Did I do this to myself ? Duct sweating...

[Agu]

Scene of the crime, 1981 block home, 1500 sq ft under air, Air con system was probably as old as the house, circa 1981.

Summer of "05 one main duct started sweating and stained the ceiling. The other main duct going to the other side of the house stayed dry.

This was the first time in our four years in the house this had happened. I at first thought it was because we had a large get together the week before and the system ran for several hours straight.

So last winter I patched the ceiling (btw I hate popcorn ceilings), and replaced the insulation in that part of the attic (I'm not so fond of fiberglass batts either)

This summer it happened again so I'm trying to figure out why after 24 years the ducts started sweating ? Then the light bulb went on. I had closed three out of six ducts on that side of the house to force more cold air into the master bedroom (The Mrs likes it REALLY cold when she's sleeping).

I suspect that closing the vents caused that duct to chill down so much it started sweating. Is this a possible scenario or do I need to look into other causes ?


Thanks for your time,

[a/c-harris]

Scene of the crime, 1981 block home, 1500 sq ft under air, Air con system was probably as old as the house, circa 1981.

Summer of "05 one main duct started sweating and stained the ceiling. The other main duct going to the other side of the house stayed dry.

This was the first time in our four years in the house this had happened. I at first thought it was because we had a large get together the week before and the system ran for several hours straight.

So last winter I patched the ceiling (btw I hate popcorn ceilings), and replaced the insulation in that part of the attic (I'm not so fond of fiberglass batts either)

This summer it happened again so I'm trying to figure out why after 24 years the ducts started sweating ? Then the light bulb went on. I had closed three out of six ducts on that side of the house to force more cold air into the master bedroom (The Mrs likes it REALLY cold when she's sleeping).

I suspect that closing the vents caused that duct to chill down so much it started sweating. Is this a possible scenario or do I need to look into other causes ?


Thanks for your time,

Yes this is a possible cause. Systems are designed to pass a certain amount of air, so when you damper down ducts or close grilles it could create this problem. You have possibly raised to air pressure (static) in your system causing a problem. I would also look to make sure that the insulation has not fallen off of the duct that is leaking and that it is still in tact. BUT the best solution would be to call a professional and get it properly fixed, that way you wont keep wasting time repairing your ceiling and damaging your unit. If the Mrs wants more air into her room, maybe the ductwork should be modified to make that happen. shutting off ducts is not the answer.
Just call a contractor, its not that expensive for a service call to have someone check it out.

[beenthere]

What harris said.

Also check/ er have checked, the insulation around the duct, it might have mold from getting soaked.

[dash]

Could be a couple of different things;

The outside of the duct insulation needs to be "vapor sealed" or moisture will condense in the insulation ,rendering the insulation useless and cause sweating.

Low air flow due to dirty coil,blower,filter,etc., will cause air to be colder then designed for,and ducts will sweat.

[Agu]

I've been doing a ton of reading here trying to figure out my problem. I've already decided the insulation needs to be replaced, again.

Since the original sweating incident I've made two significant changes. All windows and sliding glass doors (single pane and leaked like a sieve) were replaced with new insulated glass units. In addition the antique compressor and air exchange units were replaced by a professional. I now have a Trane 3 ton XL16i compressor and matching air handler. My electricity usage dropped by 40% and the air con barely runs compared to the old system. However the vents were still closed off, and the ductwork still sweated.

Will a static pressure test determine if my ductwork is adequate now that the vents are open?

[beenthere]

Yes, it can tell you if your duct work is sized ok or not.

Also, ask your contractor, what cfm per ton he has it set for. Too low can also cause this even if the duct is sized right.

[a/c-harris]

you really need to stop trying to figure this out yourself and get your contractor involved.

[Agu]

you really need to stop trying to figure this out yourself and get your contractor involved.

Tried that already and called the company that installed the system. He came in, looked at the spot in the ceiling, and declared I needed all new ductwork and returns. He said closing off three vents wasn't a problem. He also said running an additional vent/return wasn't an option because my ducts were "leaking".

Did I mention he never made it into the attic, and that he never actually inspected or tested anything. He doesn't do installs so he never was in the attic for the original install.


