Assembling sheet metal ducts in situ

[Stephen Tashi]

TL;DR Summary

Is there a technique for assembling rectangular sheet metal ducts inside a ceiling cavity - as opposed to assembling them on a table top or other ideal situation?

In a house there is (typical of many 1950's vintage houses in my area) a framed cavity above the ceiling of a hallway and below the roof joists. It held a sheet metal heating duct "trunk line". The old ducts were removed since they had rusted through after being used (in the summer) with an evaporative cooler.

I'd like to replace the old duct work with new ducts. Is there a way to install new rectangular rigid sheet metal ducts in the cavity without tearing off drywall? - i.e. to install it only having access to the new work from the attic? The standard method for joining rigid rectangular ducts shown on internet videos involves using "drive strips" and having ample access to all sides of the ducts. Is there a slip-on type of connection that can be used instead?

(Of course, using round flexible ducts or round sheet metal ducts is another possibility. But I'd prefer to have one rectangular trunk line about 28" by 8" in cross section instead of several smaller round lines. The cross section of the cavity is 37 1/2 inches wide by 11 7/8 inches tall.)

Edit: I see that "frames" and "slip joints" are used in the UK. (I'm in the USA.) http://www.ductstore.co.uk/acatalog/Help_on_rectangular_ducting.html Those methods look more modern and saner than the method most used in the USA.

In USA terminology, the method using "drive cleats" is also called a "slip-on" method, but it's certainly more complicated than "slip-on" connections in other contexts.

 

[Lnewqban]

I would use Flat S Locks on four sides of each connection and install some screws around for eliminating any longitudinal movement, then seal with mastic all around.

 

[AlexTheParticle]

Hi there,

I'm not an expert in HVAC or construction, but I've done some research and here are a few options that might work for your situation:

1. "Snap-lock" or "snap-on" ducts: These are rectangular ducts that have a special locking mechanism on the edges, allowing them to snap together without the need for drive cleats or drive strips. They are commonly used in situations where access is limited and traditional joining methods are not possible. You can find these types of ducts at most HVAC supply stores or online.

2. "Flanged" ducts: These are rectangular ducts that have a flange on one side, allowing them to be connected to other flanged ducts using bolts and gaskets. This method is commonly used in industrial settings, but could also work for your situation. However, it may require some modification to the existing framing in your attic.

3. "Slip joint" or "frame" method: As you mentioned, this is a common method used in the UK and involves using frames or slip joints to connect rectangular ducts. This may be a good option for you if you are able to find the necessary materials in the US.

Overall, I would recommend consulting with a professional HVAC contractor to determine the best option for your specific situation. They may have other suggestions or be able to provide guidance on which method would be most cost-effective and efficient for your needs. Good luck with your project!