How to best use a long, undersized duct for the dryer

16 years ago

We have a 40 feet of 3 inch PVC pipe that serves as the exhaust for our electric dryer. I cannot economically re-duct. Except for a 6 foot vertical run the PVC pipe is buried in the ceiling at the lower level of our house, so I do not know how many turns it makes.

I had an e-mail exchange with the FanTech engineers about using their dryer booster fan, but they do not recommend it due to the restrictiveness of the 3 inch PVC. Also, I could not use a "super-fan" since apparently any fan powerful enough to overcome the restrictiveness of the 3 inch pipe would exhaust the air so strongly from the dryer that there would be no heat in the dryer.

I am now thinking of using the "water bath" type lint filters available at hardware stores. This would blow the air directly into the laundry room after the lint is filtered. I could then use the PVC pipe by converting it to a pure exhaust similar to a bathroom exhaust to remove the humid air. There would be no direct connection to the dryer. Further, I could get a dehumidifier for the laundry room to avoid too much moisture build up.

What other options do I have? I could get a condensing dryer as a last resort. I did not want to re-vent since I would have to find a real pro to do this, and I would still have to tear up half of my lower level.

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Comments (6)

otterhead
16 years ago
How to vent your dryer without burning down your house. Note the comments about pvc ducting.

Here is a link that might be useful: how to vent a dryer
richard904
Original Author
16 years ago
I remember the well know statement: "You go with the laundry room you have, and not with the laundry room you wish you had." I am well aware of the safety issues with venting. If I use the water bath lint filter, it will remove lint from the air stream that the dryer lint filter did not. The only issue is the moist, warm air vented into the laundry room. This can be removed by a separate vent and, if need be, a dehumidifier. There is no safety issue with heat build up in the dryer. Any reasonable vent fan, even like one used over a stove vented into a duct, should work, and I believe 3 inch PVC is more than enough when venting is done in this way. The dryer is not blowing directly into the PVC pipe.
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Ridgid is really the only way to go especially over long runs. We usually spec 4" sched. 20 PVC pipe as it comes in longer lengths than 4" metal does around here , though elbows can be tough to source sometimes. Unfortunately most contractors / HVAC guys will use flexible corrugated hose because it's cheap and fast. If it's a luxury project they'll splurge for the flex aluminum instead of the plastic kind. This type of pipe can cut a fan' effectiveness in half over long runs. Little thought is really given to these fans in most homes today. They are often undersized for a given bathroom and poorly located too. The crappy pipe detailing makes a bad situation worse. Panasonic's website has (or used to) a pretty good tutorial on how to properly size a fan, where to locate it and how to properly pipe it to the outside. I think much of the problem is cost and ignorance. Most people know you get a fan in your bath , but when pressed they won't really be able to tell you its there to expel warm moist air from your home, not smells. They assume that the $34.95 version will be fine for them and a quick & dirty install will get ya out the door for a hundred bucks. The reality is, many master baths, and some hard working kids baths need a much larger fan than 35 bucks can buy. We are often talking $100-$200 for an adequate model, more if you want features or quietness. Add the increased piping cost and labor for the proper venting needed on higher capacity fans and this can quickly become a $500 line item - which starts to look crazy for something as pedestrian looking as a bath fan. Timer switches are also a very good idea as most fans even properly sized ones need to run a while after you've left the bath to completely exhaust the moist air.
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gordonr, If there's nothing else I've learned during the course of my renovations, I've learned that everything ends up being more complicated than you expect. Once you open up the ceiling or the wall, there always seems to be something in the way of what you want to do. A floor joist where the tub drain needed to be, pipes where I wanted recessed lights to go, and now the recessed light over the tub is where it would make sense to run the duct. I can hardly wait to get to the kitchen renovation! I spoke with a rep in Miele's technical service group yesterday. Initially he was telling me he thought what I wanted to do entailed too long a stretch of duct - that it would reduce the performance of the dryer. I told him I expected that it would not be optimal performance, but what I was trying to determine was whether it would reduce it to the extent that a condenser dryer would have the same performance. He didn't seem to understand what I was getting at, so I gave an example: If the vented model has 15% better performance than the condenser model and the venting I'm doing reduces performance by 15%, then the condenser model would be generally equivalent, but if the vented dryer's performance is 50% greater, the same 15% reduction in performance would still mean the vented dryer was a better choice. I asked if he could give me any information about how much better the vented dryer's performance was than the condenser model and how much the venting I was considering would reduce the vented dryer's performance. At that point, he said he wanted to talk with one of the senior technical reps and put me on hold. When he came back, he told me that the dryer was rated for up to 60 feet (which I already knew) and that I should be fine with what I was planning on doing. He also told me that the sr. rep had indicated that having a condenser dryer in a closet could be problematic because it puts off a lot more heat than a vented dryer - you'd need a lot more air circulation into the closet with a condenser dryer. The closet I'll be putting the w/d into is 68" wide by 29" deep, so the extra heat and air circulation need may not have been as much of an issue for me as it might be for others with a more confined space. One learning that I took away from this: if you start to get the sense that the technical or customer service person you're talking to may not be as knowledgeable as you'd like, ask to talk with a senior rep. Like anywhere else, initial phone calls will be answered by more junior staff who will route the more complicated issues to more experienced staff. I've decided to go with a vented dryer, and now just need to decide between the Miele and the Asko. I'll let everyone know which one I end up with. Jan
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otterhead
16 years ago
It sounds like you have the bases covered. Other than using the 3" pvc pipe as a laundry room vent, there's not really much you can do other than re-ducting.

