Venting for Dryers

I received an email from someone who was looking into dryers.
They wanted to know how venting works.

Since I wrote a long response, I decided to publish it here.

This is basic venting 101 for any laundry room or laundromat.

For more absolute specifics, take my information to your HVAC company to double check for your specific building.

Good morning Everet!
How the venting through the outside wall should go for your dryer vents. And also how the intake for fresh air should be cut into that wall. Placement, size, etc...
Thank you very much!



(Everet response)
Here is another long answer email.
I know everyone hates math, but I had to include it for this one.
Feel free to give me a call for more explanations on this.
There is make up air and exhaust air.

As for makeup air the rule of thumb is 1sq inch of make up air per 1,000 BTUs for the dryer.
Lets pretend a 30lb stack dryer is 144,000 BTUs and 

it is 190,000 BTUs for 50lb stack dryers.

You want to install three of the 30lb stack dryers and four of the 50lb stack dryers.
How big of an opening do you need for make up air?
For the 35lb dryers:
144,000 BTUs x 3 stacks = 432,000 BTUs
432,000 BTUs/1,000 BTUs =432 BTUs x 1sq inch =432sq in of make up air
To get the square of something it is the Length x Width
Pick standard number for width (lets say 24 inches but it can be whatever you want)
432sq inch/24 inches = 18 inches
So the make up air opening needed is 18 inches by 24 inches for the three 35lb stack dryers. More air is better so I like to go a little bigger so say 24 inches by 24 inches. 

For the 50lb dryers
190,000 BTUs x 4 stacks = 760,000 BTUs
760,000 BTUs/1,000 BTUs = 760 BTUs x 1 sq inch = 760 sq in of make up air needed for the four 50lb stacks
760 sq inch/24 inches = 31.66 inches (round to 32 inches)
Opening needed 32 inches by 24 inch or to go a little bigger
36 inches by 30 inches. For bigger dryers I like to add a little extra.

The larger the dryer the more air it needs.

The numbers above are minimums and general guidelines.

Your specific dryers are going to be different so check the labels.

For exhaust air are you planning to have the dryers vented individually or run the exhaust into a common duct and then the duct goes outside?
Pretend the dryer exhaust duct is 8 inches, What size hole in the wall do you need?

1. The dryer exhaust venting in this case is 8 inches in diameter. Typically the hole in the wall is just a smidge over 8 inches in diameter and that accounts for the width of the sheet metal of the duct going out of the wall. Usually there is a little vent hood over the outside opening to keep wind, rain, birds out. It is best if each dryer can be vented individually, however that isn't always possible. So if each dryer cannot have its own exhaust opening...

2. The dryer duct work inside the building connects to a common "box" of "take off" and that take off covers a hole in the wall roughly the same size. Up to four dryers (preferably less) have the exhaust venting connected to this take off. No matter how many connections to the take off, be sure to stagger or offset the holes cut into the takeoff a little bit so air coming out of dryer A doesn't get blown back down into the exhaust of dryer B.
The more dryers you vent into the take off, the larger in diameter the takeoff and hole in the wall has to be to move the air through it

Exhaust air vents should be above the height of the make up air intakes. You don't want exhaust vents to be below make up air otherwise your dryers will be sucking in exhaust air. Exhaust air vents usually have some sort of vent cover to keep rain and wind and animals out.

Make up air intakes sometimes do and sometimes if it is a bigger opening, they are just flush with the side of the building with no hood over them. Once you put louvers or some sort of covering over the air intake you need to make your intake hole bigger to accommodate for the air blockage caused by the louvers. Check with your HVAC contractor for local codes.

Usually they want to see two openings for make up air behind the dryer line (in this case one that is 24in x 24in and one that is 36inx 30in but some will let you have one big opening.

The make up air vents can be lower in the wall closer to the ground (just don't make them level with the ground because snow will pile up and block air), mid way up the wall, or in the ceiling of the area behind the dryers, wherever you want them to be within code. You can put louvers over the make up air opening that you can close to block out the air on the coldest of days, but when you do that you need a bigger opening because louvers can block up to 50% of the air depending on what you use. Your HVAC company can suggest several types. There are systems that when your dryers are running the louvers open and close when the dryers are idle saving you some of the cold air behind the dryers.

Notice all of the combination of 45 degree and 90 degree elbows in the vent lines in the pictures below. When installing dryers try to keep the venting as straight as possible coming out of the dryer to the outside of the building. Every elbow that is included you lose efficiency of your dryers and your dryers have to work harder and it takes longer for items to dry.

I hope this helps your understanding of venting and planning for your new or replacement dryers.

Ev
A Heck of A Nice Guy