1

Hard to describe this in the title. What I'm referring to is how segments of duct overlap each other where they are connected. Ideally, as with plumbing, the tube the flow enters should be overlapped outside of the tube that flow enters.

Maybe some ASCII art can help:

   CASE 1: THE CORRECT WAY TO DO IT
   -------------------
--------------
-->-->FLOW-->-->
--------------
    ------------------

The two ducts are close to the same diameter. I understand the overlap should be as it is in that ASCII art so that flow does not come up against the seam of the overlap, as it would in the opposite case:

   CASE 2: THE WRONG WAY TO DO IT
---X----------
 --> X------------------
-->-->FLOW-->-->
 --> X-----------------
---X----------

In case 2, flow (lint, for dryer ducts) can get stuck on the inner overlap as it comes up against it. If the overlap were as it is in case 1, the flow of lint has no seams to get stuck on.

So, I think I got all that right. Thing is, doing a dryer install I find that some existing ductwork is fit together such that the joint going to the outside vent connects with a rigid piece with the overlap shown in case 2. I tried changing that, but I think I'd need to replace the part, as the rigid smooth piece just does not fit within the knurled end of the joint it flows into. There is also one other part of the duct extension that's going in, where the pieces fit together in such a way that it's much easier to use case 2 than case 1.

Thus my question: it is not ideal, but is it reasonably safe for case 2 to exist in a ~8' dryer duct? Wherever possible connections would be done with case 1 overlaps. Just trying to get an idea of if case 2 is an absolute no-no, or just not a best practice.

As an aside, I had a similar situation as case 2 with a drain hose for something, and with some plumbers tape (and a tray underneath to catch drips if they did occur) the connection has remained leak free for a while (months, not years). So again, while it isn't ideal, in some cases it isn't a big deal.

2

You basically answered your own question.

No one here should answer you and say "it's safe" and even if they did, would you blindly trust that as the final answer?

Is it safe to walk over Niagara Falls on a tight rope? If you are Nik Wallenda, yes. Should he do it? That's debatable, but he did - and indeed, many safety precautions were taken.

Dryer vents are NEVER EVER EVER "Install and forget." All families drying clothes are different sizes, and the clothing they own are different materials, and qualities. Even the dryer itself has a YMMV (Your Mileage May Vary) effect on the amount of lint passing into the tube. Eventually, the tube will have to be cleaned.

Technically speaking, at any given point in time a dryer tube with lint in it could catch fire (think cause and effect/affect; intentional pun). So anything you do that negatively impacts the desired laws of physics (as you have correctly described their relevance in your pictures above) increases the mathematical likelihood of an eventual fire by some factor.

The real point is that you should install it the right way. Regardless of if you follow through with installing it the right way, you should develop a schedule of checking it and cleaning it at regular intervals.

I do not sell nor necessarily advocate this product, but products like this are made that are supposed to give you feedback on when it might be time to clean your duct work. I bought this device and in about 6 months will be checking my recent duct work install to see how clean it is and probably clean it for good measure.

Whatever you choose to do, I recommend you go with what gives you peace of mind and just be sure to check it at a regular interval regardless.

  • While I appreciate you pointing out the clear line of safety, and giving the good advice of "in any case, cleaning at appropriate intervals is necessary to reduce risk", I think more nuance could be given to the gradient of safety. Specifically, after having someone more experienced take a look at my setup, they pointed out that in some cases the joins between ducts will have a part that overlaps as shown in Case 2 and that's OK if the interior 'seam' is smooth. To further reduce risk, making those kinds of joins in an accessible spot make good cleaning routines easier to act on. – cr0 Feb 10 '18 at 20:51
  • There is no reason to have the joins be reversed. Case 2 should not exist. If it does, then the wrong parts were used, or the parts were used wrong. From a gradient perspective, it's not something easily given a number and because all cases are unique, any assertion of how black or how white of an answer it COULD be is hearsay. The best practices are there for a reason. The parts are not expensive. For 8' we shouldn't be talking about much money. – noybman Feb 11 '18 at 5:24
  • Oh, and: "after having someone more experienced take a look at my setup, they pointed out that in some cases the joins between ducts will have a part that overlaps as shown in Case 2" is not true, it should never be this way, it often is, but when it is, it is wrong. Even when smooth, still wrong. Also "making those kinds of joins in an accessible spot make good cleaning routines easier to act on." is presumably not true, Since the ducts need to be slid or separated from one another by the same distance to undo the overlap, it should be entirely irrelevant. – noybman Feb 11 '18 at 5:29
  • 1
    I have seen those used many times, My father has used them in the past (probably 10 years, and never cleaned out the tube either), its still wrong. Note, the amazon product page does not describe it as being for Dryer applications, and also note that they do sell plastic dryer ducting and it's advertised as for a dryer. Then go look up some articles on dryer fires, and dryer ducting safety. Both items are not the right solution for a dryer application even if a retailer wishes it to be true. There are non dryer applications those parts are suitable for. – noybman Feb 14 '18 at 1:49
  • 1
    The recommendations outlined below reflect International Residential Code (IRC) SECTION M1502 CLOTHES DRYER EXHAUST guidelines: M1502.5 Duct construction. Exhaust ducts shall be constructed of minimum 0.016-inch-thick (0.4 mm) rigid metal ducts, having smooth interior surfaces, with joints running in the direction of air flow. Exhaust ducts shall not be connected with sheet-metal screws or fastening means which extend into the duct. – noybman Feb 14 '18 at 1:52

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