Durability of Insulated flex duct for exhaust?

G

Girino

New member
I'm installing my CV1800 in the attic over my garage, with 6" mains ducting. I'm thinking of using 8" insulated flex duct for the return to reduce noise in the return run. My concern is that specs for the insulated HVAC flex duct max out at 200 CFM. I'm concerned that over time the plastic liner will break down under the higher airflow of a DC. Any thoughts?

I'm OK with springing for spiral duct for the return, but I'm looking at about 20 linear feet with two or three 90 degree turns, and the 90 sweeps add up for spiral pipe and it will be noisier than flex duct, so I'd rather not spend that if I don't have to.
 
Girino,

Flex HVAC ducting is ribbed on the interior. This ribbing increases static pressure. Whether on the input side or output side of the CV1800, the increased static pressure could, I suspect, reduce air flow.

I am not sure I understand your plan. If by 8” return you mean installing 8” flex HVAC duct between the 6” mains and the CV1800, that is, on the intake side of the CV1800, I would think the 8” flex might collapse or its membrane could tear. Even if it does not collapse or tear, erosion caused by wood dust and chips rubbing the side walls could cause it to fail.

If the 8” HVAC Flex duct is on the output side between the CV1800 impeller and the filters, pressure could cause the flex duct to tear, although I am not sure of the pressure on the output side and increasing the size to 8” reduces that pressure some. Erosion, though less of a problem, could eventually cause the flex duct to fail.

On the other hand if the CV1800 and the filters are located in the attic and the goal is to return air from the dust collection filters to the shop, the flex duct could work well, although using a larger flex duct than 8” or a pair of 8” flex ducts would reduce air velocity and static pressure in the 20’ run with elbows and probably reduce the sound of air rushing through the return.

HVAC duct work insulation is available at my local home center. If you elect to go with solid pipe, the duct insulation could dampen some sound. However, sound transmitted through the inside of pipe probably remains unabated.
 
Thanks, sorry I didn't make my setup clear. I have enclosed the cyclone and filters in a box in the attic so the return will be from that box back to the shop, downstream of the filters. 10 inch flex duct would fit in the attic space I have available so I might go that route.
 
See my response in a the HVAC Flex thread posted today for a low-pressure use of this lightweight duct -- I agree with JSbrow that typical HVAC insulated flex duct would not be suitable for use between the blower and the filters as it is designed for very low static pressure in home HVAC systems. I strongly suspect that the HVAC dust will burst or come loose at connection points if used as you suggest. I would recommend that you use 8" metal duct or bite the bullet and buy 8"flexible PVC duct. You can buy clear PVC hose by the foot through McMaster Carr -- here is a link.
 
I re-read your last post and realize that you are downstream of the filters -- 10" HVAC duct might work for you and it is not all that expensive. The downside is significant static pressure loss for the amount of airflow. If you have joists running in the direction of your return, you might consider making a plenum by removing the attic insulation and adding some cleats along the joists to support hardboard above any ceiling below and then cover the top with hardboard to create a smooth plenum that you can transition to from your filter box and then add a ceiling register from below in your shop to return the air. Obviously, you'd need to block the ends of the joist cavity, but this would have the lowest static pressure loss.
 
I used about 15 feet of 10" insulated duct and 8 feet of 14" insulated duct to return airflow from my external shed housing my CV 1800 back to the interior of my shop. The velocity in the duct is not excessive and I expect that it will be very durable. The pressure drop from the return duct run is negligible. I also used a short length (about 18 inches) of 8 inch flex duct to connect the discharge from my CV 1800 to the filter stack. It has been in place almost one year, and seems to be doing fine. It is set up so that it is stretched out to near maximum length, making the interior smoother.
 
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