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My existing dryer vent run enters the crawlspace behind the dryer and exits through a vent that is part of one of the normal crawlspace vents. Since the house sits on a slight slope, the crawlspace on that side of the house is very short - when I am in the crawlspace there I cannot turn over. This means that the vent is only a few inches off the ground.

I was told by a contractor who cleaned out our air ducts that this was a fire hazard as leaves could accumulate and warm air exiting the vent could cause a fire. He also noted the vent in the crawlspace was a flexible type and that was bad (I understand and agree).

First question - is it really critical to move the vent so it is more than a few inches above the ground? If not, I will replace the flexible dryer vent with the 4" galvanized tube and call it good.

Assuming that I need to move the vent, I don't think I have any good options due to the length of the required run. As you can see from my diagram below, the dryer is not against an exterior wall and there is a door between it and the nearest exterior wall. The furnace is to the right of the dryer and that prevents me easily running the vent to the front of the house. Moving the dryer is not practical because I'd have to run electricy to the new spot, and unless I also ran plumbing I'd have the dryer and washer in different locations.

I considered running a vent through the roof but I understand that is not advisable due to the height of a vertical run.

It looks like my best option is to go over the door to the left of the washer and then out through that exterior wall. The total length of that run would be about 18 feet, plus two 90 degree bends, plus two ~ 30 degree bends, which by my calculations, works out to about 33 feet of effective length. This does include an 8 1/2 foot vertical run. Is this acceptable?

If that is acceptable, then I will need to exit the exterior wall (aluminum siding), at l east 12 inches below the eaves. Any guidance on how to do this? All the online guides I see have it going through the rim joist, which won't work for me b/c the water heater is in the way.

appliances layout

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What's your climate? –  Bryce Nov 8 '13 at 15:54
    
Mid-atlantic. Snow accumulation is rare. –  Chloraphil Nov 8 '13 at 19:07

3 Answers 3

International Residential Code (IRC) says the maximum exhaust duct length is 35 feet. However, it also says that this length can be overruled by the manufacturers instructions, if the instructions are provided to the inspector.

2012 International Residential Code (IRC)

Chapter 15 Exhaust Systems

Section 1502 Clothes Dryer Exhaust

M1502.4.4.1 Specified length. The maximum length of the exhaust duct shall be 35 feet (10 668 mm) from the connection to the transition duct from the dryer to the outlet terminal. Where fittings are used, the maximum length of the exhaust duct shall be reduced in accordance with Table M1502.4.4.1. The maximum length of the exhaust duct does not include the transition duct.

Table M1502.4.4.1

M1502.4.4.2 Manufacturer’s instructions. The size and maximum length of the exhaust duct shall be determined by the dryer manufacturer’s installation instructions. The code official shall be provided with a copy of the installation instructions for the make and model of the dryer at the concealment inspection. In the absence of fitting equivalent length calculations from the clothes dryer manufacturer, Table M1502.4.4.1 shall be used.

I couldn't find anything that says the termination has to be a specific height above the ground, just that it must be 3 feet in any direction from any openings into the building. And that it should have a backdraft damper, and no screens

2012 International Residential Code (IRC)

Chapter 15 Exhaust Systems

Section 1502 Clothes Dryer Exhaust

M1502.3 Duct termination. Exhaust ducts shall terminate on the outside of the building. Exhaust duct terminations shall be in accordance with the dryer manufacturer’s installation instructions. If the manufacturer’s instructions do not specify a termination location, the exhaust duct shall terminate not less than 3 feet (914 mm) in any direction from openings into buildings. Exhaust duct terminations shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination.

The code also says that the ducts must be 4" smooth walled ducts, and sections can't be attached using fasteners that protrude into the duct more than 1/8".

2012 International Residential Code (IRC)

Chapter 15 Exhaust Systems

Section 1502 Clothes Dryer Exhaust

M1502.4.1 Material and size. Exhaust ducts shall have a smooth interior finish and be constructed of metal having a minimum thickness of 0.0157 inches (0.3950 mm) (No. 28 gage). The duct shall be 4 inches (102 mm) nominal in diameter.

M1502.4.2 Duct installation. Exhaust ducts shall be supported at intervals not to exceed 12 feet (3658 mm) and shall be secured in place. The insert end of the duct shall extend into the adjoining duct or fitting in the direction of airflow. Exhaust duct joints shall be sealed in accordance with Section M1601.4.1 and shall be mechanically fastened. Ducts shall not be joined with screws or similar fasteners that protrude more than 1/8 inch (3.2 mm) into the inside of the duct.

NOTE: Always check with your local code enforcement agency before begining any project, as codes vary from location to location.

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First of all, I find it hard to believe that a dryer vent a few inches off the ground is a "fire hazard". At worst, it is an "inefficiency hazard" if the vent were blocked by snow accumulation, flooding, barkdust spreading, or leaf accumulation. Those are still good enough reasons to try to improve it though.

I would replace the flex tube with rigid 4 inch ducting in the same place. Where it reaches the outside, put in a 90° elbow to route it upward, for a reasonable distance, like 3 feet, and put the vent flapper there. If this area is exposed to precipitation, etc., then use an additional 90° elbow.

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For condensation concerns, drilling several small (1/16 inch) weepholes at the low point of the elbow should be good enough. Dryer exhaust starts out cool and moist: as the load dries, it becomes hot and dry which also dries out the vent line.

If the appearance seems unwieldy, construct a box around it.

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Its a possible fire hazard because if the lint cannot escape due to blockage it will back up in the pipe, potentially all the way back to the dryer which can ignite it. –  The Evil Greebo Nov 6 '13 at 16:06
    
This is a good thought, but I'd worry about the warm moist air condensing inside the duct (if it's a bit chilly out). –  Tester101 Nov 6 '13 at 16:06
    
@Tester101 Are you responding to me or the answer? –  The Evil Greebo Nov 6 '13 at 16:11
    
@Tester101: amended. –  wallyk Nov 6 '13 at 16:37

If all else fails... Is this a gas drier, or electric?

If electric, it is possible to simply exhaust into the basement through a filter (to trap the lint) and/or a water trap (which tries to condense out some of the water before it hits the room). Basement humidity would be increased even with the water trap, but supposedly less so than without it. (DEFINITELY DON'T TRY THIS WITH GAS DRIERS, OF COURSE, DUE TO CARBON MONOXIDE/DIOXIDE DANGERS.)

Friends of mine have a low-tech version of this: a duct that runs a few feet from the electric drier so the air can cool a bit, then ends in a knee-length women's stocking which acts as a disposable lint filter. This definitely increases basement moisture, but they count on their dehumidifier pulling that back out of the air. Not energy-efficient, but their alternative was having someone core-drill a concrete foundation wall and they've been putting that off for 14 years now. The stocking probably doesn't meet currently recommended 5-micron particulate limits, but it may meet 30-micron, especially after it has started building up a lint layer on the inside as additional filtering.

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