Outside Ducts: Issues?

[CuriouslyGeorge]

Besides not looking pretty, is there any code issues (or other concerns) which come to mind when running a duct along the outside of a building? From the ground to a 2nd floor? I know this is atypical, and code varies state to state, but I'm doing atypical HVAC applications...

Does this set off a red flag with anyone regarding any concerns, in general?

Thanks,
CuriouslyGeorge

 

[walkes]

Other than being kind of ugly, it can be necessary due to things like working with an existing structure, or with an Architect that won't concede space for ductwork inside the building.
However, when confronted with exterior ductwork the gage of the material is heavier than interior ductwork to protect from abuse. The insulation on the duct may be a rubber based material with a UV protective coating.
Also, I would try to ensure nobody can climb the duct and endanger themselves.

 

[CountOlaf]

Ductwork run outside is not that uncommon. You won't see it too often for a commercially aesthetic facility, but at industrial locations it's done fairly often. I don't think the sheet metal gauage needs to get heavier just because its outside, unless, like walkes said, it's routed in an area where someone might climb on it (say a horizontal run on a roof). Regarding insulation, for exterior applications, I always call for lagging or field applied jackets which are made of stainless steel or aluminum, and all joints have to be caulked weathertight. This will increase the cost of your insulation (of course) but again, these kind of things are done indoors regularly in areas subject to damage, like mechancial rooms.

 

[ChrisConley]

We have an older office building in our downtown that installed an air conditioning system with two duct runs along the outside of a 12 storey building from the roof to the main floor.

I like ductwork and mechanical equipment but everytime I drive past the building I cringe.... but no code issues that I know about.

 

[misterd]

Snow and ice load? Wind load? Maintenance load? The choice of material might vary depending on your environment. Also, don't forget special support or hanger etc. These factors are well documented in "Round industrial duct construction standards", published by the SMACNA.

After reading that book, you might have a totally different point of view about the "atypical" installations... Have fun.

 

[CountOlaf]

MrD: Not that there aren't snow and wind loads, etc. to consider; but technically, industrial ductwork is usually associated with industrial processes where gases are carried at high temperatures or the particulate is abbrasive or corrosive and of course there are extreme pressure classifications. e.g., hood type systems, etc. I think our guy is talking about conventional comfort cooling/heating. By the way, why doesn't SMACNA's "Rectangular Industrial Duct Contruction Standards" or "Accepted Industry Practice for Industrial Duct Construction" apply as well if the round does?

If you're going to pick up a SMACNA book, get the important one: "HVAC Duct Construction Standards, Metal and Flexible". This one is the one most commonly cited in specifications regarding HVAC, and it contains details on supporting ducts from walls as well as a chapter on "Exterior Components."

 

[lilliput1]

Use elastomeric material with weatherproof & UV proof coating instead of jacketed insulation which will eventually leak water in causing ductwork to rust.

Remember outdoor ductwork supports have to overcome wind loads, snow loads & temperature changes.

Architects may want it hidden from view or screened.

 

[friartuck]

Rubberised insulation can inflate if not well sealed. A metal cladding is preferred to prevent this.

add a suitable drain in case water gets in.

Friar Tuck of Sherwood

 

[lilliput1]

Friartuck,

I have lost confidence in metal jacketed insulation when we opened the insulation on roof mounted condenser water piping. The piping was so severely rusted the metal pelled off in circumferential layers. The entire piping run had to be replaced. The insulation vapor barrier was compromised & the jacket trapped moisture contributing to te rusting.

Properly applied elastomeric insulation with manufacturer's recommended adhesieve should not inflate and should be vaporproof.

 

[cme]

FWIW, my specs:

chilled water piping in mech rooms gets armacell, outside gets pittsburg corning with alum jacket (make sure the PC gets sealed at the joints)

ductwork outside gets armacell with rubber roofing jacket or if its too costly, I spec armacell with factory applied weatherproof coating/mesh and exposed edges get a brush of coating (coating is by armacell)

osa ductwork in mech rooms gets armacell

just support and strap vertical ductwork outside

 

[KenRad]

lilliput1,

I have experienced the same on exterior metal-jacketed condenser water pipe, but only with fiberglass insulation. The fiberglass absorbs any water that makes its way through the jacketing, and the piping stays wet for a long time. There are enough closed-cell options for pipe covering that I don't allow fiberglass at all anymore. Armacell and styrofoam are good for low temperature; polyisocyanurate (Dow Trymer) is good for up to 300F.

A good alternative to metal jacketing on exterior piping is UV-resistant PVC, with all solvent welded joints.

---KenRad