Condensation trouble:

[JamesFarmer]

Hey Everyone,

I am having an issue with condensation in my grow space that has become a serious problem as the weather has cooled.

I run air cooled hoods and draw the air from outside. The air flows through the grow space in round metal ducting, through the hoods and is then exhausted outside.

When the outside weather warmer I did not have trouble but as the temperature has dropped I have started seeing condensation on the outside of the ducting. As we get down into the 30's & 40's (F) the condensation has gotten so bad that I have a small stream running through my grow space.

I am running a dehumidifier and holding 65% +/- and have plenty of fans.

I am thinking that I need some kind of condensate tray that drains outside. Maybe suspend lengths of rain gutter beneath the duct?

Anyone have a good solution for this?

Thanks

FJ

 

[queequeg152]

just insulate the duct.

 

[Granger2]

If you insulate the duct, what happens when the [unwarmed] cold air gets to the lamp with an abrupt rise of temp? -granger

 

[queequeg152]

If you insulate the duct, what happens when the [unwarmed] cold air gets to the lamp with an abrupt rise of temp? -granger

the difference in the insulated verses uninsulated duct AIR temps will be very small.

what will not small is the difference in the duct surface temps. with the duct surface now including hopefully... 1.5-2 inches of insuation, its temperature should now approach that of the air temp. which hopefully is far above the dew point.

the actual BTU difference in the air will be small... probably around 500 btuh. unless the air flow is massive and the duct area is massive as well.

regarding HID lamps in general.

they are actually very resistant to abrupt changes in temperature.

its cfls that perform badly in cold temps. the high temp cfls like PLL's and t5's are designed to operate at like 105 degrees. when you cool them agressivly you can literally watch the lumens drop off rapidly. dont take my word on this... get a cheap candle meter and desk fan.


HID's are insulated from temp drops out of necessity. if they were not, they would explode violently all the time from just from being molested, dropped or what ever. the outer envelope also shields most of the UV light the arc tube produces.

think how the temp of the glass envelope changes as they are first fired up in some chichago street light... going from 20f to 110f in just a few minutes.

the outer envelope is a partial vacuum. the only part of the bulb thats under pressure is the inside arc tube vial. the vacuum acts like a dewer does and insulates the arc tube from temp changes. if the partial vacuum were replaced with some gas... internal pressures would change drastically as the gas is heated savagly by the arc tube... it would have to be made very thick, else it would surly explode violently.

in some of these bulbs the arc tube pressure is quite high... so high that when they fail they can actually do some damage. i think thats why some of the CMH bulbs are wrapped in a wire girdle... to prevent shattering the outer envelope should the inside vial explode. i think the outer envelope is also a sort of last layer of protection shielding folks from hot shards of arc tube should it fail grievously... however i would not be suprised if the outer envelope could be shattered as well by an exploding arc tube.

 

[queequeg152]

if you do choose to insulate the duct. i highly reccomend that you either ....

a) just buy insulated flex duct, r6 minimum.

or

b) do a good job retrofit insulating. because any and all gaps will condense moisture happily.

moreover, your lamp hoods could conceivably condense moisture even while the lamps are on and heating the hoods.

if this is the case, id suggest lowering fan speed, or mixing in room air into the outside air stream.

you can do this with a reducing wye + a volume damper.

 

[JamesFarmer]

if you do choose to insulate the duct. i highly reccomend that you either ....

a) just buy insulated flex duct, r6 minimum.

or

b) do a good job retrofit insulating. because any and all gaps will condense moisture happily.

moreover, your lamp hoods could conceivably condense moisture even while the lamps are on and heating the hoods.

if this is the case, id suggest lowering fan speed, or mixing in room air into the outside air stream.

you can do this with a reducing wye + a volume damper.

Thank you All for your replies.

Do you have any suggestions for a reasonable method of insulating the round rigid metal duct pipe that is strapped to the ceiling?

I can lower fan speed. I would think that if I monitor the lamp housing temperature while slowly reducing the fan speed I should be able to find a sweet spot. However, the worst of the problem is before the first lamp. I wonder how fan speed will affect this spot?

