# How do I replace a damaged section of air duct?



## poiihy (Aug 18, 2015)

I found this article here which helps:
http://homeguides.sfgate.com/splice-hvac-supply-line-31457.html

Also here's another thing. I don't want to buy 25ft of air duct for $30 when I only need a few feet. Could I use some short solid metal duct piece to splice the flex duct together? Something like this?


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## beenthere (Oct 11, 2008)

Get a new piece of that size of flex duct. And a 2' long metal pipe of that size. Cut the pipe in half. Cut out the crushed section. Insert the pipe in one end, and put a new piece of flex duct on it. Do same with other end of duct. Use duct ties to fasten.


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## poiihy (Aug 18, 2015)

beenthere said:


> Get a new piece of that size of flex duct. And a 2' long metal pipe of that size. Cut the pipe in half. Cut out the crushed section. Insert the pipe in one end, and put a new piece of flex duct on it. Do same with other end of duct. Use duct ties to fasten.


Ok thanks.

But where can I find a short piece of flex duct? All I can find on HD's site is 25ft of flex duct for around $30. I don't want all that just to cut a little piece off. :\

Maybe I can use a longer piece of solid duct, cut the existing pipe, and attach the two sides to the solid duct like you said. Then reuse the old insulation of the damaged duct to wrap around the solid piece and wrap in tape to hold it together. Would this work? May be a bit of a hack job but... it would be fine wouldn't it?


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## beenthere (Oct 11, 2008)

Won't find shorter lengths of flex for sell.

The insulation must over lap the 2 ends of the flex.


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## HVACDave (Oct 16, 2007)

If your old flex is that brittle you may end replacing the entire run anyways once you try cutting in to join it together, $30 is not a very large investment to make compared to what it would cost to have someone come out to make those repairs for you.


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## poiihy (Aug 18, 2015)

HVACDave said:


> If your old flex is that brittle you may end replacing the entire run anyways once you try cutting in to join it together, $30 is not a very large investment to make compared to what it would cost to have someone come out to make those repairs for you.


It's not that old; the house is only 15 years old. I already cut into it. It's not brittle at all, it's fragile....

I cut open the duct and saw the damage... and I found that the duct is made of a spiral wire and very thin plastic film!!! :surprise: No wonder it broke. The plastic is like the plastic used for clear tape.

It says 8" so I suppose that means it is 8 inches in diameter or an 8in duct.


Pics


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## poiihy (Aug 18, 2015)

Will this solid duct fit inside the flex duct to connect to it?
http://www.homedepot.com/p/Master-Flow-8-in-x-5-ft-Round-Metal-Duct-Pipe-CP8X60/100172984


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## supers05 (May 23, 2015)

Solid pipe will work fine. Make sure you insulate it though. You'll lose a lot of heat-a/c out of it if you don't. I use metal pan, self tapping screws and foil back tape. Many people use very large zip ties or gear clamps. 

Cheers!


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## poiihy (Aug 18, 2015)

I got a piece of 7" solid duct. At first I was at the wrong place at HD and all I found was semi-rigid duct (which was $16!), but later I found the correct place and found the solid duct.

They didn't have large enough hose clamps. Largest they had was 6" but the box and label said 7". So we got large zip ties instead.


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## supers05 (May 23, 2015)

Sounds good. Post pics after you're done. 

Cheers!


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## poiihy (Aug 18, 2015)

Ok I spliced the pipe. And it did not go very well. Because the air duct went into the second attic so it went through a hole in the roof boards.

Those zip ties didn't really do anything. I pulled them as tight as possible but the flex ducts still easily slid on the solid duct. Maybe I should've used an 8" duct instead so the flex duct stretches a little to make it tight.

So I wrapped one end with the duct tape and that went fine. But wrapping the other end was hell because it was in a really tight corner! I could not fit the damn roll of duct tape between the solid duct and the roof to get it around!!! :vs_mad:
It really frustrated me so I did it later.
Luckily I had an old roll of duct tape that was a lot smaller. Although it stuck to itself a lot more strongly so it was hard to unroll. But I wrapped the duct enough to seal it.

Then I tested it and there was no leaks. I felt the output of the duct that it feeds and for some reason it didn't output very much compared to the other ducts.  I wonder why. Maybe because I made a tight bend.
I didn't bother insulating it that time because we weren't using the A/C at all that time and I was tired.
We rarely use the ℀ in that house anyway and we never use the furnace because we have no gas! And we only stay there for something like a week every 1-2 months.

