Anyone with AC running dehum during lights on?

lazaah

lazaah

Well-Known Member
I'm putting the final touches on my build and thinking about dehumidifier placement.

Flower and veg share a lung room. Flower is sealed with AC but vented in the during dark cycle.

Weighting up whether I actually need to run a dehumidifier in the flower room during light cycle, as the AC is pulling out water during that time.

Would be way easier for me to run the dehumidifier in the lung room, as having it running in the flower room will eat up precious real estate.

Whats everyone's experiences with lights on, AC, and humidity?
 
I have a 3 ton mini split and still have 2-70 pints and a 45 pint in that room. If I get too much plant mass I'm still overpowering the dehumidifiers.
 
Remember that cooling air causes the RH% to rise, so if fighting RH% and temperature, you should make sure you aren't running any cooler than you must. When it gets to where you are putting up the biomass that can outrun your environmental controls, you either need less plant mass or stronger dehumidification.

Just remember, keep the rooms as warm as possible since this means the air can hold more water.

Think about it like this, if you water your plants every day and they take so many pints of water, some runoff happens, minus the runoff, what stays with the plants roots, 98% of that water will transpire out of the plant into the air causing the dehu to have to process that much water between waterings.
 
Cheers guys.

I'm running DTW so there's no pooling water, and the dehumidifier is hard plumbed and rated for 120% of the mount of water I'm irrigating with daily.

I'm currently running 28C with an average of 70% humidity. This will need to come down during flower, so I guess it is staying in the flower room
 
lazaah said:
Cheers guys.

I'm running DTW so there's no pooling water, and the dehumidifier is hard plumbed and rated for 120% of the mount of water I'm irrigating with daily.

I'm currently running 28C with an average of 70% humidity. This will need to come down during flower, so I guess it is staying in the flower room
Click to expand...
You will find that as the biomass increases during stretch after flip the transpiration of moisture will increase a LOT. So your dehu may be unable to keep up with the demand. Generally by day 20 ish of 12/12 the plants kick into overdrive and they stay there until around day 50 ish (these numbers are for a 70 day strain and they do vary per strain). So if I am watering my plants every 24 hours on day 19 (soil is dry after 24 hours) then I will find myself having to water like every 12 hours when they start "peak drink" on day 20 ish. For those next 30 days the amount of water that needs to be removed from the air basically doubles. They speed up suddenly and they slow down suddenly. Remember that when they slow down if you keep watering at the high frequency, root rot is a likely result. I have had plants in 10 gallon pots go from taking 1.75 gallons every 24 hours speed up on day 20ish and require 1.75 gallons every 8 - 10 hours so thats even more than doubling the amount of moisture the dehu has to pull in a given time period. I ended up with three dehu's ( 2 x 100 pint and one 150 pint) in that room and still couldn't keep up with the RH% during lights out.

Lights out is when bad things happen RH wise. So lets say you are at 84F and 50% and the light go off. The AC stops fighting heat and thus doesn't pull any water, leaving it all on the dehu. The temps drop from 84F to lets say 75F, with a constant amount of moisture in the air this causes the RH% to spike to around 70% and to make matters worse the plants are still transpiring for about 20 minutes after lights out. So in this case you might see the RH% hitting 75 or 80% and thats no bueno. To make matters even worse, your dehumidifier is much less effective at pulling water out of the air when the air is cool. So you may be able to pull enough water when the air is warm but when it's cool the dehu struggles to pull half that amount. So this leaves us with one really big tool for this situation, a space heater. It will help prevent the night temp from dropping and raising the RH% on you and it will help the dehumidifier a great deal as air at 80F is much easier to dehu than air that is at 75F.
 
Renfro said:
You will find that as the biomass increases during stretch after flip the transpiration of moisture will increase a LOT. So your dehu may be unable to keep up with the demand. Generally by day 20 ish of 12/12 the plants kick into overdrive and they stay there until around day 50 ish (these numbers are for a 70 day strain and they do vary per strain). So if I am watering my plants every 24 hours on day 19 (soil is dry after 24 hours) then I will find myself having to water like every 12 hours when they start "peak drink" on day 20 ish. For those next 30 days the amount of water that needs to be removed from the air basically doubles. They speed up suddenly and they slow down suddenly. Remember that when they slow down if you keep watering at the high frequency, root rot is a likely result. I have had plants in 10 gallon pots go from taking 1.75 gallons every 24 hours speed up on day 20ish and require 1.75 gallons every 8 - 10 hours so thats even more than doubling the amount of moisture the dehu has to pull in a given time period. I ended up with three dehu's ( 2 x 100 pint and one 150 pint) in that room and still couldn't keep up with the RH% during lights out.