BTW, they weren't low bidder on the original job either.

[Dave-B]

open the vents

[HVACSvc08]

Tried that already and called the company that installed the system. He came in, looked at the spot in the ceiling, and declared I needed all new ductwork and returns. He said closing off three vents wasn't a problem. He also said running an additional vent/return wasn't an option because my ducts were "leaking".

Did I mention he never made it into the attic, and that he never actually inspected or tested anything. He doesn't do installs so he never was in the attic for the original install.


BTW, they weren't low bidder on the original job either.

That's crazy. They should have check the ductwork in the beginning of the install. He should've mention all of the things he is saying now before you had to do some work to your ceiling. Especially for a 20-plus year system.

[dad211]

is this the same guy who replaced your old equipment? if he is then whydidnt he adress this at the time of the install, if hes not then why didnt the contractor who installed the new system adress the leaking ducts ? and you should ask which comic book back he got his x-ray glasses from so i can get me some i been looking for a way to look at the ducts in the attic without actually getting in the attic would save me lots of time..

[teddy bear]

Could be a couple of different things;
The outside of the duct insulation needs to be "vapor sealed" or moisture will condense in the insulation ,rendering the insulation useless and cause sweating.
QUOTE]
This is excellent advice! A/c ducts are normally below the dew points of the air in unconditioned space. Without the vapor barrier on the outside of the insulation, the vapor from the high dew point air migrates to the cold duct surface. Colder ducts and higher dew points require more insulation with a perfect vapor barrier.

[QUOTE=dash;1684108]Could be a couple of different things;
Low air flow due to dirty coil,blower,filter,etc., will cause air to be colder then designed for,and ducts will sweat.
Yes, raising the air flow raises the duct temperature but decreases the moisture removal of the a/c. Regardless what method of raising air flow, expect less dehumidification from the a/c. Many a/c operate succesfully because of restricted ducts that lower the a/c coil temperature. The a/c removes adequate moisture for the wrong reasons, but works. Better to have large ducts, low pressure drop filters and a lower fan speed. Search for "Kevin O'Neil" for a through discusion on this problem. Insulated ducts close to other cool surfaces like other cold ducts or cool drywall tend to sweat on the outside of vapor barrier surface. Season greetings, TB

[rdpstl1]

dido open the vents

[unionguy]

one problem, many questions.
1. is the unit above the lid? if so is it level? have you verified it's not condensate from the evap. pan?
2. what is the air velocity thru the unit? is it too high and drawing the moisture off the coil? have you done a delta P across the fan to get CFM yet?
what is your air side delta T?
3. is the duct in question properly insulated? if hot air contacts the duct moisture 'may' form. the first process of cooling is dehumidification= that moisture will go somewhere.
Just some random thoughts...I'm not an expert,but I did stay at a holiday in last night.;)

UG

[Dave-B]

This summer it happened again so I'm trying to figure out why after 24 years the ducts started sweating ? Then the light bulb went on. I had closed three out of six ducts on that side of the house to force more cold air into the master bedroom (The Mrs likes it REALLY cold when she's sleeping).

I suspect that closing the vents caused that duct to chill down so much it started sweating. Is this a possible scenario or do I need to look into other causes ?


Thanks for your time,

Did you open the vent back up yet?

[Agu]

Thanks to everybody for the input. Sorry not to reply sooner but we had houseguests for the last week.

The ducts are open but there's no way at this point for me to determine if that was the cause of the problem so my plan is..

1) Call a pro to have the actual ductwork checked for leaks. If the ductwork is OK ...

2) Have the ducts tested for static pressure and have the blower checked that it's set appropriatelly for the duct work. If that's not going to work..

3) Look into having an additional run put into the far bedroom and....

4) Get the compromised insulation removed and replaced (Including a vapor barrier)

Does that sound like a good plan ?


Thanks in advance.

[Agu]

BTW, the air handler is in the garage so it's not a plugged drain or leaking pan causing the problem.

[ethical sait04]

your better off getting a good contractor to resize your ducts but you can download a 1 time version of hvac-calc online to try it yourself. But I would still recommend the contrator
tim