How to size a dehumidifier

[Guest 18340]

Hi folks
Can anybody tell me if theirs a general rule of thumb when it comes to sizing a dehumidifier? I need 2 of them for 2 separate rooms. One room is 10x5x8, the other is exactly twice as big (20x10x8).
Is it just a matter of the actual size of the room or does the (grow) equipment that's in the room play a part when it comes to size needed?
As an example, when sizing an AC one has to take into account (besides the obvious physical size of the room) how many watts of light are in there, etc.
Both rooms are air conditioned and the AC units work perfectly, but I need a dehuey for when the lights are off.
I noticed that dehuey's are sized by how many pints/water/day it produces. Is that right?
Please forgive my newbness on this subject Any help would be greatly appreciated.
Evl.

 

[Guest 18340]

Sooooo nobody uses dehumidifiers huh.

 

[Che]

You will need probably a 70 pint for the small room and at least double that for the large room.. I'm not sure of the math but that's a pretty large space with a lot of moisture to control. Also, dehu's are rated for normal use, so drying a room full of transpiring plants is a lot to ask of one. As such, normal area sizing rules likely do not apply, at least on the sq.footage scale.

I use a 70 pint for a flowering room with half that much area populated, and it will fill overnight if it's not hooked up to a drain. Running non stop it can keep it around 25-35% RH in my space.

 

[Guest 18340]

Thanks for the response Che. I do have AC in both room (one in each room) so they should help the dehuey's not have to work as hard. No? I'm askin. Actually, I'm hoping

 

[Philosophelon]

Yeah, they are generally rated at pints or liters per 24h, although some list pints/liters per hour as well. Most stores that sell (non-industrial) dehueys don't carry any that are larger than 70 pints (33 liters) per day, so I use more than one. Using multiple smaller dehueys uses more electricity than a single large industrial unit, but they are WAY cheaper ($250 vs. $1K+). Make sure you get an energy-star rated dehuey as well, I have an old 40ppd dehuey that I don't use that is not energy star certified and it uses almost as much electricity as my newer 70ppd models (6.5A vs 7A). You may be able to get away with a single dehuey in the smaller room, but you'll probably need at least two for the bigger room. Having a sealed room with A/C helps with humidity too, as the dehuey(s) should heat up the room enough during lights-out that the A/C kicks on, which removes humidity as well. As far as a rule of thumb for sizing dehueys, I don't really have a good one. I simply estimate based on past experience -- in my experience larger plants with more leaf surface area and a deeper canopy (trees) transpire more water per square foot than smaller plants with less leaf surface area and a shallow canopy (SOG/SCROG). Also, higher ambient air temp increases transpiration, and thus the demand on one's dehueys.

 

[Lapides]

I had a 65 pint LG dehumidifier running only when my lights were off. I set the humidifier at 35%, it never got above 47% RH. That was with 120 plants, 6000 watts, and a 25kBTU AC running.

 

[brie]

I use a Santa Fe dehumidifier in about 1400 sq ft its air flow is 275cfm and 100pints a day I bought it at sylvane.com It works great

 

[Guest 18340]

Hey guys, thanks for all the help with this.
This morning a bought a 6 moth old energy star rated 40 pint/day dehuey from my neighbor for $60. I'm gonna put that one in my 10x5 room.
Its fully adjustable, auto restart etc.
I set it to kick on at 55% but it hasnt kicked on yet 'cause the ac is running and keeping r/h down to 45.
I'm gonna try and find a bigger unit for the 20x10 room.
Thanks for the input everybody!

 

[whodi]

do people use dehumidifiers for un-sealed grows with inline fans.. or are they for sealed rooms with AC mainly, basements, etc..

 

[LlamaSchool]

They are fine for use with unsealed grows and necessary in my case. Of course, you will be less and less efficient the more air you are moving through the grow area.

 

[noone88]

The black LG 65 pint model that everyone seems to use (sold at Casa Depot) pulls *900* watts (I have the Kill-a-Watt).

It's cheap, but if power usage is an issue, look into the Santa Fe models. More expensive, but power usage is less.

Just like with AC's, the more powerful the unit, the less energy it will use overall because it's not turned on all the time.