As far as using room air: I don't see how I can do this without creating a vacuum in the room. That will pull outside air in and is not a good thing.

Thanks again.

 

[Granger2]

I'm not worried about the lamp exploding, I'm talking about condensation. Seems that at some point in the system, cold outside air will hit hot air, causing condensation, right? Even if you insulate all the way to the lamp, eventually cold air will hit hot air.

I have a 6" Vortex pulling outside air thru 3 600w lamps. It comes from under the building, thru the floor, and up thru 2 6" vertical metal ducts. About 4.5' up from the floor the 6" ducts enter a box with an 18" sq med duty pleated AC filter. Out of the top of the filter box is a 10" metal duct that goes to the ceiling where it is attached to a plenum box that supplies each light with air, separately. That's so hot air doesn't come out of one lamp and into another. They all get cool air. The Vortex pumps it all out of the room thru another plenum box.

Somewhere in that system, cold meets hot. I haven't insulated the ducts. I almost did, but it occurred to me that I'd rather have cold meet hot below my filter. Hopefully the condensation will occur low in the system and run back down below the floor. Otherwise, if I insulate it, I think, the condensation will occur higher in the system and wet my filter. Nothing may work if it's really cold. I don't know. I do know that my theory works in a certain temperature range. I don't know what that range is, or how much outdoor RH is involved. Guesses? Thanks. -granger

 

[wolfhoundaddy]

Green houses have to deal with this. They run a channel type metal under the gutter to catch the drips.

 

[Limeygreen]

The drip gutter is a good idea, you could also try installing a damper in the duct and limit the cold air, won't cure it but may help a little bit with the amount of condensation. Otherwise you could take a small piece of ducting from inside the room and work it back to the intake to mix with the cold air with the damper you can control how much gets mixed. This would be similar to a heat exchanger, it may help you but I don't know 100%

 

[queequeg152]

Thank you All for your replies.

Do you have any suggestions for a reasonable method of insulating the round rigid metal duct pipe that is strapped to the ceiling?

I can lower fan speed. I would think that if I monitor the lamp housing temperature while slowly reducing the fan speed I should be able to find a sweet spot. However, the worst of the problem is before the first lamp. I wonder how fan speed will affect this spot?

As far as using room air: I don't see how I can do this without creating a vacuum in the room. That will pull outside air in and is not a good thing.

Thanks again.

without unstrapping it from the ceiling? not really.

i would just lower the duct and wrap it in insulation.

this is what you want.

http://www.zoro.com/johns-manville-...JkMg8HF6znEVRVIfCHtt2RoCBpjw_wcB&gclsrc=aw.ds

you can get it at any hvac supply place. id reccomend an outward clinching staple gun and mastic tape or foil tape or scrim and mastic.

 

[queequeg152]

I'm not worried about the lamp exploding, I'm talking about condensation. Seems that at some point in the system, cold outside air will hit hot air, causing condensation, right? Even if you insulate all the way to the lamp, eventually cold air will hit hot air.

I have a 6" Vortex pulling outside air thru 3 600w lamps. It comes from under the building, thru the floor, and up thru 2 6" vertical metal ducts. About 4.5' up from the floor the 6" ducts enter a box with an 18" sq med duty pleated AC filter. Out of the top of the filter box is a 10" metal duct that goes to the ceiling where it is attached to a plenum box that supplies each light with air, separately. That's so hot air doesn't come out of one lamp and into another. They all get cool air. The Vortex pumps it all out of the room thru another plenum box.

Somewhere in that system, cold meets hot. I haven't insulated the ducts. I almost did, but it occurred to me that I'd rather have cold meet hot below my filter. Hopefully the condensation will occur low in the system and run back down below the floor. Otherwise, if I insulate it, I think, the condensation will occur higher in the system and wet my filter. Nothing may work if it's really cold. I don't know. I do know that my theory works in a certain temperature range. I don't know what that range is, or how much outdoor RH is involved. Guesses? Thanks. -granger

um... yea if you take saturated 100% humidity air and then suddenly cool it more, far below its dew point you get sudden condensation aka fog.

