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In my home the dryer vent is set up to vent out into an enclosed back porch. The problem is that all sorts of lint (and heat) get pumped out into that enclosed area and makes a mess.

Directly above the dryer there is a ventilation pipe that goes to the roof (also connected to the vent in the bathroom) that I could easily re-route the dryer vent into.

A couple of questions about this:
1) Is it a generally accepted practice to vent a dryer up through the roof, or does a dryer vent have to generally be at a level or downward slope?
2) Is it okay to use the same vent line as the bathroom or am I asking for trouble?
3) Is there a recommend maximum distance for venting a dryer that I am going to exceed going from the first floor of a home to the roof?

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I think that by "through the roof", he actually means through the roof, not through the ceiling into the attic space. –  Mike Powell Aug 26 '10 at 15:27
    
Correct. There is a vent pipe directly above the dryer that already goes up through the roof and out of the house. –  JohnFx Aug 26 '10 at 15:48
    
@Mike - thanks for clearing that up. Original comment deleted. –  ChrisF Aug 26 '10 at 21:22
    
Are you using a lint trap to terminate your dryer vent? (for example, amazon.com/Dundas-Jafine-TDIDVKZW-ProFlex-Indoor/dp/B000DZFTC6/… ) I've never tried one of these, but if it works it might solve your lint/mess problem without having to reroute the vent at all. –  The Photon Sep 6 '12 at 18:26
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4 Answers

up vote 8 down vote accepted

I would not do this for exactly the reasons you've asked about:

1) By sharing the bathroom vent pipe, you're ensuring that at least half the gunk that's being spewed onto your porch is going to be spewed into your bathroom unless you can find and install some kind of one-way flapper between the new "Y" fitting and the bathroom. If you do find and install such a thing, it's going to be difficult to keep clean and operational.

2) I'm assuming since you say "first floor" that you also have a second floor, in which case that's a long way to go to get up to the roof. It's not such a big deal for the bathroom fan to vent up there, but that's a long way for lint from the dryer to be carried, and it's going to be more likely to collect on the walls of the pipe over that long distance.

Do you have a crawlspace or basement that you could run a new pipe through?

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No basement or crawlspace. The house is on a concrete slab. I suppose I could put a wire mesh into the vent pipe just before the dryer line splices in. –  JohnFx Aug 26 '10 at 15:47
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Can you repurpose the bathroom vent pipe for the dryer, then punch a new vent outside from the bathroom? You're still running the vent a long distance but it might be worth trying. –  Mike Powell Aug 26 '10 at 15:52
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@JohnFx Don't put mesh into the dryer vent pipe. That would be a pain to clean, which means it won't be done as often as it should. This will lead to decreased dryer performance, and possibly a fire. –  KeithB Aug 26 '10 at 16:27
    
Not to mention if you use the mesh to keep lint from going into the bath, you'll block the air flow from the bath with lint, and moisture won't be drawn from the bath as it should be, which is the whole point of a bath fan. –  The Evil Greebo Sep 6 '12 at 13:43
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Going through the roof is fine, the idea is to make the run as straight and short as possible. First floor through roof is not ideal, so keep it under 25 feet.

I'd worry about return air. If you go through the roof, I would not use the same line as a bathroom (note my condensation problem).

I'll let you know how my new cap works out (if it ever arrives), you'll want something like that as well to keep the outside air out of your dryer.

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By going straight up, especially in a two story house, don't you risk increased lint buildup, which could lead to a fire? Sounds like an unnecessary risk. –  tnorthcutt Aug 26 '10 at 18:26
    
@tnorthcutt- Do we know if it is a 2 story house? –  auujay Aug 26 '10 at 19:55
    
Venting through the roof is a common practice in areas where you can't get under the house. Obviously the ideal is to stick your dryer on the outside wall and vent it 10 inches. I haven't seen anything to indicate that lint buildup is a problem as long as the run is as short as possible and pressure off the dryer is adequate. Distance and # elbows should be a greater concern than gravity. –  Steve Jackson Aug 26 '10 at 20:33
    
@auujay - It is a two story house, but the second floor is a loft so it isn't quite as high as you may be thinking. The pipe would have a run of probably no more than 15' total. –  JohnFx Aug 26 '10 at 23:12
    
If lint build up is a concern they do have lint traps you can stick in the line right after the dryer (but before it gets very far in the vent pipe). This acts as a secondary lint trap to the one in the dryer and will let you clean it out from inside the house. –  auujay Aug 27 '10 at 13:57
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Can you continue the vent on the porch to an exterior wall? Maybe put in a 90 degree bend and continue the vent pipe along the wall of the house? Here is a list of maximum vent length, down toward the bottom of the page.

I wouldn't try to share the bathroom vent. I can see several potential downfalls. The dryer may actually vent into the bathroom which will fill it with hot, moist air if not lint. The bathroom may vent through your dryer. The bathroom fan may fill with lint, making it inoperable at best, and causing a fire hazard in the worst case.

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We had to go through our roof with our dryer. I doubt it was longer than 15' but at any rate never experienced any problems.

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