drainage is LUNACY

I was thinking he'd make a great civil engineer in Las Vegas - seeing how well they designed storm water management for a whole 0.6" of rain
4" sched. 20, holes not required if piped to "daylight". If going to a dry well, I use 4" sched. 20 drainage laid in 3/4" washed gravel (holes at 4:00 and 8:00) and add a pop up overflow.

The pitch for 4" is typically 1/8"/ft.; for 3" and smaller it's 1/4"/ft.. So if the top of 2" and 4" pipes started at the same elevation, at only 20' the bottom of the 2" would be lower than the 4".

If you run to "daylight", be sure to install 1/4" screen to keep out critters, and also allow at least a 1" drop so the water (and sediment) will not slow down before it's out of the pipe.
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