Condensate inside A/C ducts

Unknown

Just playing devil's advocate, I think. Never have seen a problem like this but I guess it can happen. Good luck with it!

KB_2

Condensate inside A/C ducts

I installed central air in a ranch house a few years back. The duct insulation is R-5. I am having issues with quite a bit of water collecting inside the duct work over the winter. I know this must be condensation from indoor air going into the ducts in the attic. The grills are blocked off in the fall, but this hasn't helped. I would hate to go through the expense of changing all the ducts to R-8 and not have it solve the problem. Any suggetions would be appreciated.

Brad White_189

Arrrrrgh-8

Kevin, you have a number of converging forces at work here; moisture and insulation/temperature obviously.

If you are condensing that much, R-8 will not make much difference but will buy you a few more degrees before it occurs.

R-8 is an energy-code-minimum in most places, (edit:R-8 is used in unconditioned spaces with R-5 in conditioned spaces per most model energy codes), but no one is claiming it has any magical properties.

Think about your house and attic insulation- R-19 maybe? R-25, 30 or 38? That is what you have between you and the great outdoors. Within this, you expect a fraction of that, even R-8 to be an effective barrier between you and a cold attic.

Add to this that the ductwork is probably wrapped all around so is insulated from "house ambient heat".

The "sealed grilles" are not likely to be moisture-tight even if apparently air-tight, that much you have proven to yourself.

I would recommend making that duct think that it is entirely within the heated volume, either insulating the roof plane or wrapping the duct to at least (at least!) R-19 but with one vapor barrier on the outside. Seal the duct first after stiping though.

KB_2

Condensate inside A/C ducts

Thanks Brad. I was considering laying R-19 blanket insulation over the entire system. It's just that I'm looking pretty incompetent to the customer, but I have never ran into anything like this before. I know they have gone to R-8 on the supply side of systems, but the returns have remained R-5, so this is obviously for energy conservation issues not the infiltration problems i am having. Thanks for your input.

Brad White_189

Just be sure to

strip the ducts of the prior layer before insulating. Wrapping with another layer of R-19 alone will make things worse. Moisture and even air, can and will freely travel through fiberglass and condense, possibly, on the outside of the current jacket.

You want an insulation barrier as thick as the house at that point but also no perceptible leakage from the duct itself. That then would have as air-tight a wrap as you can make, as a vapor retarder.

I did not gather that you were the installing contractor but it is what it is. (The notion of your insulating the roof plane was directed at a homeowner for their overall benefit, not to you, a contractor, as a "fix".) Getting in there and doing it right is the only way and, well, a lesson learned. The inconvenience to the homeowner aside will be forgotten and that you stood up to do it will be what they remembered. Not meaning to lecture of course, just my $0.02.

KB_2

Insulation

I just can't reason why this house is so much different than any other I have ever done. Even the R value of the panels on the air handler has to be taken into consideration. I'm just thinking how this will look like a Rube Goldberg by the time I'm done. But I guess it is what it is. Thanks for your help Brad

Chris S

I had the same condition in an ICF house. I attributed it to the moisture of new construction, and the well insulated attic. It got better after 2 seasons- what we normally consider the settlemen ( shrinkage) time for a new home. I haven't heard anything since - the house is now 4 years old. Was I wrong about the moisture trying to get out?

Chris

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KB_2

Condensate inside A/C ducts

The house that this system is in is about 30 years old and the system was put in about 4 years ago. This happens every off season. I would definitely say it's condensate directly related to the delta T between the inside of the duct and the attic space and it will never go away until I insulate the duct work better.

Brad White_185

Another thought

Reflecting upon what Chris was saying, plus I had time to access some of my tables and resources..

Naturally we have been focusing on the temperature aspects but without looking at the humidity level in the house, pretty basic.

Even a relatively dry house, say 70F and 20% RH, will have a dewpoint of about 27.7 degrees F.

That is fairly low and to find a point in the house that is that cold, even during design winter temperatures, indicates either a serious gap in insulation value somewhere, a very high humidity within the house or a combination of both.

With 1.5" duct wrap, (compressed to 80%), 70F air in the duct, the exterior duct surface temperature would be about 56 degrees F., twice the dewpoint.

Even half-inch thick wrap, slightly compressed to 3/8", would get you a sheetmetal surface temperature of about 41 degrees....

