moisture in duct due to humidifier...insulate inside duct?

Nom_Deplume

Hi...we have hot water heat, so we're humidifying with an Aprilaire 800 steam humidifier running through the AC ducts.

I can see (through a hole in the duct covered with plexiglass) that there's a bit of condensation just above the steam output and a little water dripping to the bottom of the duct. It's not a lot of water, so I was wondering if some adhesive closed-cell sheet insulation to cover the area where the condensation is happening would be a good solution.

Because of its low conductivity, the insulation should not cool the steam and condense it the way bare sheet metal does. It would be kind of the reverse of insulating AC ducts on the outside to prevent condensation from outside humidity---I would insulate on the inside because that's where the humidity is.

But I am not an expert. Many thanks for any advice.

PS The air being humidified is at room temperature, as is the duct itself. No heating is involved except for boiling the water to make steam.

mattmia2

A better solution would be to run the blower on low when there is a humidity call if it isn't already. If you insulate, insulate the outside so it gets warmed by the steam but the moisture won't get in to the insulation.

aircooled81

I suggest double check the instal instructions, if you find humidifier is too large for your blower system, talk to OEM to see if it can be derated. Or consult HVAV technician on how to move more air.
FPM is the speed atwhich the air moves through the duct. the slower the air, the longer of a straight duct run you need for the air to absorb the moisture.
OEM has some installation instructions, https://rp.widen.net/view/pdf/uspzrlun0i/aprilaire-800-series-steam-humidifier-installation-guide-B2206277.pdf
double check your dispersion tube is orientated in appropriate location, looks like they want the holes pointed up. If it's in a horizontal duct they say mount low.
The output of your size humidifier will dictate minimum absorption distance from obstruction like an elbow in duct. faster moving air will allow for shorter distance to obstruction.
I believe that if the moisture is able to hit cool surface, it will condense regardless of how much insulation you use. I dont think a different product will condense less, maybe it absorbs moisture better but temperature is temperature, if the temp of the surface moisture comes into contact with is at dew point - the water vapor will condense.
you might have too much humidifier for the volume of air available, you might have an elbow directly upstream of the dispersion tube and there's a dead spot for air movement? Dispersion tube could be pitched wrong direction and condensate is spilling out the last hole or two? If the humidifier is above the duct and hose pitched down, you probably do get condensate dribbling into the dispersion tube rather than back to the humidifier.

EBEBRATT-Ed

The air flow has to absorb the amount of humidity your putting in the air. You need more air flow or less humidification. No insulation on the inside it will get moldy. Insulate the outside if you think it will help.

standard procedure on a commercial job is to put in a section of stainless steel duct with a drain fitting on it

Nom_Deplume

Thanks for the helpful replies! I am actually aware of the airflow and absorption distance (straight run) requirements. I had to teach our contractor, who did not know any of it when they did the original install. The installation is now fully complaint with the Aprilaire 800 requirements. There's only a little bit of condensation now.

Regarding "I believe that if the moisture is able to hit cool surface, it will condense regardless of how much insulation you use"---lets say you have an AC duct that is sweating on the outside due to summer humidity. If you insulate it on the outside, will it still sweat on top of the insulation? No, because the insulation surface (bad heat conductor) doesn't get cold like the metal (good heat conductor) does. My proposal is the same thing, inside out.

PS Stainless duct makes a lot of sense, of course.
PPS Re. "it will get moldy", I am proposing closed-cell neoprene sheet, not fiberglass or porous foam. It is the same material used for weatherstripping, i.e. it is used in wet locations.

mattmia2

Insulation over a cold AC duct needs to have a sealed vapor retarder on the outside or be a vapor barrier in itself or you will get condensation within the insulation.

Even the steel duct can grow mold. Foam in a spot that gets wet even if it is closed cell would be a problem.

Nom_Deplume

mattmia2 said:

Insulation over a cold AC duct needs to have a sealed vapor retarder on the outside or be a vapor barrier in itself...

Neoprene is a vapor barrier.

mattmia2

Are there settings for delay, for the blower to pre and post purge for a minute or 2? Maybe that would help warm the duct and help carry away anything left in the hose.

As I said, the insulation either needs a vapor retarder or be a vapor retarder itself.

Still, don't put it inside, if it gets wet you are asking for any number of nasty surprises. Put it on the outside and try to keep the metal warm enough to not condense. Also look carefully in the manual about where the wand is installed and how the hose is routed.

You might be able to bump up the low speed blower setting too.

Nom_Deplume

Good points. There is no post-purge but I could rig up a timer relay with a delayed-off function.

The fan is on the highest setting. I can block part of the duct to increase the fpm (the fan runs based on holding a fixed cfm.)

pecmsg

Nom_Deplume said:

Good points. There is no post-purge but I could rig up a timer relay with a delayed-off function. The fan is on the highest setting. I can block part of the duct to increase the fpm (the fan runs based on holding a fixed cfm.)

Do not block any of the duct!

pecmsg

What RH are you trying to maintain?
as the outside air gets colder the RH set point needs to go down to prevent condensation inside the home. 

I the heater a heat pump of gas/oil furnace?