Blow'n in the wind. Cheap flexible duct to discharge to out doors

[Bruce Seidner]

My local plastic pipe dealer, who treats me with respect and honest bottom line prices, is out of 8" 3034 sewer/drain PVC. Ferguson, a national chain with knowledgeable and friendly salesmen comment that their cost is more than what I pay from this company. I have to wait a couple weeks for the next delivery and I need about 50'. But at $3.50 a foot versus $6 a foot, I can wait and have plenty to keep me occupied. However, in the interim, I have decided to bypass the indoor filter and discharge to the outside. It occurred to me that the requirements of discharge out of the blower are very different than intake into the blower. Also this will be light dust, rather than material laden air, that has been through the cyclone or Thien separator, whichever I can get to fit in my limited crawl space.

I can't think of a reason not to use HVAC uninsulated, plastic or aluminum, 8" flexible duct to discharge to the great outdoors and share my newly liberated dust, soon to be mulch, to the side garden. This is my longest run and is about 30'.

Is there any reason that I should spend the money on PVC when I don't need to worry about the duct collapsing under suction? Also, the cost between 8" and 12" HVAC flexible duct is negligible. In my small brain this would more than make up for the resistance introduced by those pesky spirals and the dreaded turbulence they introduce. The discharge on my blower is an 8"x5.5" rectangle and there are transitions that could be adapted and reinforced that would make a handy job of this. Plus there would be no fittings to fret about. I can route the duct in a gentle and pretty straight line to the wall and a cement block that is a 3 space vent that I can enlarge without worry that my home will collapse.

What am I missing? Anyone else using this stuff?

[Ray Newman]

Where are you located?

Besides the possible EPA , State, and local ordinances about discharging dust into the air, would you be removing heated or cooled air from the shop?

[Ole Anderson]

I used a piece of the 7" diameter corrugated aluminum flex from HD at the inlet to my 2 HP Oneida and it works fine, haven't collapsed it yet with all blast gates shut. And in your application with it under a slight pressure you would have no problems. But I would use snap lock steel for the straight run, leaving the flex for the bends, cheaper and less losses that way. 8" should work fine, as losses at the pressure side are less consequential as those on the suction side.

And by the way, my local mill, which has been there for at least 40 years and is located on a busy state highway, only has a cyclone, no baghouse, and they haven't been shut down by the EPA or the state so don't worry about it. Now that doesn't mean you should aim it at your heighbors's house either.

[Myk Rian]

And by the way, my local mill, which has been there for at least 40 years and is located on a busy state highway, only has a cyclone, no baghouse, and they haven't been shut down by the EPA or the state so don't worry about it.

Would that be Armstrong? At least they don't have any close neighbors.

[Bruce Seidner]

I used a piece of the 7" diameter corrugated aluminum flex from HD at the inlet to my 2 HP Oneida and it works fine,

I am looking for the least expensive way to make a long run out the house from my blower outlet. If this aluminum holds up on your inlet should be no problem for my outlet.

What got me onto this was the need to replace a broken and totally rusted HVAC run from a fiberglass plenum which I did with insulated fiberglass flexible duct that I got at Lowes.

This stuff is about a buck a foot for 8" diameter duct and took all of fifteen minutes to install. But that is merely handling the duties of HVAC.

There is no free lunch. I'll plus/minus the options.

Thanks

[Van Huskey]

The Al version will likely last basically indefinately, I am actually more curious about the plastic version. IF you plan to use it to get by for a while I say go for it since the worst that can happen is a dust leak, by far not the end of the world. If you plan to try to avoid the PVC altogether I would just go with the aluminum since I am pretty sure it is a one time deal.

PS when I first read this thread I envisioned 20 extra feet of the plastic duct sticking out a window dancing like:

[keith ouellette]

My entire shop is piped with 4" thin wall pvc waste line. I have a 2hp collector out side of my shop in a closet. The waste line is probably much cheaper than any metal and mine was actually cheaper than the common 4" black flex tube.

[Bruce Seidner]

The Al version will likely last basically indefinately, I am actually more curious about the plastic version. IF you plan to use it to get by for a while I say go for it since the worst that can happen is a dust leak, by far not the end of the world. If you plan to try to avoid the PVC altogether I would just go with the aluminum since I am pretty sure it is a one time deal.

PS when I first read this thread I envisioned 20 extra feet of the plastic duct sticking out a window dancing like:

There is a lot of CFM lost to turbulence in a pipe and as in your example of the sky dancer there be much flapping that would go on. So while this is not critical to heat or cool a room and as HVAC duct is easy as pie to install, I have backed off the idea for my home run out the house. I will see what the PVC will cost when my ship comes into the supplier who is currently out of 8" D3034 or I may upsize the duct to 10" aluminum flex to compensate for the resistance of the ridges.

Funny you should have a picture of a sky dancer. I picked up one of those fans for very little money and it is 18" diameter and a 3/4hp fan that pulls 7amps at 3405RPM. I have been trying to figure out what fabric to use for the filter and plan on using it for the whole shop fan filter. It is a bit noisy but has 3 speeds. This reminds me that I need to call American Fabric and get their recommendations.

[Van Huskey]

I think using the plastiv HVAC ducting would be a poor choice long term, maybe not if the run was just a couple of feet from the cyclone to the outside but I still am curious how it would do in a longer term longer run. My gut says no but my mind wants to see it none the less. I just wasn't sure if you were just planning to use it until the PVC came in or not. Given that 10" Al HVAC pipe will give a 60% increase in cross sectional area I would think you will have more than compensated for the surface ripples, even if you overcompensate and the rate of flow drops significantly what you are trying to keep in suspension at that point is so light it won't cause issues. I can't remember who but someone once postulated in a thread that increasing the size of ones flexible drops to machines by 1" might be a positive since the surface undulations reduction of flow in the flex hose might cancelled out. In other words for flow a 7" flex hose might act like 6" smooth pipe. I have always thought that this was also a potential good idea but unlike your case you would have to be concerned about the air speed dropping to much, especially in a vertical drop.

If you get the fan built into a air filter I imagine it will work very well.