Flow Through PVFC Pipe

[dafish]

Hello gentlemen. My thanks in advance for you advice!

I'm installing my own radon system. I know enough to accomplish this safely, and I know code. What I don't know is CFM airflow implications as I exit the house.

I'm going to be running 4" Schd 40 PVC over roughly 10' of horizontal and 10' of vertical (there is a little more, but it gets complicated and isn't a factor here). So fairly short all things considered.

However, I'm going to have to penetrate two engineered floor trusses, and I'll have to use two 90's to get above the bottom of the floor joist and out the holes. I'd much rather do that at 3" PVC, and my general contractor agrees. I'll still reinforce those beams. FWIW, I'm going to see if I can use 4" 90's and then come down to 3" to penetrate the trusses, but that's a space issue and may or may not work out.

My problem: I tend to think a short run of 3" PVC won't hurt much, but I'm thinking ya'll will know better than I. I've seen this calculator, and I suspect I should assign 2 PSI number (it'll be a vacuam, and less then 2" of water column)

1. https://www.rapidairproducts.com/tech

And I've seen this:



But I've no idea how to identify the total flow implications. I've been told by one gentlemen that any amount of 3" reduces the entire section to 3" flow. I don't believe that's correct, for the velocity should simply increase through the short 3" section, but I'm well out of my depth here.

Thoughts?

Thanks guys!

D

 

[dafish]

Anybody?

 

[Jeff Handy]

Post some pics and draw a sketch.

Otherwise your post sounds pretty hard to follow, at least to me anyway.

Don’t forget that rain will be falling into the pipe, so everything has to pitch and drain back to the slab.

 

[Jeff Handy]

You can drill through engineered joists, but only in certain places, in certain sizes, etc.

You should hire this out!

 

[dafish]

Yes sir.

Pitch: Yep, 1/4" per foot was the plan. Thanks for the reminder. I'll finish in a foot or so of horizontal out to the roof eave then finish in a 45 degree down so I keep crittters out.

Pic folllowing as a soon as I get my scanner behaving.

 

[dafish]

You can drill through engineered joists, but only in certain places, in certain sizes, etc.

You should hire this out!

Thank you Jeff! I do appreciate the voice of wisdom. My general that build my home has literally seen it, knows the plan. He advices me to glue in additional width, 4' to each side, to the laminate beam. I'm going to overkill that and do that, using 3/4" laminated board as he suggests and then I'm going to do the full height of the beam (so including the top and bottom of the beam. The "Outer" supports will also be 2" wider then the beam itself. Yes, it will then drop down 2" more, but oh well.

So that's 3" of additional support beam over 8' of the beam, plus 1.5" of beam that is 2" deeper. Sure, it's overkill, but it's not going anywhere this way.

And with all that I'm STILL uncomfortable, which is why I don't want to use more than 3" PVC. The hole will have to be 4" for that, so...

 

[dafish]

Image. Crude, but perhaps shows the area of concern. As Jeff has called out, I want to minimize the hole size through the floor joists. However, I'm clearly working to maximize CFM through the system and don't want to do all that and then loose it over 2' of 3" pipse.

Yes, I know on the left I'm going down to 3" after the "Y" connection, but I'm also connecting a second radon collection point farther away (that I'm stuck with, so not point in getting into it). For me the primary collection point will be the sump pit. (shown at the bottom of the drawing).

 

[dafish]

BTW, thanks guys, for looking at this and sharing your thoughts!

D

 

[Jeff Handy]

Can’t you choose a different route, in a joist space, and go out through a rim joist, and not go against the floor joists?

 

[dafish]

Hello again Jeff, and again, thanks for chiming in!

Yes I could. I've been torn, so I'll share the pro/cons and you tell me what you think.

Side exist:
Coming out the side places it right next to power, is easy to get over and away from openable windows, and is only a foot above grade. I can install the fan 3-4 feet higher and still have a highly serviceable install. It's also relatively sheilded aesthetically by bushes and trees. However, I need to re-enforce the floor joist (joists?) I'm penetrating. CFM over the run, (except for whatever the 2' 3" section I've asked about) looks to be in the 2800 CFM capable area.

Please note floor joist language. I may be (am) explaining things poorly. One of what I've drawn as a floor joist sits on the foundation/sill plate. It carries no span load at any point, I don't think it's called a rim joist though, so for lack of better wording I've call it a floor joist. (you may call me an idiot if it helps.) of course I can only re-enforce one side of it too, but again, not much load there.


Back exit:
There is only one other option, and it's a bit of a dog. It adds another 10' of riser height to get above openable windows , and another 40' of horizontal to get there.. This hits CFM a fair bit. It's almost cut in half, down to 1500CFM. It comes out 8 feet above ground, so service isn't ideal, and power has to be brought to it. Installing at that height will be less fun too. Aesthetically its a loser, as it has to exit right over a walk out basement door. The wife is not a fan. (yes, I see the pun).

So yea, I too didn't like penetrating the floor joist, still don't, but of the compromises it seems far the less onerous.

 

[dafish]

And back to my question:

Over what will be an equivalent 40' (including of 90' bend corrections here) using 4", how much does 2' of 3" hurt?

 

[Jeff Handy]

Sounds like the alternate route is way too long.

Are you planning to use aluminum gutter down spout sections as the vent outside?
That is what is used around here.

And it helps disguise the vent, it just looks like a down spout.

 

[dafish]

I may have answers for the next guy.

savings everybody the math, it looks like 3" vs 4" is has a 4 fold increase in pressure loss. A bid deal over a long run, but over 2-3'? Insignificant.

I go back to the fan vendor I'd been speaking to who swore if I used any 3" if reduced the entire run down to 3" capability. Politely, I think I've found that's BS.

Links for anybody else that may follow:

https://www.pei.org/wiki/inches-water-columnhttp://www.kylesconverter.com/flow/liters-per-minute-to-cubic-feet-per-minutehttps://www.copely.com/tools/pressure-drop-calculator/https://www.rapidairproducts.com/technical-faq/flow-rate-calculator
My thanks to all!

D

 

[dafish]

Hello Jeff. I'd seen those on the internet, and I may just do that. I've been doing a lot of searching and it looks to me like the fan vendors insistence on 4" is a little overdone. Sure on a long run you're trapped, but on a short run it's not that bad. So yea, converting to downspots looks like a pretty good plan. Thanks for suggesting it.