 

[Lazyman]

Yeah, they are generally rated at pints or liters per 24h, although some list pints/liters per hour as well. Most stores that sell (non-industrial) dehueys don't carry any that are larger than 70 pints (33 liters) per day, so I use more than one. Using multiple smaller dehueys uses more electricity than a single large industrial unit, but they are WAY cheaper ($250 vs. $1K+). Make sure you get an energy-star rated dehuey as well, I have an old 40ppd dehuey that I don't use that is not energy star certified and it uses almost as much electricity as my newer 70ppd models (6.5A vs 7A). You may be able to get away with a single dehuey in the smaller room, but you'll probably need at least two for the bigger room. Having a sealed room with A/C helps with humidity too, as the dehuey(s) should heat up the room enough during lights-out that the A/C kicks on, which removes humidity as well. As far as a rule of thumb for sizing dehueys, I don't really have a good one. I simply estimate based on past experience -- in my experience larger plants with more leaf surface area and a deeper canopy (trees) transpire more water per square foot than smaller plants with less leaf surface area and a shallow canopy (SOG/SCROG). Also, higher ambient air temp increases transpiration, and thus the demand on one's dehueys.

Yep, I agree with this. I have a 30X20 room and use two 45-pint dehueys I got off craigslist, plumb the drains into a discharge bucket or into your res. I like two small ones better than one, cause if one malfunctions the other one can still help somewhat, vs. being totally down. The number/size of reservoirs, grow medium, and number/size of plants all affect humidity, so that's probably why there's no formula for it. Hope that helps!

 

[grow nerd]

I think Philosophelon pretty much nailed it.

 

[grow nerd]

I had a 65 pint LG dehumidifier running only when my lights were off. I set the humidifier at 35%, it never got above 47% RH. That was with 120 plants, 6000 watts, and a 25kBTU AC running.

I'm guessing guessing that the 25k BTU A/C was pulling out quite a bit of water, too. Probably more than your 65ppd dehumi?

 

[NOYB]

do people use dehumidifiers for un-sealed grows with inline fans.. or are they for sealed rooms with AC mainly, basements, etc..

IME dehumidifiers are only of value in an open system during lights off if you stop ventilation during this time. If you keep the fans running then a dehumidifier is useless in an open system.

 

[Guest 18340]

Definately helped, Lazyman. I was wondering what/if any effect my rez's would have.
The 10x5 has 2 55g blue drums, the 20x10 has 4. They all have non-removable lids, with 2 threaded holes on the top. The medium will be coco in both room. 2x600w in the 10x5 (over a 3x6 table) and 8x600w in the 20x10. Though, all 8 won't be running at once, only 4 at a time.
The number of plants in each room will vary from grow/grow so thats the only unknown factor for me.
I've been running my 10x5 all day with the AC and dehuey on and the thing only kicked on once, and that was when I left the door open for 5 minutes while I was in the room. Nice.
RH has been 40-50- all day.
I like the idea of running two, I always like to have a back up of everything

 

[whodi]

They are fine for use with unsealed grows and necessary in my case. Of course, you will be less and less efficient the more air you are moving through the grow area.

how will i be less efficient the more air i have moving through the grow... i was always taught more air movement is better.

 

[NOYB]

how will i be less efficient the more air i have moving through the grow... i was always taught more air movement is better.

The more air you move the less efficient a dehumidifier will be in this situation but the better the environment will be for your plants...typically...because of the improved air exchange. The less air you move in an open system the more humid the air will be because your plants basically 'perspire'.

 

[Lazyman]

Hey Evlme, yeah I was thinking a 45-pint for the small room, and a 90-100 pint for the big one (or two 45's) should do the trick. As long as your reservoirs are covered that will help a lot, I'm trying to find covers for my grow bags so I can keep bugs out of the coco and reduce transpiration/drying of the roots too. Just make sure you have an overflow fitting on your reservoirs before you start dumping the dehueys into them, but otherwise it's GOOD water! Keeps more of a closed-loop system too.

Noyb, I think your statement depends on outside RH, if it's normally higher than indoors you'd be absolutely right. Where I live my outdoor RH is usually lower, so bringing in outside air helps reduce my indoor RH.

 

[NOYB]

Noyb, I think your statement depends on outside RH, if it's normally higher than indoors you'd be absolutely right. Where I live my outdoor RH is usually lower, so bringing in outside air helps reduce my indoor RH.

Only time I've ever used a dehuey, and I've only run open systems, has been during the lights out cycle with no ventilation. Had one setup, 4800 watts and hydro, and at one point outside was pouring down rain and fog, my air exchange was about every 60 seconds in an 18' x 8' x 6' space, yet still my flower room RH lights on did not go over 55%...with no dehumidification. That was not by design and spent awhile trying to figure out how that happened. I've also reduced RH by canopy pruning without, what I felt, sacrificing yield.