Ducting Question

[Shopshirt Mofo]

I've been a long-time lurker and have always been in awe of the knowledge base on the site that I haven't felt like I had much to contribute. That being said, I have a question about HVAC Ducting.

We recently purchased a home with an old ductboard system in the attic.

The ducting has been patched and still had some splits that I've temporarily repaired but there are still some leaks around the vents and I think the fiberglass is deteriorating etc...

What I would like to do is replace the ductboard with metal round duct of equivalent size. Are there any pitfalls to replacing the original ductboard with metal by doing the appropriate conversion and installed in the same layout and as the original?

 

[matt_i]

So when you put ducting into the "cold side" of the attic (thinking winter) then it loses a bunch of heat. In summer, flowing AC it can condense on the exterior and introduce water drips in a place you don't want it.

There's insulated round duct, or if you go with rectangle duct it can be "wrapped" in 1" EPS foam insulation (with the silvery skin both sides). I did the latter in my shop build.

The round metal is going to be hard to wrap with anything.

Also look at Carlisle HardCast Foil Grip tape. Very good at sealing both the ductwork and the insulation for air leaks.

Menards has a whole online catalog of metal ductwork. A lot of it is special order to the store, but its the most comprehensive catalog I've yet seen for a non-pro.

 

[Jackfre]

I would consider the metal in whatever configuration a substantial upgrade. If the duct board is deteriorating it is blowing fiberglass particles into the living space. Not good. Do the research, sizing and insulation well and you will be happy with the system. Also, do not assume that the original is correct. As long as you are doing this go at it with a clean sheet on the sizing.

 

[readhead]

Replacing the duct board with similar sized round duct will improve static pressure and may quiet it down some. As mentioned above check the sizing. This may be an opportunity to improve delivery. Consider dampers in the branch runs to help balance. Depending on the size of the house you may want to zone the system. There are plenty of insulation wrap options for round duct. On a very basic level this is DIY but it may be worthwhile to have some HVAC contractors look at it.

 

[OccupantRJ]

I did my shop system in the attic with round duct. It is relatively easy to work with. I wrapped it in 4 foot wide duct insulation after totally sealing it with brush on duct sealer.

After taping the insulation joints with UL listed metallic tape, I then went an extra step and lightly banded the duct insulation every foot with soft vinyl banding. I did not want to have to worry about insulation coming loose at a later time as I am not getting any younger. I have used the round metal duct on two systems so far and would recommend it.

I did my homework on airflow and sizing, and the system seems to flow nicely.

 

[Shopshirt Mofo]

Thanks guys...whatever I do would be definitely insulation wrapped.

 

[metlmunchr]

You can buy sleeve type fiberglass insulation for all common sizes of round duct. It eliminates 90% or more of the required taping. Makes the duct installation a bit slower since you work the sleeve onto the pipe as you go, but its a hell of a lot simpler than getting a good wrap job on previously installed pipe.

 

[Ohmthis]

Any idea how old the ducting is? Duct board has a finite life span, as I think you are finding out. Metal is in my opinion sooo much better that what you have. Correctly designing the new duct layout is extremely important to the ability of the system to work properly. My advice is to have the duct system professionally designed. There are online services for this if you find it difficult to find someone locally. It would **** to go through the time and investment of putting a new system in, to have it be not quite right. Good luck and post up any questions you might have.

 

[LS6 Tommy]

I'm not saying round is a bad choice, but keep in mind, it is more expensive than square.

Tommy

 

[walta]

Duct work in an attic is a very bad idea. All duct work leaks. The only question is how much, weather it is 5% or 40% it is pointless waste.

You sound like a good candidate for mini split system.

Walta

 

[bobbyjean]

box duct,duct mastic(sealer),foil tape.....
it's a sizable job to take on...is it ac only,flex runs to outlet's,central return?

 

[nsula_country]

I'm not saying round is a bad choice, but keep in mind, it is more expensive than square.

Tommy

Snap lock round is cheaper than square. Spiral tube is expensive.

CT

 

[readhead]

Round is less expensive for KD pipe and the aspect ratio is better that square.

 

[nsula_country]

Round is less expensive for KD pipe and the aspect ratio is better that square.

Correct, KD Pipe = Snap Lock. Very easy to work with.

CT

 

[LS6 Tommy]

Snap lock round is cheaper than square. Spiral tube is expensive.

CT

Just proves that I'm more of a service guy than a tin knocker!

Tommy

 

[PWC Repair]

Is the original system a sort of chase that runs directly over the hallway with a dump into each room basically right by the doorways?

 

[danski0224]

Snap lock round is cheaper than square. Spiral tube is expensive.

CT

Straight 10 foot lengths of spiral is cheap, pricing is not much more than 26 ga snap lock round.

They get you on the accessories like elbows, couplings and so forth.

That said, when the parts and labor are considered for an installed and sealed job, the rubber gasketed spiral and fittings is faster than going back and sealing the non-gasketed stuff.

 

[nsula_country]

Straight 10 foot lengths of spiral is cheap, pricing is not much more than 26 ga snap lock round.

They get you on the accessories like elbows, couplings and so forth.

That said, when the parts and labor are considered for an installed and sealed job, the rubber gasketed spiral and fittings is faster than going back and sealing the non-gasketed stuff.

Good to know. I will have to look into the rubber gasket fittings. I have only seen the hard fittings that require sealing.

CT