Installing Clothes Dryer Ducts


Dear NH,
I am installing a gas clothes dryer and associated vent duct. The standard is 4 inch metal rigid duct, but I already have a large amount of 6 inch metal duct in my possession. I will need to run approx. 8 feet of 4 inch duct from the dryer at a 45 degree upward angle. At that point I plan to connect the 4 inch duct to my 6 inch rigid duct that will run horizontally for 18 feet to the outside of the home. The table in the instruction booklet allows for a max. of 44 feet of 4 inch rigid metal duct with two 90 degree turns. My install will have 2 45 degree turns and a 6 foot vertical climb. Will there be a problem with using 6 inch rigid metal duct instead of 4 inch duct? Will using the larger duct size increase or decrease the maximum length of duct that can be used? The install booklet also mentions to use a manometer to measure system back pressure and that it should not read higher than 0.75 inches. Where can I find an inexpensive manometer if I need one?
JB from San Francisco, CA

Increasing the duct size will not negatively affect clothes dryer performance. Actually, as vent pipe size is increased, back pressure is decreased, resulting in better exhaust and quicker drying. Measurement of system back pressure would be unnecessary in this case. The overall effect will probably be a more efficient dryer.

If your ducting travels through unheated space, you may have to take
additional steps to deal with condensation caused by the hot moist dryer exhaust meeting cold metal ducting. Under some conditions, condensation can be enough to cause water damage if it were to leak from the ducting back into your living space.

Compensate for this if possible by sloping the ducts down towards the outside so the condensed water, if any, can drip outside and not collect in the duct. You can also wrap the ducts in insulation, which would minimize condensation by allowing them to more effectively hold the heat. Also, always run the dryer long enough so that the last few minutes push only hot dry air into the duct. This will help to clear out any remaining moisture.

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