Hey guys, just thought I'd pass on some info I've gleaned over the years.
Sizing a dehuey is fairly straightforward, but if it's in an air-cooled room, it's not so easy.
Plants transpire 96% of the water you give them, so the math is pretty easy. If you use 10 gallons of water a day, you need a 9.6 gallon (8 pints per gallon equals 76.8 pints of water to remove every day.
That would put you at around an 80 pint dehuey, right? Easy enough, or you could use two 40 pint units to spread out the work and have a little fail-safe in case one dies (the small home units from Kenmore etc last about 2 to 3 years usually before death.)
Now, if you have a sealed room with AC, your AC unit will actually do a fair amount of dehumidification all by itself. Check the rating on your unit, as they often list how many pints per day they remove. Subtract this amount from your daily dehumidification needs.
However, if you have an air-cooled room, then what?
This is where it gets complicated. We'll assume that you're running your lights at night, because I don't think anyone runs lights during the day in air cooled rooms.
So, if your outside air is hot during the day (dark period for plants) then the air is either dry, or its humid. If it's dry, you are probably ok to use that air (and air exchange) to dry your room out during the dark phase. As long as the air isn't much over 90* F and the plants have plenty of water to drink, your RH should be easily controlled with fans, and not need dehumidification during this period. You may need it at night if your fans aren't running much (like the outside temp drops a lot) but in general, you don't want to be running intake/exhaust fans and dehumidifiers at the same time. One will always work better than the other, and it takes time to treat and dry the air.
If you have a hot humid climate, you likely have AC, and may need supplemental dehumidification. It's probably best to shoot for sealed rooms in these climates as there's nothing really beneficial about the outside air for you to use.
What about cold (snowy) climates? Since snow and freezing temps tend to reduce RH to nothing, or close to it, it can be tricky to air cool these grows. In snowy areas, high RH isn't usually an issue, usually it's too low. Humidifiers are often used to keep the RH above 30%.
Most of these climates are relatively easy to deal with, since AC or the outside air are beneficial enough to preclude the need for dehumidification. However, cool, humid climates present a bigger challenge.
Cool, wet, weather like we have in the Pacific Northwest winters, mean we can't use AC, but we do air cool our rooms because it's in the low 40s at night, but 100% humidity. Great for cooling lights at night, but shitty air for keeping RH down in the dark period.
To an extent, the electric lights we use do a pretty decent job of drying the air themselves, but we still need to do air exchange to keep temps stable (and keep the room at negative pressure to avoid odors.)
I find that if your intake and exhaust fans are sufficient, dehumidification isn't usually needed during lights on. Using bluetooth weather sensors from Oregon Scientific, we charted our temps and RH and could see how it fluctuates from hour to hour.
Shortly after lights on, temps rise and the RH briefly spikes (plants warming up and transpiring.) Then the RH drops rapidly into the low 50s, which is acceptable, and close enough to ideal so as not to worry.
My big flower room may be extreme though, as we have 30 x 1000w DE lights, and 8000cfm exhaust, with 5000 cfm intake. So I can change the air every 30 seconds if everything is on (which it usually isn't this time of year.)
So, we finally get to the problem. Cold rainy days (dark photoperiod) and 100% humidity.
Now, you can run your fans to try and keep the RH down. It may work, but it probably won't, even with no intake fan going. Your carbon filters will clog up with all the moisture in the air, and won't last as long as a result, and any air you bring in is probably more humid than what you're trying to blow out.
So you need to probably shut the exhaust fans off, and use a dehumidifier, or maybe several.
To keep odor down, you can use a scrubber fan and filter to simply circulate room air. Or if you want to get fancy, use a couple motorized dampers on your exhaust duct (one Normally open, one Normally closed) to route the exhaust air back into your room during the dark period, then back outside on a timer when lights come on.
Anyway, plants transpire 24x7, but they do transpire faster in warm dry air than they do in cold humid air. During lights on, the fans will handle the humidity, but lights off belongs solely to the dehueys.
Let's say that your plants give off half their transpired water during the dark period (which is probably very high, but that's the idea) and that half is what we need to calculate.