Lights out is when bad things happen RH wise. So lets say you are at 84F and 50% and the light go off. The AC stops fighting heat and thus doesn't pull any water, leaving it all on the dehu. The temps drop from 84F to lets say 75F, with a constant amount of moisture in the air this causes the RH% to spike to around 70% and to make matters worse the plants are still transpiring for about 20 minutes after lights out. So in this case you might see the RH% hitting 75 or 80% and thats no bueno. To make matters even worse, your dehumidifier is much less effective at pulling water out of the air when the air is cool. So you may be able to pull enough water when the air is warm but when it's cool the dehu struggles to pull half that amount. So this leaves us with one really big tool for this situation, a space heater. It will help prevent the night temp from dropping and raising the RH% on you and it will help the dehumidifier a great deal as air at 80F is much easier to dehu than air that is at 75F.
Click to expand...

What size room was that in though Ren? This is a 4x4, dtw coco, 50pint/day dehumidifier, and 13k btu mini split. 9 plants in 3gal pots watered 6 times a day.

The minisplit is on a diy controller so I can maintain 25C at lights off (its early summer here atm anyhow). During lights off the room can be vented to the lung, which can then be exchanged with outside air if needed.
 
lazaah said:
What size room was that in though Ren? This is a 4x4, dtw coco, 50pint/day dehumidifier, and 13k btu mini split. 9 plants in 3gal pots watered 6 times a day.
Click to expand...
You can see my rooms in my thread but it's definitely bigger than your space.

Thing is though if you are venting the tent to and from a lung room you need to condition that entire volume of air. Simple fact is a 50 pint a day dehu rarely actually pulls 50 pints. If you quantified how much water you have as runoff and subtracted that from the total volume of water input, you will get a pretty good idea how much water is being transpired in a 24 hour period.

If you have LED lighting then the AC isn't going to pull much water as it's not fighting a big heat load. What sorta lighting is being used?
 
How does your controller work, is it possible to switch mini split to it to dehuey mode for the first hour of lights off?

Or maybe cycle it between heat and dehuey? During lights off.
 
Renfro said:
You can see my rooms in my thread but it's definitely bigger than your space.

Thing is though if you are venting the tent to and from a lung room you need to condition that entire volume of air. Simple fact is a 50 pint a day dehu rarely actually pulls 50 pints. If you quantified how much water you have as runoff and subtracted that from the total volume of water input, you will get a pretty good idea how much water is being transpired in a 24 hour period.

If you have LED lighting then the AC isn't going to pull much water as it's not fighting a big heat load. What sorta lighting is being used?
Click to expand...

It's a box build with 90mm wool insulation right through. 12x5x6. Entry into lung room, pass through 4x4 veg to get to 4x4 flower at the end. Each section is partitioned with panda film and zips.

Flower lights are 2x GLA 420 units, dimmed to a combined 480w (for now). Veg, for mums/clones/short grow, is adjustable shelves lit with vero10s.

Minisplit is in the flower end. Single fan for veg/flower with extraction zones set by electronic damper. Lung room has it's own extraction fan.

At the moment there's 4 in veg and an autoflower in its 2nd week of flower in the flower chamber. Combined they are probably drinking a gallon a day atm, and thats coming out from the dehumidifier.

I'm trying to guage whether I need to build a recess to house the dehumidifier in the veg chamber but open to the flower chamber to keep humidity down during lights on, or if ill get away with using the dehumidifier in the lung room during flower lights off and using my controller to cycle between cool and dehumidify during lights on. Without the AC cooling, the room easily rises to 40C.

Apalchen said:
How does your controller work, is it possible to switch mini split to it to dehuey mode for the first hour of lights off?

Or maybe cycle it between heat and dehuey? During lights off.
Click to expand...
The controller will work however I program it to. Its a series of ESP32's and a Raspi running home assistant. I won't need to run the AC as dehuey during lights off as the room will be vented to the lung room or ill use the dehuey I have. It's what to do during lights on that I am trying to figure out.
 
Are you running CO2 in flower lights on? If so your temps should be higher, like 29 to 30 C, and therefore you VPD required a RH of 70%.
 
tilopa said:
Are you running CO2 in flower lights on? If so your temps should be higher, like 29 to 30 C, and therefore you VPD required a RH of 70%.
Click to expand...

Not yet. I'll be setting up CO2 after my next round, as I would like to get to know my strain a bit better first
 
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