Help me with my Humidity issues pls. 8kw direct exhaust passive intake

so have built out half my garage into a 12x22' 8kw room. i have a 1700cfm exhaust for temp control. the intake is passive through two 12x24 hvac returns that draw air from the unconditioned side of the garage. as air is sucked passively through these returns fresh air from outside is drawn into the unconditioned portion of the garage via soffit vents.

the problem is that when this exhaust is running full blast to keep the room under 90 F in middle of summer the humidity in the room is only around 25% RH. even with two 1.4 gph humidifiers blasting full force its only able to raise the RH up 1 or 2 RH percentage points.

for some reason the humidity outside the garage could be 50-60% but as it gets drawn into the flower room its only 25% rh. Am i creating this issue by passively drawing air into the unconditioned part of the garage first where temp fluctuations cause the rh to drop significantly? would i fix this issue by directly drawing fresh air in from outside?

Do you have any AC running? Also is your lung room smaller then your 12 x 22 room? I'd try putting your humidity gauge at different locations in lung room, maybe you have alot of humidity at ceiling or floor that a ceiling fan would mix depending on which direction you run it.

no ac is running. i have an insulated room built within an uninsulated garage. essentially the room direct exhausting creates a vacuum passively pulling air from the uninsulated part of the garage which then pulls air from outside via the soffit vents.

p0opstlnksal0t said:

no ac is running. i have an insulated room built within an uninsulated garage. essentially the room direct exhausting creates a vacuum passively pulling air from the uninsulated part of the garage which then pulls air from outside via the soffit vents.

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Is your lung room uninsulated?

I wouldn’t worry about the humidity if your plants are healthy. My grow room is about the same humidity as yours and never had an issue

correct. the "lung room" is just the uninsulated half of the garage that was not built into the flower room.

I may also be drying out the air with the 8x 1kw DE lights. the fix may be to hook up a 1200cfm intake fan directly pulling in outside air and then stick a swamp cooler in the room as well. then eliminate the passive intake from the "lung room" portion altogether.

adower said:

I wouldn’t worry about the humidity if your plants are healthy. My grow room is about the same humidity as yours and never had an issue

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i havent grown in this room yet during summer so i dont know what it would produce. winter was much easier since it was pulling in 0 degree air lol

p0opstlnksal0t said:

i havent grown in this room yet during summer so i dont know what it would produce. winter was much easier since it was pulling in 0 degree air lol

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I doubt you’ll have issues. I have the same type of setup as you but with 10k worth of light. Winter is nice though cause you don’t have to worry about ac costs. If anything, less humidity is better. No worries about pm.

..try pulling in the air directly from the outside and see what happens ?
-good luck!

ya i think ill try just adding an intake fan pulling in direct outside air first. this would be the easiest option.

p0opstlnksal0t said:

correct. the "lung room" is just the uninsulated half of the garage that was not built into the flower room.

I may also be drying out the air with the 8x 1kw DE lights. the fix may be to hook up a 1200cfm intake fan directly pulling in outside air and then stick a swamp cooler in the room as well. then eliminate the passive intake from the "lung room" portion altogether.

Click to expand...

Test the humidity in flower room with lights out see what you get, maybe it is the lights drying your rooms humidity out. I dont ever see below 40% humidity in my room but its 70% humidity outside now. I dont really see a issue with low humidity for you, i expected from title of thread to find you were having high humidity issues.

lights out the exaust only runs around 5% air exchange rate and the rh gets up to around 40-50% when lights come on then the room heats up and the tstat kicks the exhuast dampers open full bore

I think my biggest issue is not having a fan suck in direct air from outside. with the 1700cfm exhuast and all passive intake creating a heavy vacuum in the room the air is drying out faster than what the intake is pulling in. it is like 65RH outside right now and with my fan exhausting and passive intakes only RH in the room is only 38% This is with the lights off right now!

I have a feeling the passive intake and high vacuum created in the room with the air coming in from the unconditioned portion of the garage all culminates into a scenario giving me extremely low RH conditions and not enough air exchange.

How about this?

Lights pull from lung room through sealed ductwork, then exhaust as one circuit.

Secondary exhaust that vents the froe space separately. You can run the secondary that vents the room at a lower CFM to mitigate neg pressure

Requires a second exhaust fan, but more flexible