thats pretty rare in hvac systems though. you do see it in airplanes though.. where they are pulling a huge amount of air from outside, chilling it and redistributing it inside.

but taking inside air... at say 60% humidity and pumping it down cold duct work? its not going to form fog or anything like that... what will happen is you get sweating on the inside or outside of the duct.

i dont really understand your concern to be honest. it sounds like you are drawing outside air and pumping it outside? if so there is no drastic mixing of hot and cold air of varying humidities.

perhaps if you draw a picture of it or something, i could understand better, but what you want is to insulate all the way to where the air is ejected from the conditioned space.

when you are drawing cold air from outside where air outside the duct is warmer(warmer air holds far more moisture btw), the surface of the duct drops below the dew point of the air outside the duct and this air outside the duct is condensing onto the surface of the cold duct.

when you insulate cold ducts, you are trying to keep the warm air outside the duct from sweating onto the cold ducts surfaces, not the other way around. at least when the air in the duct is cold, and air outside the duct is warm. when conditions reverse, the opposite happens.

when you are pumping in hot air through duct inside a cold environment... its reversed, the hot air itself is condensing onto the INSIDE of the duct.

your lights are hot right? yes they are, but they are not adding any humidity to the air stream.

actually by adding pure heat to a cold body of say... 20% humidity you actually lower the Relative humidity assuming everything stays the same pressure.

what im saying is... putting pure heat into your lamp cooling duct is not ever going to cause condensation because it will increase the airstreams ability to hold water vapor. what causes condensation is removing heat from saturated air.

again, generally speaking you need to insulate all the way to the roof cap or where ever its being exhausted.

 

[packerfan79]

I had this same problem. I got insulated 6in ducting from home Depot. 25 bucks or so. I had condensation on my blower the ducting, I even had a bit of water on the inside of the hood. I think it would be easier and more effective to not air cool the lights but air cool the room itself. I don't know what you're weather is like. My fall and winter weather is down to the teens and 20s at night, and up to 60s during the day. it is very dry here, which threw me with all the moisture condensating

 

[exploziv]

His condensate is on the outside of the duct, inside the growroom. What happens is that hot air with a high RH meets a surface that is under the dew point temperature (meaning it can't hold as much humidity as before. So that water vapor is condensated on the colder surface. If he insulates the duct, condensation can still occur on the hoods, if the bulbs inside aren't enaugh to keep hood temp over the dew point.
A solution would be kind of a lung room where the air gets a bit hotter than the dew point temp. Other solution would be designing a system of drip catching gutters. I know this has been discussed already in the thread, just wanna make sure you understand your problem so you can solve it.

Good luck.

 

[queequeg152]

regarding condensation on the lamp hoods themselves.

fwiw, there is no reason why you could not use the same material to carefully insulate the hoods as well... its just going to take a long time to measure and cut the gores out to the right sizes.

its very important to protect the fiberglass/mineral wool face of this insulation less it become an airborne irritant. if you go this rout, i suggest you apply spray adhesive as you would for any other elbows or random fitting and pay careful attention in taping off all exposed insulation.

there are so called duct 'buttering' compounds available for sealing and gluing cut surfaces , however they traditionally are for rigid duct insulation such as duct board or the fancy acoustic panel materials.

 

[Granger2]

Hold everything. I got my wires crossed about hot air holding less water than cold. Also, I didn't explain very well. Thanks for attempting to help yous guys.

I can't/won't draw a draw a diagram. I'm too inept. It would take me all night. Let me think this thru and maybe work on an explanation. Thanks. -granger

 

[growshopfrank]

I had this same problem. I got insulated 6in ducting from home Depot. 25 bucks or so. I had condensation on my blower the ducting, I even had a bit of water on the inside of the hood. I think it would be easier and more effective to not air cool the lights but air cool the room itself. I don't know what you're weather is like. My fall and winter weather is down to the teens and 20s at night, and up to 60s during the day. it is very dry here, which threw me with all the moisture condensating

This is it add a charcoal filter upstream of the hoods and draw room air out. If your climate gets real cold operate the fan hooked to a thermostat, preferably one that will ramp speed up or down with the temperature change.
Gotta love it when it rains in your indoor garden