3 inch dryer vent?

We left our old washer and dryer in the house in Florida so I just bought another washer and dryer for this house.

Poking around the laundry room today, I noticed the dryer vent has a larger 4" flange attached to the wall but a smaller 3" pipe in the wall. There was a ton of lint between the 2 ducts.

The house was built in 1977. I've never encountered anything other than the standard 4" dryer ducting before. Was this common in older homes? Can I just get a 4" to 3" duct reducer and use that?

Yay home ownership 

I suspect it has worked since 1977 so it's probably good. Just make sure it's unblocked and has no screws pointed into the duct. And be diligent about cleaning your lint trap. 

The new dryer has a 4" connection for the vent, yes?  How easy is access to the run of 3" pipe?  Attic?  Crawlspace?  If the dryer wants 4" and the outside vent is 4", why not replace everything in between with 4"?  Sheet metal vent pipe/duct should be available at most any retail home center.

Given that the cross-sectional area of 3" duct is a lot less than that of 4" duct, I would guess the dryer would perform better and more efficiently with a 4" vent.

In reply to 1988RedT2 :

Unfortunately, the laundry room is dead center in the middle of the house and the dryer vent duct runs up through the wall, through the attic and out the roof.

I agree that the 4" duct would flow a lot more and probably improve the dryer performance. I wish it would be easier to replace the entire duct run with 4" duct 

My concern is that the two different diameter ducts aren't sealed and it's blowing lint into the interior of the wall. 

I once had to figure out the dryer vent runs on a 5 story Holiday Inn. Roughly 45' of run is the maximum distance. You subtract so many feet for each 45 or 90 in a run. So, if the run is short (10 feet and a few direction changes) you should be ok with the 3" pipe.

Otherwise, replace with 4" as 1988RedT2  suggested.  Worst case get a booster fan and stick with the 3".

Remember kids, dryer lint fires are no fun!

And let this be a lesson. Building codes are there for a reason.

In reply to stanger_mussle (Supported by GRM undergarments) :

Stupid question: I get that the laundry room is in the middle of the house, is the laundry room directly below the attic? 

If it is let me suggest running a new 4" metal pipe (not aluminum flexible duct, 5 foot lenghts of split seam round duct) duct system up the wall in the laundry behind the dryer, up into the attic and then vent it out the roof.   It's OK to use a short length of the flexible duct to connect the dryer to the main vent duct, if you use large hose clamps where it connects to the dryer and the hard duct it makes it easy to remove and vacuum out with a shop vac.  

My little tip for dryer vent cleaning - not specifically related to this: Every year, I disconnect the dryer from the wall and stick the leaf blower in there. Use a towel or two to seal it up, then fire up the leaf blower for several minutes. Amazing the amount of stuff that comes out of the vent in the outside wall!

VolvoHeretic said:

And let this be a lesson. Building codes are there for a reason.

2nd lesson would be building codes have changed in 47 years. I'd bet in 77, 3 inches was code. 

In reply to Steve_Jones :

You could be right, but I was working construction in the late 70's and I don't ever remember 3" dryer vent. I was trying to look it up but as usual, strange things I want to know about never show up.

Edit: Of course, back then worrying about codes wasn't anywhere in my job descriptions.

Quite likely somebody realized a 4" duct won't fit in a 2x4 wall and this was the solution. 

I'd be tempted to commence removing sheetrock and building a little pilaster in the back wall of the laundry room with a couple 2x6 straddling the new 4" pipe. Lots of dirty work going through attic and insulation and sheetrock.. but peace of mind.