SuperDuct...can I join metal ductwork to it?

Hi All, Construction of my basement 1 rm studio is underway, and I hit a major DOH moment when I realized that I had forgotten about the HVAC truncs. Fortunately the ceiling isn't up yet so all hope is not lost. I'm looking at replacing my main send and return truncs with John Manville Superducts...but keeping the existing sheet metal feeds that come off of these lines (ones that go into the studio will pass through a series of baffles). Does anyone here know if metal feed lines can be attached to the lined superduct , or does the whole system need to be superdcut to work? Obviously, going with superduct everywhere will be quieter, but is it possible to mate metal ducts to the superduct. I imagine if it is possible that I will get some benefit, as these lines are about 14 feet long...there should be some attenuation. Also, doesn anyone have any rought $ on the costs of superduct? Cheers Kris

Okay, did more quick research. Looks like there might be a local supplier for the Johns Manville duct liner Linacoustic R-300....the rigid fiberglass product that you can fit into ductwork. Obviously since this stuff is meant to fit into a metal duct, there's no problem doing metal duct feeds to it. Has anyone here used duct liner....probably. If you have, and you're reading this thread, please let me know what you think of it, how easy/hard was it to use, and how effective is it? Cheers, Kris

But if you just use the liner, you would still have a continuous connection through the metal ductwork... Me thinks it might be better if you at least partially used the far less rigid superduct... or the OC equivalent...

If your concerned about the AC working properly with two different kinds of ductwork, that's not a problem. As long as you can mechanically join the two and keep the air from escaping, the AC will work fine. What you want to make sure that you do however, is do the best you can to make the downflow seam so that it doesn't protrude into the duct, thus impeding airflow. If you are concerned with sound transmission issues with joining the two different types of ducts, I'm thinking that you're on the right track with the ductliner in that it will considerably help the metal part of the duct. Generally the metal duct has very little insulation on the inside. Sometimes it's just enough to keep it from sweating on hot days. Scott

plus6vu wrote:

If your concerned about the AC working properly with two different kinds of ductwork, that's not a problem. As long as you can mechanically join the two and keep the air from escaping, the AC will work fine. What you want to make sure that you do however, is do the best you can to make the downflow seam so that it doesn't protrude into the duct, thus impeding airflow. If you are concerned with sound transmission issues with joining the two different types of ducts, I'm thinking that you're on the right track with the ductliner in that it will considerably help the metal part of the duct. Generally the metal duct has very little insulation on the inside. Sometimes it's just enough to keep it from sweating on hot days. Scott

The thing that bothers me with this pproach is that you are ow shrinking up the duct size - which means increasing the static pressure on the system - which might be already maxed out based on the motor/fan - when you use insulated duct the inside of the duct is the same size as the metal - when you use duct liner you increase the duct by the liner thickness so as to maintain the same design parameters. Thus -with 1" liner - for a 12x20 duct - you would construct a 14 x 22 galv shell. I would not proceed with liner until I had a mechanical engineer verify that my system could handle the added static pressure while still maintaining my building requirements. BTW - the answer to the original question is "yes - youcan transition from duct board to metal duct. The installation manual is clear on how to acheive this. Take care, Rod

Hi Rod, thanks for the input.... I'm waiting for a return phone call from the local distributor of the Johns Manville product with regard to your concers. The same thing occured to me on Friday...since I'd be reducing the cross sectional area of the duct, what is the impact on pressure? we'll see what he's got to say on that.... For now, it looks like I might just not worry about the ductwork at this time, and worry about fixing it at another time. Yeah, it'll be more expensive to deal with it later, but at least the ductwork is contained within a boxed in section of my ceiling, so the damage done when I have to replace my ducting will be localized. I'll probably be installing a HRV in the studio to provide fresh air, and am considering building removeable vent covers for the connection to the main HVAC system. That way I can disconnect the room from from the main HVAC when I'm going to be loud (like rehearsals), and run the HRV (connected only to the studio) for fresh air. We'll see what's necessary... Cheers, Kris

drfrankencopter wrote:

I'm waiting for a return phone call from the local distributor of the Johns Manville product with regard to your concers. The same thing occured to me on Friday...since I'd be reducing the cross sectional area of the duct, what is the impact on pressure? we'll see what he's got to say on that....

Kris, I wouldn't put a whole lot of stock into what he has to say......... he is a salesman - sells product to make or line duct - that doesn't make him an expert in HVAC design - What matters is your existing fan coil system - what it's design limits are for static pressure - what the existing static pressure is - and what it would be if you shrink the duct size. Typically most residential systems do NOT have a lot of slop in the design...... this because they aren't usually designed by engineers - but rather by the contractors installing them - and they want to get your house cooled and heated for as little money as they possibly can as to improve their profit margins. So they do not plan for people adding onto these systems unless they are specifically told by the homeowner to do exactly that.. Your equipment has listings on it - and the actual static pressure can be measured at the unit to determine what is available (if any). Sincerely, Rod