Anybody know how to make residential ductwork?

[rtimgray]

I'm working on my house expansion, and one of the items that I'm going to have to deal with sooner or later is a new HVAC system. As far as the unit goes, I think I can pick up a unit online or at a local distributor pretty cheap. The problem comes in with the duct work.

When we built our first house, I got the unit from a local distributor but then had to pay an ungodly amount for the custom-made trunk line going from the air-handler and running down the length of the house. All of the branch lines (the round duct) was pretty reasonable because I could buy it at a local hardware store for pretty reasonable prices - it was just the trunk that was so expensive.

I've probably got upwards of a year before I will be needing this stuff, so I've been doing a little research about alternatives. Here's a couple of my ideas:

1. Buy some equipment and make my own sheetmetal ductwork - I'm assuming this would require some kind of brake, but I don't know what kind. Also, I don't know what tool/tools makes the bends/laps that are used to join sections.

2. Use a different material than sheetmetal - I've found a little bit of stuff about fiber ductboard and it seems it just requires a few specialized hand tools, but I know nothing about this stuff - has anybody used it for anything?

I would appreciate any insight anybody can give me. I can tell you that I have had great difficulty with local HVAC providers (won't return calls, provide estimates, etc.) and would prefer to avoid them if possible. I picked up a manual on ductwork design, so I think I can handle the actual design/layout of the ductwork, and I've installed it all before in the first house - it's just the fabricating of the ductwork itself I'm missing.

Thanks for any help you guys can provide.

 

[chrisw212000]

Sheet metal brakes of any size cost a good bit. I've done some duct work with my brakes and had good results. Going down in size as you go tends to keep velocity up. I'm no expert by far. Maybe someone else will chime in.

 

[s219]

I don't think you need to use sheet metal -- our house built in 2000 has a pre-fab pressed foam/fiberglass/foil trunk that has the weight and consistency of a heavy cardboard.

A large (at least 16-24" dia) insulated flex line goes from the main blower to the trunk, and then smaller lines run from there to each room.

I had to run an additional branch line to our bonus room when I finished it, and was able to cut a new hole in the trunk with a steak knife. Connected the new line and then taped the joint with aluminum tape to seal it up.

I assume the main requirement with the pre-fab trunks is that you can get them into the space where they are needed. Ours seems to be sized to fit through a typical attic hatchway.

If pre-fab wasn't an option, you could consider building the trunk out of any suitable materials. It has to be as close to air-tight as possible, rigid, insulated, and amenable to attaching branch lines. A reflective exterior will help reduce radiation losses, but probably isn't critical. Who knows, maybe you can build the trunk yourself, and then wrap it in the same insulation they use for sheet metal ducts.

Someone smarter than me can maybe comment why sheet metal was originally used for ducts. Could have just been that it made the most sense when sheet metal was cheap.

 

[tractchores]

In some of the newer construction they're now using PVC piping instead of ducts. I don't know that the hvac unit is any different, but it delivers a high velocity of air through the pipe, so you get the same heating / cooling effect, but its much easier to install and takes up less space in the walls & ceiling.

Hi Velocity Small Duct Heating and Cooling Systems

 

[BeezFun]

I have my ducting made up at the local sheet metal shop, then install it myself. I take them cardboard templates and a drawing of what I want, they're real good about helping me get what I want. I've also had them make roof flashing the same way.

I'd be careful using materials like wood or anything that can support mold. Depending on your climate you can get some moisture condensing inside and on ductwork. I'm sure if you think it through you could make it work, but I'd stick with something designed for that kind of application.

 

[rtimgray]

Thanks for the replies so far. I'm really curious about the fiber ductboard because I've done some more reading on it and it is evidently approved for use as HVAC ducting and code acceptable in most locations (code is not a concern in my case, I just want to have a good material). I've looked at some of the cut sheets that show installation instructions for ductboard and it seems pretty straightforward, if you have the correct tools. The material itself also seems relatively inexpensive (compared to what I have paid for ductwork in the past), but I haven't found a local retailer yet. As with anything big and bulky, shipping/freight can eat up a lot of material savings quickly.

I appreciate any other comments and ideas that anyone has. Thanks again.

 

[dfkrug]

When we built our first house, I got the unit from a local distributor but then had to pay an ungodly amount for the custom-made trunk line going from the air-handler and running down the length of the house. All of the branch lines (the round duct) was pretty reasonable because I could buy it at a local hardware store for pretty reasonable prices - it was just the trunk that was so expensive.

I have done retrofit HVAC, and my new house design and installation.

You are correct that you can get insulated round Al duct at HD or Lowes.

If you go to a sheet metal manufacturer/HVAC materials supplier, you
can get that, plus rectangular ductwork and all the fittings, elbows, etc.
The prices are quite reasonable.

The only custom sheet metal work I needed has been the intake plenums
at the furnace, and the outlet plenums. You can make these up yourself
out of raw sheet steel, but you will need a brake to bend at least some
tabs. What I did was draw up what I needed and took it to the above
shop and they fabbed it in a couple of days. Typically only about $100
for labor and materials. At that price, it made more sense to have
it done there, as BEEZ has also discovered.

 

[nikdfish]

Another angle to consider might be one of the "ductless" heat pump systems like the Mitsubishi "Mr. Slim" (there are plenty of other manufacturers as well) to just add one or more new zones in the expansion area.

They only need a 3" hole/passage to the air handler location, which can be at a wall/ceiling junction or above the ceiling. Various configurations are available, single or multiple zones, visible vs. hidden, etc. They are available with some very high efficiencies - up to something like 28 seer. This makes them eligible for some of the tax credits & rebates available for improved HVAC efficiency.

We are considering going that route if we add the attic & basement to the house's conditioned envelope if the current system doesn't have the excess capacity to accommodate the add on.

Nick

 

[rtimgray]

Thanks for the suggestions so far. I've pretty much ruled out mini-splits if for no other reason than just the high cost of the copper linesets and the cost of a technician to connect them and install the refrigerant.

I'm still investigating the ductboard, but now I'm back to thinking about sheetmetal again. I've found a good local source for sheetmetal. I can pick up a cheap brake that will let me bend it in 3' sections, and I also found the tools that will let me make the bends for the tabs for the drives.

I could make the main ductwork out of to big "L" sections with tabs, but I think I would have to join them together along their length with sheetmetal screws. The manual that I have for ductwork shows using sheetmetal screws to connect sections of round duct together, so would there be any issue with having the sheetmetal screws protrude inside the trunk line? Just wondering what you guys thought about that.

Once again, thanks for the comments and suggestions. I appreciate all of the input that you guys have share.

Good luck and take care.

 

[rkeith]

Ductboard doesn't require any special tools, just a utility knife to cut it and HVAC tape (silver). I used ductboard when I built my house and it was easy to use and doesn't sweat! I then just installed collars in the duct board. I used flex between the collar and the register that was in the room.

This is important if the sheet metal will go into or through your attic. Attics are hot and the sheet metal will get hot thus the cold air will be warmed up as it travels through the duct. This is why ductboard is insulated on the inside and shiny on the outside (it reflects heat).

Just my :2cents: