1 year old Trane XL19i- Air handler in attic w/horizontal layout. Installer connected 18" flex supply duct with galvanized steel sheet & collar at end of air handler. They didn't insulate the sheet metal @ air handler box. I found condesation collecting on bottom of air handler box. Water causing rust & finally finding path out of box. Drip tray is handling condensation from coil just fine.
What is the correct method to prevent the condensation, insulated inside, outside or both? I thought I'd use reflectix & waterproof Alum. tape. Should get the original installer to fix it or correct it myself & know the job is done right?

This may not be the only problem. How did this pass inspection?

This is system replaced a 13 tear old Carrier AC last year. There was no inspection done. Do you have any ideas on preventing the condensation problem? Thanks.

If you insulate inside the duct, you'll reduce its cross-sectional area more than you think, which could affect the performance of the unit. My guess is the best thing to do is insulate the outside of the aupply outlet. I would use styrofoam, at least 1/2 inch, up to a maximum of an inch, all the way around the outlet, held on with a good tape. That should solve your problem without affecting the performance.

I'm assuming it's strictly an A/C unit and not also a heating unit.

Insulating the return duct on the outside as Richard suggested is quite necessary. I doubt that this is the source of your condensation. Be sure that the condensate drain is clear, properly trapped and pitched. Be sure there is only one filter in the return and that it is clean. I'm not familiar with model numbers, but it sounds like a variable speed drive. Of course this will at lower speeds reduce the temperature inside the airhandler, but unless you have exceptionally high humidity levels, I doubt that the cabinet insulation or the return duct insulation are the culprit.
Good Hunting!

Of course if it is indeed the supply duct that was left uninsulated; then by all means this will create your problem. Be sure to use insulation that is properly rated for the higher temperatures that your duct system will have during heating season.

just to add. Even if you decided to use a larger metal duct for the job do not insulate the inside. AS a mold remediation company once the ducts get dirty they cannot be cleaned. Forget about the claims that say they can spray it and it will kill the mold etc. just does not work.
next off I assume by the post your getting moisture on the air handler itself under where it sits above the emergency drain pan. This means that the main supply duct or the return duct/air filter if its in the duct and not in air handler has been sized wrong. As the air flow drops because of duct friction, wrong size duct dirty filter, the air supply temp drops as well. Not enough to freeze but enough to cause the little insulation that the manufacture installs inside of the unit not to be sufficent enough to prevent this condensation from occuring. Thus when the air temps in the attic become warm and moist condensation collects on the outside of the unit.
Another reason for this to occur is a refergerant leak. As the gas escapes or if it was not properly charged the coil temps go down and this will occure as well. If the leak continues the freezing will eventually stop as well as the cooling of the sytem.
Not knowing the size of the ac system I cannot tell you if the duct is correctly sized. How many tons of AC was installed. If you do not know, provide the model number of the outside unit as that will reveal its size. I then can tell you what size duct should have been installed in the unit.

First, let me thank you all for your help. My location may have a bearing on correction that I need. I live in a suburb to Phx, AZ. We're in our Monsoon season, so we're fanally seeing some humidity. Thus the AC portion (coil box) of the Air Handler is removing the moisture from the conditioned air & the drip pan in the coil box is working well, as is the drain piping. The problem that I saw happening is that at the end of the coil box has sheet metal w/ a 18"collar attached to it for the transition to the 18" flex duct, which takes the conditioned air to a splitter box. The sheet metal at the end of the coil box is where the condensation is accumulating inside the coil box & then dripping down to the bottom of the coil box where ther is no drain or pan. It finds its way out after causing some rusting in the coil box. The sheet metal isn't insulated on either side, which I thought possibly is causing the condensation to accumulate on it inside the box. The seams @ the sheet metal/box may leak some air too.

HCC to answer your questions. The unit is a 5 ton unit serving at 2267 SF ranch home with vaulted & 10' ceilings. The return has a 18"x18" tee on it, the 1st leg is feed by a 14" flex duct from a the original 20"x 30" filter 10' away, the 2nd leg of the 18" tee is feed by a 14"x14" tee which the 1st leg is a 14" flex duct from a 16"x25" filter (added) from the east end of the vaulted central hall at the other side of the house 35'+/-, the 2nd leg of the 14" tee is feed from a 12" flex duct from a 20"x20" filter (added) 7' away. It's located on the west end of the same vaulted central hall. Is that what your looking? or too much?
I'll get the installer out to verify the unit is work properly with no leaks.

Duct sizes look ok, Five ton units require at .1 residental friction loss a 18" round duct supply and return. I know the AZ area well with weather as I own a home in Tempe. Any duct that is not lined should be wrapped with at least 1 1/2 inches of duct wrap insulation. All exterior duct should also have a foil or plastic vapor barrier around it. If any exposed insulation is visible moisture will collect between the insulation and the duct and condense causing mold and rust.

AS Gary Miller stated, be sure that your condensation drain is trapped properly. Many contractors fail to install the correct trap size and type and water is sucked back up into the air handler rather then properly drain to the outside. That will cause a lot of water to leak out of the air handler.

