Duct and Cover

I just bought a bunch of ducts at the big box to tackle my RMH project. They happened to be on sale even. But after coming home with a bunch of "Ts" and end caps, I noticed the Ts all have two receiver 'female' ends and one crimped 'male' end. The caps are females to fit over male ends. This means that if you use the cap on the only option available for a T section, you end up with a 90* turn that has two female ends and no male, requiring a double male adapter piece to be added to continue the duct run. Sorry if that sounds confusing, I'm not familiar with conventional terminology for ducting. That also makes it difficult to search for information like this, hence my line of questions.

Is there a reason for normal HVAC use that the third port would always be a female? Are T pieces that have 2 male ports and 1 female commonly available? I found caps that have a male fitting through the big box store. They are special order and end up costing a few cents less than the female cap and male to male adapter, but it seems worth it to me not to have an extra jenky connection to worry about.

Now for the real mind blower- does anyone make a standard duct piece that has 3 paths but none of them straight? I could use cleanouts in a couple of corners with the opening to the top, and a T doesn't provide this. Yes, I can put a 90* elbow with a T right before or after, but that might mess up the proportions I'm looking for, makes access to one of the runs more difficult, requires buying two special join pieces rather than one, and creates an extra join.

And while we're on the subject, would there happen to be anyone making fixed bent ducts at a comparable price? Amazon has some 4 and 5" 90* bends, but nothing larger. I'm seeing some from UK producers at reasonable prices, but they don't have the crimped end fittings and most are out of stock. Seems the majority of the time I'm either looking for a 45* or a 90* bend, and having something pressed to make those angles would be easier to work with and produce smoother flow to the gasses inside.

Maybe I should have started with the big daddy meta question here- how are the round HVAC ducts commonly used in US RMHs standardized? Is there some body that regulates this or did they develop through normal commerce? I thought Wikipedia would mention something like this but didn't see anything directly referred to in the articles I browsed...

I can't really speculate on whether there's any standardisation in the industry. Where I grew up, HVAC tradespeople came to a job with premade plenum and transition pieces fabricated at the shop, but there was inevitably lots of on-site customisation with aviation snips and rivets (and sometimes Colourful, Descriptive Language). If you have a pair of needle nose pliers and a bit of patience, you can crimp a bare end of sheet metal duct to fit into a receptacle. Another thing you can do is get some flat sheet and make your own transitions. I recently did this with some custom ductwork I built to install a couple of mass-produced heat transfer kits into my house (spoiler alert: this job also involved Colourful, Descriptive Language).

Go to a site like this one and select the shape you're trying to make, then enter the dimensions. It will spit out a plan that you can use to make a part that does exactly what you need. A couple pieces of flat steel and some clamps will help you make nice creases (so will a bench vise), and aluminium tape hides a multitude of sins. Besides, it's all going to be covered in mass...you just don't want any smoke or CO leakage.

A decent hardware store will have in the stovepipe/ductwork section a pair of duct crimpers, for maybe $10-$15. It is like pliers but with one beak on one jaw and two beaks on the other, to make a crimp in the end of a straight duct. Repeat around the whole circumference to get a crimped end. You may also find a multi-crimper to do more at a time, but that of course would require more effort to do.

Coydon Wallham wrote:Is there a reason for normal HVAC use that the third port would always be a female?

My guess, because it is easy to crimp an end if you need it to be a male but if it is already crimped and you need the female you are out of luck.

That is probably also why all the caps are female, they make one product and it is up to the installer to crimp the cap if they need to.

Yup, just crimp a female end to turn it male.  You can do it slowly with two needle nosed pliers or the fancy crimping pliers.

I've never seen a three ended duct piece that doesn't have a straight thru path.  Not much need for that in the ducting world...

Fixed duct pieces are fairly expensive so that's why most people use hvac fittings.

On the subject of crimps, does it matter for RMH ducts/exhaust pipe which way the crimps face to the flow?