For the ductwork to see a temperature of say 28 degrees to be conservative, with a zero degree ambient in the attic (and how likely is that?), the effective R-value would have to be seriously diminished at least in spots. (It may not be all of the surface, just a few choice misses spots anyway.)

Calculating the effective "balance" dewpoint assuming 70F air floating up into the ductwork and at zero degrees around it, you would have to be about 70% RH "in the duct".

A slight cooling within the duct would elevate the RH (but not the absolute or net humidity quantity), but to even get that close, there has to be an elevated humidity in the home and at least some "gaps" in insulation integrity.

As stated, even compressed insulation would keep the duct above dewpoint, assuming normal (low) winter RH's in the 20% or even 30% range.

Point being, you have some "cover" (so to speak) and some wiggle room as to cause.

I suggest some data loggers as a place to start, monitoring duct RH and temperature, house RH and temperature, over time. Do this either before or after the "fix", although it would have been great to do this over the past winter. Oh well.

KB_2

Condensate

I had also inquired of the resident if he was using the bathroom exhaust during showers, as the return is in the hallway near the bathroom. He blocks off the all the grills with plastic, but now I have to wonder if restricting air flow through the ducts all together is actually worse. The worst of it was the first year in one takeoff that was running close to the soffit vent. I relocated that run and it was much better, but he said he still gets a little water coming out of a few vents when he goes into the attic and lifts the flex runs. This is not a common practice by most homeowners, but I have to wonder if what is happening here is not as unusal as I first thought. In most cases the moisture in the duct would dry up on its own if we didn't go looking for it. As I recall I know I've seen evidence of moisture on schematics glued to blower housings in air handlers in the past. Just another thought. I will try to resolve this any way I can though and thanks Brad and Chris for all your help.

Greg_41

Duct condensation

If there is a central return, the thing to do is to wrap the filter in a dry cleaning bag and put it back in the filter rack. This will stop air flow. Just make sure to note the removal of plastic before cooling season. I have seen this situation a couple of times w/ hydronic heat and a/c air handlers in the attic.

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Mr Spam

Condensation

I am a homeowner. Had the exact same problem. Could have gone into the water business. Bedroom celing caved in. Water stains at other places in the house on ceiling. Massive amount of water in the flex.

Installer tore out entire system and insulated main lines. Also replaced flex with another flex that had more insulation.

All vents were replaced with one's that have a rubber gasket around the piece that closes. I close them off every fall. All returns have a 1/4" whiteboard placed onto them using magnets and weather strip.

When my installer replaced the ductwork I think the main thing he did was to have the take off flex ducts angled first up hill from the main trunk and then turm back down toward the ceiling vents.



hope this helps. This went on for 3 seasons of trying differnt things before the sealed vents and angled flex fixed it. Leaving the system open during the winter only made things worse.

Unknown

Are we,,,

100% sure this is not due to water being slung off of the evap and into the duct or a full pan from a partially blocked drain or incorrect level? Given the relative dryness of the Air during the winter months and the fact that the registers are blocked makes it seem unlikely that it is moisture condensing in the ducts. I've seen plenty of attic systems but can't recall any that had the condensation problem you describe. Just my $0.02 of course...

hvacfreak

.10 @100ft supply and 600fpm return

I have to agree with the " velocity carrying condensate from the coil / primary pan thread ". I'm pretty much on the commercial side of HVAC , but in 18 years of sidejobs and " favors " , I haven't seen a residential duct system sized as it should be ( subject line ..and 5 years of legitimate residential sheet metal fab / install ). We cut metal by hand back then , lol.

But if it is sized correctly , you can't go wrong listening to Brad W , man I wish I could work on one of his jobs. - M

Mr Spam

Condensate inside A/C ducts

> I have to agree with the " velocity carrying

> condensate from the coil / primary pan thread ".

> I'm pretty much on the commercial side of HVAC ,

> but in 18 years of sidejobs and " favors " , I

> haven't seen a residential duct system sized as

> it should be ( subject line ..and 5 years of

> legitimate residential sheet metal fab / install

> ). We cut metal by hand back then , lol.

>

> But

> if it is sized correctly , you can't go wrong

> listening to Brad W , man I wish I could work

> on one of his jobs. - M

Mr Spam

Duct condensation

"the relative dryness of the Air during the winter months and the fact that the registers are blocked makes it seem unlikely that it is moisture condensing in the ducts............"

