What should the temperature be in a duct?

elliottheperson

Hi folks--
I'm trying to solve a water entry/condensation problem at my grandparent's home (which was literally TLDR in another post).

The ducting is all rigid and external on the roof (here's a quick video of the roof ducting), and an HVAC technician used his temperature gun and showed me that when the AC unit was off, and it was about 70 degrees outside, the temperature inside the duct near the AC unit was 100 degrees farenheit, and just about the same temperature in the duct boot/supply that is in the ceiling next to the fireplace where I'm getting a ton of condensation/moisture.
I wonder if the condensation is coming out of the supply register because the internal temperature is about 60 degrees farenheit-- a 40 degree differential.
So--what should the temperature of ducting be when there's no fan turned on, and the AC is off?
If the ducting is this hot, how should it be addressed? Should I try to wrap the duct boots? Wrap that metal box between the joists (it's a flat roof) that connects the duct boot to the register? Wrap all of the rigid ducts?

Thank you.

mattmia2

Dewpoint is what you need to look at. If the air encounters a surface that is colder than the dewpoint of the air then it will condense on that surface.

Looking at your other post I strongly suspect it is a leak in the ducting or the chimneys or the various things that connect to them but you have about 100 potential leak points in that roof. I would recommend you take @Larry Weingarten up on his offer of an independent consultant to try to narrow it down to fixing a few of them instead of all of them. This problem may be so expensive to fix that it may be more cost effective to get rid of the rooftop system, repair the roof penetrations, and add a min split system for cooling and heating.

Adding another post is just going to make this more confusing, it would be better to add to the original post.

elliottheperson

@mattmia2 Thanks for your reply... I have taken Larry up on his offer of an independent consultant. I was wondering if the problem does narrow down to dewpoint (and I think people might have gotten lost/turned off from my other long post). I appreciate it.

ChrisJ

I can't imagine air conditioned air causing condensation inside the ductwork.
Outside yes, but inside? Doesn't make sense because I don't see that ductwork being colder than the evaporator.

But warm humid air in the winter could.

clammy

I once had a boss tell me decades ago that the only time to use lined duct work is for people you don’t like . Exposed ducts on roofs are costly to properly insulate and make water tight correctly . I do see a lot of a peel and stick stuff used by a lot of contractors on exposed duct work problem is it gets worm and falls apart . The proper way is to insulate w 1 inch ridgid duct board and have rubber roof membrane installed over or use duct wrap and have a sheet metal dog house cover installed over . Either way it’s not cheap personally I prefer using a r 8 duct wrap and then have a dog house made and installing it over the wrapped duct of course a bend bracing is installed as to not compress the duct wrap . A properly insulated and dog house covered exposed duct is the correct and an old timer Dutchy trick . Now a days as I stated a peel and stick product is used and usually when I see it the duct work and basically the whole job is usually just garbage again quick race to the bottom cheap price followed by cheap work w garbage workmanship or quality and zero pride nor quality just get the money .
Peace and good luck clammy

ChrisJ

@clammy I had a professional engineer tell me to use lined duct work for acoustic reasons.

I didn't, but I highly respect the engineer that recommended it.

EdTheHeaterMan

I never liked the duct liner idea. I recall a fireproof insulating material used up until the 1970s on all kind of heating systems and building material. I just remember working with it a lot without even a dust mask as a young heater technician. Then all of a sudden "STOP USING THAT STUFF!" and it cost a fortune to remove it from all sorts of building material. Now Lawyers make millions on medical class action litigation. I am wondering if I will see the new lawyer TV commercials "Was your home built with Fiberglass Insulation inside the walls and ducts? Call us to get hundreds of dollars so we can make millions"

I said that over 30 years ago and have never installed a duct with liner inside it.

clammy

For noise on undersized return ducts and a cheaper install on supply ducts because guys hate insulating ducts plus 99 % don’t do it properly . In life those who care don’t just install crap they install with pride and look more at what ever they install as a form of art work w everything being done properly not get the check and go or a one day wonder . Yeah these happen all the time and some time correctly but they’re always a short cut .
Same goes for soft start blowers they can about due to so many head aches occurred by manufactures due to undersized supply ducts and banging and customers complaints .
Like I said internally lined duct work on supply ducts is garbage and returns in un conditioned space is fantasy and does just about zip .
Engineers sizing duct work that’s nice ,hire the right guy and you should not have to second guess and hire an engineer .
Peace and good luck clammy