Probably not. You're going to be facing more friction loss from the interior surface of the duct as a whole than the little zones of turbulence introduced by junctions. But once again, tape is your friend. Use lots of tape if you want things to be as smooth as possible.

I think (but don't know) that on horizontal runs you want the crimps such that the airflow doesn't get snagged on their ends as it passes.  So male into female.  But, on vertical runs you want it the other way so that condensation runs down the inside of the pipe instead of out the seams and down the outside of the pipe looking ugly.

Okay, so when making a clean-out, is there any advantage to putting the crimp on the cap vs. on the 'T' opening?

Once I start assembling, what are options and their advantages for securing the pieces together? I gather that 2 self-tapping screws on opposite sides is the most basic option.

Tape would add security to the seal, but I'd like to avoid it if I could for the main part of the run, only using tape around the core and barrel and vertical exhaust. How common would leaking be in pebble or plain earth masses without tape?

When I picked up a crimping tool, I saw next to it was another tool with a description about putting dimples in the duct ends to get them to lock together better. Has anyone here tried such a tool?

Okay, so when making a clean-out, is there any advantage to putting the crimp on the cap vs. on the 'T' opening?

I think fitting a cap inside of the 'T' opening would be easier, especially if the 'T' is surrounded by cob.

Most of the caps here only fiction fitted no screws or tape.

Untaped joints in a pebble mass might leak enough to be detectable under certain conditions. Cold mass and some weather patterns will make poor draw or even backdrafting likely, and that's when you could get CO into your living space. Cob will at least provide a barrier to gas movement.

Phil Stevens wrote:Cob will at least provide a barrier to gas movement.

I hear it's the natural builders duct tape...

Yes indeed Coydon..... in a world full of high tech solutions to all of our complex problems, isn't it nice to know there is (and always will be) a most simple and natural material that is often freely available to us all with so many uses.

Running into a new issue, is there an art/science to forming bends with the adjustable elbows? A 90* obviously only has one arangement, but a 45* has a few choices. I also could use some 27.5s and other odds ones, but what I've come up with is looking a little too Doctor Suess, should I be able to turn two corresponding sections to keep the elbow on the same plane at any angle? Unfortunately I've set the ones I have in the sand and they all have become difficult to experiment with, having a tendency to jam here and there...

Looks like to get any angle between 0 and 90, and to keep it in the same plane, you have to adjust each joint an equal amount.

Mike Haasl wrote:Looks like to get any angle between 0 and 90, and to keep it in the same plane, you have to adjust each joint an equal amount.

Nice concise video. I read the comments on Youtube and he also says the elbow joins are not airtight. Do people usually tape each seam in the elbow once it is adjusted properly? Maybe it should be stressed for pebble style heaters...?

You could definitely tape the joints.

There is also a flexible mastic that you can get, either in a caulk gun cartridge or in a pail for brushing on. Lots of HVAC installers use this stuff, as it's quick and easy to apply, and meets code in places where that matters.

mastic vs tape blog post

Phil Stevens wrote:You could definitely tape the joints.

There is also a flexible mastic that you can get, either in a caulk gun cartridge or in a pail for brushing on. Lots of HVAC installers use this stuff, as it's quick and easy to apply, and meets code in places where that matters.

mastic vs tape blog post

I have a big roll of 300* foil tape, that should be good enough all of the elbows. That mastic doesn't look like stuff I want to mess with if I take mine apart in a year or two to move or reconfigure...

My duct problems keep getting more esoteric. Bought some 3" x 48" 30 gauge duct to make a pocket rocket with. I was trying to snap the seam together into the round and my thumb would dent the wall before the seam would snap together, and the rest of the length would jump around like a snake. I found a video showing a guy doing 4" and using a buddy to pound the seam into a better angle on the edge of a vise. I got help from a neighbor and was able to snap it together with just a whole lot of strain on our thumbs. For future reference, anyone know a shortcut for a single person to manage this Herculean feat?