Why does my dryer duct have such a long vertical portion?

Question

My dryer duct follows this path:

1. Flex duct leaving the dryer
2. Four feet of vertical rigid steel duct down into crawlspace
3. Eight feet of horizontal rigid steel duct, immediately below the floor
4. Outlet approximately at-grade

90% of the vertical portion is above the floor. Since the dryer's outlet is near the floor, it seems like I could get more effective venting if I cut the vertical portion down to 1' and shortened the flex duct accordingly. Why is there a four-foot vertical portion? Does it have some rodent-control purpose since the outlet is at-grade?

Answer 1

Well first, the outlet being at-grade is a no-no for those pest reasons. They don't have to get up the vertical pipe to cause problems; a mouse or bird can nest in there and cause problems. They did it anyway because it was easier to run it down through the crawl space then up to the ceiling (where, if this is your garage, they'd then have to build a sealed bulkhead to prevent exhaust gases from your car getting into the vent line). If this is your situation I'd look at replacing the vent cap with a "critter-proof" model; there are several basic ideas, some more effective than others but all better than the single-flap standard model.

As for the vertical portion, they probably placed it high to allow for a stackable W/D. A higher placement also allows the flex vent more room to, well, flex, no matter where the dryer is. While it may seem logical to put the inlet to the vent directly behind the outlet from the dryer, doing this can actually be a pain, as the flex line then has to be fully compressed in line with the inlet and outlet, in order to avoid crushing it when the dryer is moved into place. It's really hard to get six feet of flex line to do that (and you need six feet of flex line so you can pull the dryer out enough to get behind it with the line still connected).

You can probably take out a couple feet of the rigid vent to lower the inlet. I'm not sure you'll really get more efficient venting, but it won't cause any more rodent problems than you'd already have with this setup.

Answer 2

It is possible that the previous dryer was a stacked unit with the dryer mounted above the washer.The general rule for dryer vents is not any longer than necessary.Two concerns I have are the flex hose if you can replace it with an elbow do that if not use metallic flex not the plastic type.The second concern is if you live in an area that sees snowfall higher than the outlet it could become blocked.As far as rodent control check some of the new style outlets that are energy efficient by having a tighter seal than the older flap type,some point up and use a plastic ball as a seal.

Answer 3

On your length, per your description you should be OK.

504.6.1 Maximum length. The maximum length of a clothes dryer exhaust duct shall not exceed 45 feet (13 716 mm) from the dryer location to the outlet terminal. The maximum length of the duct shall be reduced 5 feet (1524 mm) for each 45 degree (0.79 rad) bend and 10 feet (3048 mm) for each 90 degree (1.6 rad) bend. The maximum length of the exhaust duct does not include the transition duct. &e.. Table 603 4 for gage thickness

Naturally you don't want to be near as long as what is considered the OK length, but you also don't want a 2 foot run either. I don't think a couple of feet in your run will change much.

The height of the exhaust is also code;

504.6 Domestic clothes dryer ducts. Exhaust ducts for domestic clothes dryers shall be constructed of metal and shall have a smooth interior finish. With the exception of the transition duct flexible ducts are prohibited The exhaust duct shall be a minimum nominal size of 4 inches (102 mm) in diameter. The entire exhaust system shall be supported and secured in place and shall terminate not less than 12 inches (305 mm) above finished grade. The male end of the duct at overlapped duct joints shall extend in the direction of airflow. Clothes dryer transition ducts used to connect the appliance to the exhaust duct system shall be limited to single lengths not to exceed 8 feet (2438 mm) and shall be listed and labeled for the application. Transition ducts shall not be concealed within construction and must remain entirely within the room in which the appliance is installed. Exception: Where the duct termination is less than 12 inches (305 mm) above finished grade an areaway shall be provided with a cross-sectional area not less than 200 square feet (186m.£.) The bottom of the duct termination shall be no less than 12 inches (305 mm) above the areaway bottom

All this and the manufacturer of your dryer is how you have to have it so if the manufacturer say shorter then it is shorter. It just cannot be longer than standards.

As for the critters, get a pest free dryer vent closure.

Code is from North Carolina, which they are using the 2006 International Mechanical Code.