I forgot to bring the reciprocating saw to cut the roof boards between the main attic and second attic. So I had to cut my new access hole with a drill and wallboard saw. (The hole is in the top of the last picture in a previous post on this thread). The original access hole (not sure if it was meant to be an access hole) was at the bottom in the very tight corner and a pain to get through, and it required squeezing over the duct to get through it. Which is why the duct broke. It's now a lot easier to get to the second attic now that I've punched a new access hole.

The hole for the air duct did not allow me to fit the solid duct while having it at a small angle to be parallel to the original duct. I tried to widen the hole but found that a joist (or whatever you call the woods that hold the roof panels & shingles up) was in the way so even if I removed the plywood panelling around there, the joist would still be in the way. So it ended up like that.
I needed a more flexible duct but the semi-rigid duct of the same size is like twice as expensive!!! And it's a lot lighter and thinner and more fragile!! Why so expensive?! :vs_mad:
Instead of this hack work I could've just taped the old duct and be done with it.

Damn I hate air ducts. Why do they make them out of extremely thin plastic like the cheap clear tape from dollar tree?! They easily break and you would have no clue that there is a leak! Why don't they make them of a more durable material like the rubbery plastic and fiber-reinforced material used for duct tape?
So after you break them you have to buy a replacement. In which you find out that air ducts are so damn expensive!
Then once you get an expensive replacement it is a PITA to work with, _especially_ in tight locations. You might break it even more in the process.
And they steal lots of space in the attic and get in your way, making it difficult to get around and do work (which is why you break them). Plus the big fat machinery.

Why do people use centralized heating systems with crappy PITA air ducts?! Why not just have an ℀ unit in each room? It's so much easier and simpler. All you need is dedicated circuits going to each unit. They're more efficient because they heat/cool the air right in the same room. With a big fat centralized box you need a big huge powerful fan to pump air through the ducts, and then you loose the heat or loose the coolness (have heat leak in) over the length of the ducts. And with all those ducts you can have failures anywhere and you would not have any clue.
Another benefit of a decentralized system is you can heat or cool only the rooms you want, to different temperatures, if you want. So to save money an entire family can stay in only one or two rooms and only heat or cool those rooms. Or, if they want to heat/cool the entire house, they can control all the units with a main thermostat.
And also if one ever fails, it is a minor issue. If you're central ℀ fails on an extremely hot or cold day, you bake or freeze.


Why do people have centralized systems? Is it cheaper?

I would never install a centralized system. Ridiculous piece of junk.
Same goes to water heaters...


Ok here are the pics.










Here's a comparison between the big roll of duct tape and the tiny one that saved my behind. (the small one was a little bigger; I used some).


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## poiihy (Aug 18, 2015)

Oh I didn't get the full picture. To the left of the splice on the left there is a rather tight bend up to a y-splice.

Damn dust photobombed the picture. 

About the dust, that attic is SOOOO DUSTY! The insulation is made of some recycled paper crap and consists of bits of paper and paper fibers. I can see words on the larger particles. Occasionally I'd find a very large particle and even a piece of plastic. I once found some piece of plastic with some part of nutrition facts on it. It was apparently packaging for food.

And on top of that is a thousand years of dust. On the original undisturbed insulation that is. The dust is a layer that is a few millimeters thick. Whenever I would dig up the compressed insulation, a huge dust storm goes everywhere. Good thing I have a nice 3m face mask with an output vent. After I am done working in the attic, the dust mask is full of dust, which I would wash off. (Could you imagine if I didn't have a mask?) There would also be dust and insulation particles everywhere in the house, especially the bathroom where I shake off the dust.


That insulation is crazy. If I work on something, then work on something else for a little bit and come back to the first thing, I'd find it covered in insulation and dust again. And another thing, it is very easy to loose stuff in the insulation. They just magically disappear and you cannot find it anywhere! Luckily I didn't loose any tools up there. I did loose some a few times but I always found it the next day of working. I lost a screw once, though. I was installing a Westinghouse Saf-T-Bar for a new ceiling fan (my first one) and the screw disappeared. I did not know if it fell through the hole in the drywall or just got lost in the insulation. I couldn't find it anywhere, so I had to quit for the day until I got a new screw to install next time.


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## user_12345a (Nov 23, 2014)

Central system is cheaper to install and maintain, but yah, systems such as yours are crap to be blunt. Air ducts should be 100% metal, installed in a conditioned space, not a boiling or freezing attic. Central forced air works great if done right. The savings in the winter from keeping unused rooms cooler aren't that high because the rooms aren't insulated from each other. 

