A/C ducting - Probably simple questions

[MV Wanderlust]

The a/c in our salon has a funky smell. After eliminating everything else, we've determined the 6" hose from the a/c unit to the vent is probably mildewed inside and should be replaced. We plan to replace it with some insulated hose to help fight the condensation issue.

When inspecting the routing of the hose, to see what we're up against, the end of the hose makes a 90 degree turn and is then simply forced into a 6" hole in the cabinetry behind the wall panel. The air then flows through a small chamber in the panel and makes a 90 degree turn to go out the vent at the end of the panel. There is no routing or channeling of the air at the end of the panel, like a plenum or anything.

Is this normal construction for marine a/c, to just cut an opening in the panel and shove the duct hose in it? My first thought was to stop at Home Depot to pick up an aluminum 90 degree elbow so the 6" hose doesn't have to turn. I was going to spray paint it with Rustoleum paint in an attempt to ward off marine corrosion. But then I started wondering why such ducting wasn't already there and decided to turn to the generous brain trust on the forum.

Should I avoid the aluminum elbow and just cram the end of the hose into the opening in the panel? What do others have?

[Cheechako]

Aluminum should be fine unpainted, especially inside.

[redsky49]

"Should I avoid the aluminum elbow and just cram the end of the hose into the opening in the panel? What do others have?"

No, you shouldn't.

Best practices and standards should be observed as much as is possible.

If I understand your situation, you are using your boat's wooden casework/cabinetry to transfer conditioned supply air, from the discharge end of the duct to the supply register/grille. Is that correct?

If so, wood is a poor material for ducting conditioned air, and should be avoided. Not acceptable by any building code I am aware of.

Instead you should, from the discharge outlet of the AC unit, extend, in a gradual fashion, flexible wire-reinforced duct to either a plenum or back box for the air outlet. 45 degree bends are better than 90 degrees. Bends too close together never allow the air to straighten out and should be avoided.

Pre-made back boxes are available here:
Home

Any sheetmetal fabricator would be able to fabricate a custom piece for you. While the best materials would be either aluminum or stainless, galvanized steel should serve well for many years. These materials will also be less likely to harbor bacteria and can be successfully cleaned.

Ductwork, either flexible or rigid, should always be insulated when passing through non-conditioned space. For the short section you are describing, insulation would be optional. Ducts should never extend through machine spaces.

As for odor, the first place to look are those surfaces that are frequently wet - condensate pan, evaporator coil, etc. These should all be cleaned at least annually with an anti-microbial agent. The entering side of the coil will always accumulate lint and dust, and will be the first to start little colonies of bacteria. Vacuum when dry.

Also make sure the condensate is not discharging into the bilge. It should go to a dedicated sump and be pumped overboard. Condensate from an air conditioner is not the same as distilled water. It contains trace metals and bacteria.

[thinwater]

Actually, you really need the insulation. The lack is what cause the smell. Yes, most marine AC is run with hose and no rigid duct. Home Depot probably sells the duct.

Do make certain that you seal the vapor barrier to the tube with duct tape at both ends; humid air must be kept out of the insulation.

There may be other serious installation errors. Is the return air draw from bilge spaces? This may smell if damp, and can be dangerous if it draws from engine spaces. Where does the condensate go? Are the drains clear? Are their any seawater leaks on the unit? These can destroy it quickly.

(I now see Redsky hit many of the same points... because they are standard concerns.)

[mickeyelam]

The first point I'd make is that, if you're going to use duct tape - and you probably should - for this installation, be sure to get the good stuff. Normal duct tape (the cloth/adhesive) type we all think of first when someone says "duct tape" isn't really suitable for use in duct work. While it's a bit more expensive, you'll find that an installation will perform MUCH better if you use one of the UL listed/approved tapes such as:

Nashua Tape 2.5 in. x 60 yd. 324 Amp Premium Foil UL Listed HVAC Tape-1207794 - The Home Depot

It seems strange but the adhesive on "normal" duct tape doesn't hold up well when subjected to the warm/cool humid cycles present in an HVAC system.

The second point here is that you may have something else going on if the INTERIOR of your salon HVAC duct is mildewed. Air flowing through a nominally functioning A/C duct should be cool (obviously) but also dry, as the A/C acts much as a dehumidifier does. Replacing the existing duct with an insulated type will help to prevent condensation on the OUTSIDE of the duct, but condensation on the inside of the duct should be limited. The usual situation is which one sees condensation on the inside of an A/C duct occurs when the A/C unit fan ceases blowing simultaneously, of nearly so, with the cutoff of the compressor unit. Such a situation allows cool air to remain in the duct to be slowly replaced by warm, humid air that then results in condensation forming on the cool walls of the duct.

I'd certainly investigate options for running the fan for some period of time after compressor cutoff as an idea for preventing future re occurrence of this issue. The amount of time for fan blowing after compressor cutoff will, of course, depend upon a number of factors such as duct length, ambient temperature and humidity, etc.

[svHyLyte]

Originally Posted by redsky49

...
As for odor, the first place to look are those surfaces that are frequently wet - condensate pan, evaporator coil, etc. These should all be cleaned at least annually with an anti-microbial agent. The entering side of the coil will always accumulate lint and dust, and will be the first to start little colonies of bacteria. Vacuum when dry...

This! Particularly in Southwest Florida including Sarasota, Bradenton, Palmetto. If you do not have/use on already, install a filter on the intake side of the A/C system. In our case we use a simple 12" x 12" disposable filter from Ace Hardware on the inside face of the return air grid that we change every few weeks during the summer months. Further, periodically use a good coil cleaner on the evaporator grid to remove debris that becomes a medium for bacterial growth. Once cleaned out you can spray some 90% alcohol from the drug-store on the coil grid with a hand pump sprayer to dry it out. We have also found that periodically adding some Raritan CP (an enzymatic cleaner) to the condensate pan helps keep that clean and odor free. The "stuff" isn't inexpensive but one doesn't need much and it does work well.

FWIW...