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So while redoing some flooring in my laundry room, I discovered that whoever built my house in 1995 got real lazy and used a flexible foil pipe to connect the dryer vent in the wall through the floor and out the back of the house. After opening up the floor in a couple spots to run a new pipe, I also discovered that the foil pipe had caught fire at some point and burned a big chunk of the pipe away. So for who knows how long, the dryer was just venting into the floor/ceiling of my house.

Any who, the problem I have now is there is a sewer line running inside of the same ceiling bay as the dryer vent and the distance between the top of the pipe and the bottom of the subfloor is barely 3". Code requires a 4" metal pipe for dryer vents but that's impossible given the space I have to run it through. Rerouting the dryer vent would be extremely difficult and not ideal since it's already a straight shot from the laundry room to the vent outside. Any ideas on how to fit a 4" pipe through a 3" space?

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Matt Whitehead
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Alternative solution: replace your dryer.

You can't fit a 4" dryer vent through a 3" space. That's just basic geometry.

Fortunately modern heat pump tumble dryers don't need a vent since they run the air in a closed circuit, capturing the moisture out of the air before reusing it to dry the clothes.

The elimination of the exhaust vent not only simplifies installation — you can basically just put the dryer anywhere as long as there's a power outlet nearby, no ductwork or plumbing required — but also eliminates the need to clean the vent, and the potential fire hazard if you don't, as well as the risk of moisture damage from incorrect installation. Since you write in your question that the your old dryer vent had already caught fire(!) sometime in the past, and has since been venting into the ceiling space (do inspect that for mold and rot!), this sounds like something you'd probably appreciate.

Plus, as a bonus, heat pump dryers consume a lot less power — some sources say as little as one third of the power usage of an old vented dryer — since by drying and recirculating the air they're also capturing and reusing the heat in it, instead of just pumping all the hot air out of the exhaust duct.

Of course, the major downside is that a new dryer will cost money — and a nice modern one from a reliable manufacturer will naturally cost more than an old clunker or the cheapest no-name bargain option. But since you'd be looking at some non-trivial remodeling costs anyway if you were going to try running a new vent for your old dryer, avoiding that hassle will offset at least some of the cost. And you'll also get a new modern dryer that's likely quieter, more user friendly, less power-hungry and easier to install than your old dryer.

Ilmari Karonen
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You don't have options here. :-(

1. Rigid ROUND Metal Duct is REQUIRED

You can't get creative with the profile unfortunately. It must be 4 inch diameter and listed for purpose. Round rigid metal duct is required by the model code and almost every locality in the U.S. as a result.

2. Total Run LENGTH is Extremely LIMITED

You get 14 feet, less with complex turns, so you can't take a big detour to get there.

3. Your plumbing is highly constrained

Plumbing drains need to slope towards the house's ultimate outlet and so can't be lifted or re-routed easily. They're also subject to vent pipe connections within certain lengths resulting in a typical pipeworks that can't be moved up or down or rerouted easily (or at all).

You have to get through.

Go under the drain pipe by penetrating the ceiling and building a soffit box in the room's ceiling below.

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These don't have to be ugly:

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You can get creative with lighting and other details:

Example (not mine), from THIS:

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...to THIS:

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Much better, no?

2019 California Mechanical Code 504.4.2

Type 1 clothes dryer exhaust ducts shall be of rigid metal and shall have smooth interior surfaces. The diameter shall be not less than 4 inches nominal (100mm), and the thickness shall be not less than 0.016 of an inch (0.406mm).

By the way, that referenced thickness is 26 gauge sheet metal, but I typically see manufacturers using 24 gauge (very slightly thicker).

DrFriedParts
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At risk of offending the gods (I’d like to comment but can’t, apologies) in addition to all the fine solutions I feel obligated to add: You have a very dangerous problem to address. I had a similar issue in my home, where the prior owner intentionally (and stupidly) had simply vented the line into the attic: you now have a huge fire hazard. All that collective lint is a potential incendiary device at risk from the smallest spark. You need to do your best to vacuum out that lint any way you can. It’s amazingly flammable stuff.