Mini-split a/c with multiple heads is very, very efficient, but also expensive and loaded with electronics. Some may not like the look of the heads.

Sorry to say:

The joint you created will fail in under 3 years.

Duct tape dries out and isn't suitable for air ducts at all.

The proper way to do it is to is to use straps, then seal with something like foil tape. Would probably use some metal pipe clamps to go from metal to flex. The metal must be insulated, otherwise you'll get water damage, high heat gain/loss.

Would pull back the outer jacket/insulation and pipe clamp the actual thin flex to the metal, then cover the connection with the outer jacket/insulation and tape with foil or if you're going plastic to plastic insulation, maybe with red tuck tape.


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## poiihy (Aug 18, 2015)

user_12345a said:


> Sorry to say:
> 
> The joint you created will fail in under 3 years.
> 
> ...


Yeah probably. I probably have to redo it anyway. it's a lousy splice because things went wrong and now the air output is weak probably because of the tight bend. I need flex duct. But I don't want a whole 25'. Maybe if I can find a short piece somewhere, then I would splice it in.


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## poiihy (Aug 18, 2015)

How is a mini-split more efficient than an in-wall ℀ unit? It has to pump the cold liquid farther. The only reason I see why they have the separate units is so that it is quiet.


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## user_12345a (Nov 23, 2014)

Metal is fine, key is to use proper elbows so there aren't sharp turns.

Also the shorter and fewer turns, the better.

I know it's tough to work with air ducts in tight/dirty spaces spatially trying to get stuff to fit together while sweating and getting pissed off. done a little of that, granted not in a attic.


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## user_12345a (Nov 23, 2014)

> How is a mini-split more efficient than an in-wall ℀ unit? It has to pump the cold liquid farther. The only reason I see why they have the separate units is so that it is quiet.


The loss from pumping is minimal.

The liquid feeding the indoor unit is actually warm, it only gets cold when it hits the metering device and the pressure is greatly reduced, boils off at a low temp (boiling point proportional to pressure) picking up heat from inside, returns to the compressor as a low pressure, cool vapour. Only gets hot again once compressed. *Temperature is the intensity of heat, not the amount of heat - volume, state (liquid/gas), pressure greatly influence total heat content, call enthalpy.*

The mini-splits are designed with larger coils to reduce the amount of work the compressor has to do (has to do with compression ratio, getting into refrigeration theory here) compared to through the wall or window units. The space for coils is very limited in window/wall units.

The mini-splits also have variable capacity compressors, they can can modulate to meet the needs of the space. Running the compressor at a lower speed most of the time saves energy; just the physics of coil size vs compressor capacity. 

Even the least efficient conventional straight central a/c units are more efficient than wall or window units, but duct losses often wipe out any savings.


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## beenthere (Oct 11, 2008)

poiihy said:


> How is a mini-split more efficient than an in-wall ℀ unit? It has to pump the cold liquid farther. The only reason I see why they have the separate units is so that it is quiet.


They can't make the coils of an in wall unit big enough to get the 20Plus SEER a mini split can have. Unless you want a 3'x3.5' hole in your wall for a 2 ton unit.


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## supers05 (May 23, 2015)

Welcome to HVAC. This is why some people change through the nose for simple work. (It sucks) 

Cheers!


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## user_12345a (Nov 23, 2014)

Working with air ducts really sucks.

Service is probably not too bad.


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## poiihy (Aug 18, 2015)

supers05 said:


> I use metal pan, self tapping screws and foil back tape.


How would you use screws? You'd screw through the thin flex duct and into the metal duct? If so, how can you do that with such a thin film duct? I would imagine it would tear apart in no time.


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## user_12345a (Nov 23, 2014)

Screws are for metal, not for flex at all.

Metal ducts are screwed together, then sealed. Flex is strapped/clamped.

Duct tape? Good for everything but air ducts.


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## beenthere (Oct 11, 2008)

user_12345a said:


> Screws are for metal, not for flex at all.
> 
> Metal ducts are screwed together, then sealed. Flex is strapped/clamped.
> 
> Duct tape? Good for everything but air ducts.



Screws in conjunction with large flat washers work great on flex to metal connections. The washer clamps down on the wires of the flex.


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## supers05 (May 23, 2015)

I mostly use semi rigid flex, or fab5 which is considerably stronger then what you're using. Either way, with the stuff that you're using, larger washers work great. It must be accompanied with foil back tape, or it'll rip through. (foil back is like duct tape but uses aluminum foil and a much better adhesive) 

Cheers!


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