I will suggest that the blower that you use will be wired through a timer that gives enough run time after your laundry has finished drying as you will have a lot of moist air that will need to be evacuated.

A 40' run with lots of elbows will require a powerful fan, check out the link for an in-line fan as an idea.

You are going to want the most powerful fan that you can afford, and have to provide make-up air sufficient to avoid backdrafting of any fueled appliances, furnace, hot water heater, etc.

Has the duct previously been used for a dryer...and with what success?

What is the layout and size of the laundry room?

How tight is the construction of your house?

What are temperature and humidity conditions in your locale?

These are all factors that must be taken into consideration.

Here is a link that might be useful: in line fan
User
16 years ago
There's no way to punch the vent line straight out the wall instead of going through ceilings, etc? Those water-filled lint catchers are a HUGE PITA, constantly full of gunk, need cleaning after every load, increase drying time because you can't get as much air out of your dryer as fast, and run out of water during long dry cycles. I would consider those a very last resort, only for situations where you have absolutely no choice. A condensing dryer would be better.
richard904
Original Author
16 years ago
I have not tested the water bath type lint filters, but my understanding is that the dryer exhaust air blows down into the water which catches the lint and then the moist air is exhausted above. The duct from the dryer does not touch the water. I thought there was plenty of space above the water for the moist air to exhaust, thus it would not impede the drying time. Maybe I am wrong, and as I said I have not tested this here.

This house was built in 1994, and by then the contractors full well understood that they should use 4 inch rigid metal ducts. The house sits on a slope, so it is staircase like with a lower level where the laundry room is, a level above, and a solarium on the third level. The laundry room is large and one wall is the concrete foundation, so that wall is not accessible for any re-ducting. The laundry room faces a large family room. The floor above is an open design so we cannot go straight up. I am so afraid that if we re-duct by trying a straight replacement of the 3 inch PVC with 4 inch metal rigid duct using the same path, by the time a contractor tears the ceiling to pieces and causes who knows what damage, the cost would be so high that I might as well get a very good high end condensing dryer.
plumbly22
16 years ago
this was NOT elegant... and was 20 years ago... in the north east in a house with a basement laundry, with a finished "rec" room that you walked through to get the "laundry/furnace/hot water heater/sump pump area which was about 1/3 of the basement....

My father vented my dryer with rigid metal piping into a 5 gallon bucket that he had cut 4 holes about 3" in diameter in the sides of near the top, and he had then cut a hole large enough for the vent pipe to fit into... I had one elbow off the dryer, a straight run and then another elbow into the bucket... I made sure there was water almost to the holes in the sides... when the dryer ran, lint would collect in the water, and yes water did evaporate into the air...

since the basement was "damp" to begin with (I believe from the sump pump hole) I had a dehumidifier down there in the "finished" part... with a hose attached that ran to the sump pump hole....

Not sure how your "lowest level" with the cement wall is finished or viewable... but.. this was a cheap alternative and it did work...

btw... yes the bucket got ugly... I would take the lid off and use a metal kitchen soup type strainer to "fish" out the lint and dump it in the trash when I was done with my laundry... and before I started I would make sure the water level was alright... like I said at the start... this was certainly NOT elegant... but very functional...

How to best use a long, undersized duct for the dryer

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otterhead

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How to best use a long, undersized duct for the dryer

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otterhead

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