"100% sure this is not due to water being slung off of the evap...................."

I'm really a novice to this but in my situation the above 2 situations were not the cause. I went through 3 winters dealing with(or putting up with)this problem. The 1st winter, not knowing any better, I left all of my diffusers open as well as all of the returns. During the winter months, it's true, the air is relatively dry BUT it is warm air that is entering the duct work. The ducts are in the attic and they are cold. Freezing temps in my area. Now, what happens when the warm air hit the inside of the cold duct? Condensation. Add to this the occupants taking a shower every day (family of 4) and now you are not only putting warm air into the cold duct but your adding the moisture from 4 showers a day to it.

During the winter months in this situation, the A/C is not running. So the warm air just collects inside the ducts and condensates. I have HWBB. Most of the water in my ducts was in the area of my bathrooms BUT I also have enough condensate elsewhere in the ducts to stain my ceilings that are nowhere near the bathrooms. My house is 2500 sq ft.

You may think that the condensate would be a minimal amount but if you have a solid 3-4 months of temps below 40 degrees, you'll make water and lots of it.

Each spring, my installer would do a "fix" so I had to wait till the next winter to see if it worked.

Winter 1 - vents left open. Result: Lots of water. Installer came back in the spring and sealed the main line joints. Didn't touch the flex.

Winter 2 - Closed off diffusers and closed off the returns. The diffusers were the kind that had a knob that you turned to close them off. These were not air tight at all. Result: still had water but not as much as the 1st winter. Installer came back in the spring and double insulated the main lines. Didn't touch the flex.

Winter 3 Closed off all diffusers and returns. Result: Water in the main line and also in the flex. The flex near the bathrooms were full of water. Installer came back and tore out everything. He then installed a double insulated main duct line. All of the flex take offs are angled towards the attic roof a little bit and then turn down towards the ceiling keeping an angle. Nowhere is the flex laying flat. I guess the theory is that if warm air does enter the flex, It can't go downhill to the mains. Also by having an angle toward the ceiling, any condensation that would form would be minimal.

In addition to the above, the installer also replaced all of my ceiling diffusers. The new ones are plastix and have a rubber gasket where the baffle meets the side of the round duct so that it is an airtight seal.

It's now been 4 years since winter 3. I have not had any condensation problems. I close off all of the diffusers and returns in the fall and open them up in the late spring.

btw: my installer was great. I had him come back later and install an hvac system in my basement (another 2500 sq ft.) No problems there at all but all of the ductwork is inside the conditioned space of the house.

KB_2

Condensate inside A/C ducts

I would say it is definitely not water off the evap coil. During the operational season there is no problems at all. It is always coming out of the off season that the issue rears it ugly head. This is all great feed back. I actually do strictly service work and I always sub out my installs. Aside from this issue, the install looks like a great job, but I am ultimately responsible and want to make this right. The heat load was done with RHVAC software and the duct work was all sized accordingly. No rules of thumb were used. Thanks to all for your help.

Paul Fredricks_6

Years ago we put in an A/C system in an attic. That winter the water problem started. We tried closing vents but nothing helped.

This house had electric baseboard heat. While trying to find the cause, we found that the entire house had been wrapped in poly prior to the sheet rock going up. The idea back then was to make the house air tight for better energy savings. What it also caused was any moisture inside the house to stay trapped in the space with no way out. By installing a central air unit in the attic we gave the moisture a place to go and condense.

The cure was to install a fresh air inlet fan. I think we used Tjernlund's Inforcer.

Problem solved.

As Brad stated, humidity level is the key!

Unknown

Interesting...

I'm kind of curious why this doesn't happen more often with these kinds of set ups.

Paul Fredricks_6

I suppose it's just a matter of how tight the house is. I bet it also comes into play how many people live in the house (how often the outside doors are opened. If it's anything like my house, the kids leave the doors open year round:-).

It comes down to humidity levels, dew point and duct temp, and moisture migration.

Unknown

Yup,

I guess so. I wonder how many out there do this but remain undiscovered.

Paul Fredricks_6

Al long as the water doesn't make it out of the registers, no one would know. We dumped gallons of water out of the flex, but if the problem were more borderline they may never have known.

Unknown

Still learning...