Replies

1. 
   AM | Jan 06, 1999 05:24am | #2

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   The vent run sounds like a big-time fire hazard! You need to have the shortest run possible to the exterior. You definitely do not want a dryer exhaust line to slope upwards (not even straight pipe) because the weight of the column of air as well as the lint accumulation would really cut down the flow rate. I'd strongly recommend moving the dryer to an exterior wall. Inconvenience in rearranging the laundry room or re-routing utilities would be overshadowed by the real risk of fire and/or carbon monoxide poisoning.

2. 
   Guest_ | Jan 06, 1999 09:51am | #3

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   I've seen several different brands of inline dryer-vent fans advertised, ostensibly for just such a situation. Anybody know if they work, or if there are problems?

1. 
   Guest_ | Jan 06, 1999 12:18pm | #4

   *According to the Uniform Mechanical Code (sect. 504.3), the dryer exhaust duct must not exceed a total horizontal and vertical length of 14 feet including two 90 degree elbows.There is discussion (thru ICBO) about changing this regulation, as it restricts where laundry rooms are positioned, and is excessively restrictive, especially when compared to dryer manufacturer's installation recommendations.

3. 
   Mike_Bertrand | Jan 06, 1999 08:54pm | #5

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   I had a dryer with a 30' run in a rental property that I have. The dryer would take a long time to dry the clothes. I installed an In line fan kit in the run and that fixed the problem. Mine works great!

4. 
   Guest_ | Jan 07, 1999 02:18am | #6

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   Check out Fantec fans available from EFI and other places. Find EFI in FHB back pages. Fantec has great fans and some kits just for dryer ducts. Whenever I order from EFI the product is there the next day at no extra charge. No I don't own stock. Just been dealing with EFI and Fantec fans and never been let down.

1. 
   Guest_ | Jan 07, 1999 02:48am | #7

   *Thanks for the great posts, everyone. I did locate an in line fan today, installed it, and it vents perfectly.

1. 
   Guest_ | Jan 07, 1999 04:21am | #9

   *The current International Mechanical Code contains the length limitation mentioned above. I just read the ICBO December 1998 magazine's "Codes Forum". It mentioned that the proposed change allowing ducts from clothes dryers to be installed in accordance with the appliance manufacturer's installation instructions (M38-98) was approved as modified.For those who have trouble with your inspector, you may have to mention this to them to get longer ducts approved.

5. 
   Guest_ | Jan 07, 1999 04:21am | #8

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   Is there any limit to the length of a dryer vent pipe?

   We have installed an upstairs laundry room, in an interior room, and the vent is currently routed up through the attic, over to the gable end and out a discreet dryer exhaust flap. It is a total run of about 40 feet with only a couple 90's, in other words, a pretty straight run.

   The vent pipe is heavy duty aluminum flex, but to preserve the structural integrity and reduce pinholes, it was not flexed anywhere near its full length. We tested the dryer today, and we are not getting a lot of vent pressure out the exhaust flap. All the joints were made with plastic collars made to fit the flex, and sealed with aluminum/foam sealant tape. I doubt there are any leaks, but the column of air is just not moving very well. I am not smelling fumes inside the laudry room, so it must be going somewhere. Too much static pressure?

   Any suggestions, or am I just paranoid? I would sleep better if I knew the customer and his family are safe. Thanks in advance

1. 
   Guest_ | Jan 06, 1999 03:50am | #1

   *Scooter: You are right to wonder and worry if you have very little air coming out the vent. The dryer will always put out some air, even through a duct that is too long. But as the air flow drops off (unable to push through the restriction of a long pipe), where does the rest of the air go? If it's a gas dryer - losing combustion products (especially carbon monoxide) inside the house envelope is really bad. If it's electric and you have a leak, you are losing humid air (also bad, but not fatal). Or the thermostat in the electric dryer is keeping the heating rate low and the clothes take forever to dry. I've had that happen - thought "What a crappy dryer!" until I unkinked the exhuast duct.There are definitely limitations to how much duct work a dryer can push through. It ought to be in the installation instructions that came with the dryer. Or call a retailer or the factory and ask them to RTFM to you.Some factoids that the manual might not cover: One size larger duct carries the air MUCH farther. If you are allowed, for instance, 30 feet of 3-inch straight duct, you could use 130 feet of 4-inch duct, or 385 feet of 5-inch duct. Those same ratios apply, whatever your allowance is stated as. 3-inch:4-inch:5-inch = 23:100:300 feet of run.Smooth wall duct is much more efficient than rough wall duct (like the flexible and insulated flexible styles). Depends on the surface roughness (average height and spacing of the ridges) but something like 2-3 times difference in allowable length.90's make a difference! A smooth, long-sweep 90 (like an ABS DWV sewer pipe) adds 15 pipe diameters to the effective length. Tight 90's (like a copper 3/4" 90) adds 30 pipe diameters and duct 90's are at least as bad as copper fittings, flow-wise. So a 90 in 4-inch duct adds 30 x 4 inches = 120 inches or 10 feet.Equivilent Length of Straight PipeDuct Size 90 453-inch 8 feet 4 feet4-inch 10 feet 5 feet5-inch 12 feet 6 feet It may seem like a 5-inch 90 hurts you more than a 3-inch 90, but that's 12 feet OF 5-INCH DUCT, versus 8 feet of 3-INCH DUCT which is much more restrictive. If you can do a jog around a beam with four 45's instead of four 90's, you've saved a lot of frictional loses in the air flow.So measure your proposed run, add in the additional length for each 45, 90, reducer, etc., and then convert to the duct size quoted in the manufactures allowable duct length (using the ratios given above). If it exceeds the allowance, use smoother pipe, bigger pipe, and less turns until you've found an acceptable configuration. Think of air flow in a pipe as driving down the interstate. High speeds and high flows are achieved on smooth, straight, uniform roads. Imagine driving a "highway" through France. Narrow, one lane, 90-degree turns, squeezes in and out of alleys, potholes, and bumps. Give those air molecules a short, smooth ride! -David