I'had the Trane installer check out the sytem & diagnose the dripping problem. #1 Overall the system checks out fine & operating as designed. #2. He said the problem is that the blower is blowing some of the dripping condensation beyond the drip tray in the coil box. He says that the water is blowing into the duct & making its way to the bottom of the box.(I can confirm the water is at the bottom of the box.) #3. He said the fix is to install a extension to the drip tray that would need an extension of the coil box made. The extension tray angles up at approx. 45 degrees & drains the water back to the std. tray & then out. The added box would be about 25" long & match the coil box opening. Their going to quote me price, probably $300+.
Any comments? Does this sound like he's on track? Thanks.

Wrong, The drip tray has been designed by Trane not to blow off water into the air stream. Teh reason that this is happening is the tray is filling to high with water. That is a sign that either the air filter is plugged or undersized the return air duct is undersized or restricted in some manner the condenate trap is wrong or a combo of any one of these. First of all you should not be paying for this if the system is new as it was incorrectly installed. If the drip pan is wrong, which it should not be. Trane should be paying for the change. Last off. Which is what I assume this is and you can correct me if I am wrong. This system has been working ok for a fairly long time. All of a sudden there is an issue with overflows and dripping ducts. Even though AZ has been having some additional humidity. It always drys the air and condensate forms. I would have to ask myself why is it happening now when its been working fine all a long? I suspect the condensate drain is not working correctly. It is either plugged or something happened to the pitch of the drain pipe. To understand this I will explain how it works.
The coil should be a draw through system. This means that the fan part of the evap unit ***** the air through the cooling coil part. When this happens a negitive pressure is created around the coil, drip pan, and back into the duct which causes the air to be pulled into the air handler. As water begins to condense on the coil it runs down into the collection pan which is designed to contain the dripping water as it collects. The condensate then drains through a hole in which a trap not unlike what you would see under your sink. Without this trap air would be sucked up into the hole and not allow the water to drain out. The air would then pull the water up and out of the pan into the air stream. So if the trap is missing which is an installation error, the water would not drain and then overflow into the air stream causing the condition you have. If the trap was installed but has a blockage somewhere in the drain area, the water would then fill up and overflow causing the issue you have. So It has to be one or the other. Unless the air supply duct to the blower is undersized or the filter is plugged. That would then cause excessive negitive air pressure in the blower cabinet and suck the water up the condensate even with the trap installed.

Did he check to see if there were two filters installed? In some cases the manufacture installs a filter in the blower cabinet, but the installer places a in ceiling or wall filter in the system. After a while even with the filter in the wall or ceiling being changed the filter in the cabinet plugges causing the condensate to back up and over flow. You cannot have two filters inline as it creates to much negitive air pressure in the coil box. That to besides causing the issue your having will cause the unit not to properly operate.

Do not pay this person the modify the drip pan, It is designed to work from Trane. It sounds like he does not know what is wrong. Or if he installed the coil within the box he did it wrong from the first place. ( sometimes the coil has to be flipped depending on how the air handler is being used) If it is indeed the drip pan, then Trane should be fixing this at no cost to you because of a manufacture error, which I doubt.

Something sounds very wrong. You may need to find a different Trane dealer to provide a second opinion. First of all the XL19i is an air conditioner (condensing unit).The air handler that I am familiar with to complete this system is a variable speed drive that is quite capable of handling any and all condensate produced if the drip shield on the coil is installed per manufacturer's recommendation.It is usually necessary to move, remove or add baffles etc. when converting an air handler from factory setup to one consistant to the requirements of the installation.
Not trying to argue, but I generally use a much lower static pressure in my return. ctually, I use a velocity of 500-700 fpm on residential jobs. I find that I get more even coil loading and quieter system operation.

Just another quick note. the ***** in the last post should read draws or pulls the air through used the word that began with S**ks and it got dropped.

I find the coil loading interesting. Although it has been a few years sense I was in the business and at that time variable air speed compressors and air handlers were just coming out. How does the unit not freeze with the higher return static pressures and less air flow over the coil? Is there a freeze stat that controls the condensing unit speed on the compressor?

HCC, to clear up what the Trane installer said the water is coming from: He says, Nearest the outlet side of the coil box as the condensation is dripping off of the coil some drips of water are being blown past the edge of the drip pan & finding it way to the bottom of the coil box, then leaking out. He didn't mean the water is coming from the drip pan. The drip pan & condensation line are working fine. Any water in the pan is drain out as designed. I poured a gal. of water/beach mix down the drain line, it's not clogged & the tech. verified it. Does the installer's explaination make sense?
I orginally thought condensation is forming on the uninsulated sheet material inside the & at the end of the coil box where the flex duct is connected. I saw the condensation was driping down the sheet metal to the bottom of the coil box, then leaking out. Possibly my explaination is wrong too.

If there really is blow-off from the coil then there is a velocity problem (too high). Sometimes a variable drive is configured too fast resulting in blow-off. A speed that is too slow could result in condesation forming inside
of the cabinet in areas that are not meant to handle condensation. Try extending blower operation after cooling system cycles off or possibly run the blower constantly.
Good luck.
PS I personally disagree with altering the original system design. Those engineers (equipment designers) aren't perfect but I'd ask them for suggestions first.

I agree with GaryMiller, If blow off is occuring with the coil, there is something else wrong. Air belocity is the probable cause. Many installers choose a higher fan speed to make up for poor installation and design practice.