So half of 76.8 pints for our first example is 38.4 pints, which doesn't sound so bad, certainly cheaper than an 80 pint unit. It's likely that 40 pint unit is going to run pretty hard 12 hours a day in flower, and if it ends up not being enough, you'll be able to tell in about a week.
I've had rooms take that long to dry out, just because moisture had saturated wooden beams and boxes in the room to the point of being almost dripping, so the dehuey might have a lot of catching up to do.
I do like Santa Fe dehueys (made by Thermostor, as are the Quest line dehueys) as they tend to be very efficient. The more pints removed per kwh, the better. Ideal Air dehueys suck, they rate their water removal at crazy high temps and RH, and they use 2-3x as much power as a Santa Fe per pint. Not worth it.
Dehueys also work best when mounted high up, as warm, humid air rises.
Don't forget to account for the condensate, or the water from the dehueys. Condensate pumps are handy, but the water is also quite clean and generally reuseable if desired.
I don't use dehueys in early flower or veg, but once flowers start to get bigger than a gumdrop it's time to start keeping them dryer.
I also use sulfur burners and a variety of chemicals to ward off PM and botrytis (black mold) so those can help reduce your need for dehumidification a bit (but not completely.)
Mnually operated dehueys are ideal, as we can turn them on and off with timers, or external humidistats or controllers as needed. Electronic controlled dehueys often don't restart once they are powered off.
Keep your dehuey filters clean, and try to prevent running them for 100% duty cycles. If they never shut off, you need another one. Or a bigger one.
Any questions?
Ok class dismissed!
Sizing a dehuey is fairly straightforward, but if it's in an air-cooled room, it's not so easy.
Plants transpire 96% of the water you give them, so the math is pretty easy. If you use 10 gallons of water a day, you need a 9.6 gallon (8 pints per gallon equals 76.8 pints of water to remove every day.
That would put you at around an 80 pint dehuey, right? Easy enough, or you could use two 40 pint units to spread out the work and have a little fail-safe in case one dies (the small home units from Kenmore etc last about 2 to 3 years usually before death.)
Now, if you have a sealed room with AC, your AC unit will actually do a fair amount of dehumidification all by itself. Check the rating on your unit, as they often list how many pints per day they remove. Subtract this amount from your daily dehumidification needs.
However, if you have an air-cooled room, then what?
This is where it gets complicated. We'll assume that you're running your lights at night, because I don't think anyone runs lights during the day in air cooled rooms.
So, if your outside air is hot during the day (dark period for plants) then the air is either dry, or its humid. If it's dry, you are probably ok to use that air (and air exchange) to dry your room out during the dark phase. As long as the air isn't much over 90* F and the plants have plenty of water to drink, your RH should be easily controlled with fans, and not need dehumidification during this period. You may need it at night if your fans aren't running much (like the outside temp drops a lot) but in general, you don't want to be running intake/exhaust fans and dehumidifiers at the same time. One will always work better than the other, and it takes time to treat and dry the air.
If you have a hot humid climate, you likely have AC, and may need supplemental dehumidification. It's probably best to shoot for sealed rooms in these climates as there's nothing really beneficial about the outside air for you to use.
What about cold (snowy) climates? Since snow and freezing temps tend to reduce RH to nothing, or close to it, it can be tricky to air cool these grows. In snowy areas, high RH isn't usually an issue, usually it's too low. Humidifiers are often used to keep the RH above 30%.
Most of these climates are relatively easy to deal with, since AC or the outside air are beneficial enough to preclude the need for dehumidification. However, cool, humid climates present a bigger challenge.
Cool, wet, weather like we have in the Pacific Northwest winters, mean we can't use AC, but we do air cool our rooms because it's in the low 40s at night, but 100% humidity. Great for cooling lights at night, but shitty air for keeping RH down in the dark period.
To an extent, the electric lights we use do a pretty decent job of drying the air themselves, but we still need to do air exchange to keep temps stable (and keep the room at negative pressure to avoid odors.)
I find that if your intake and exhaust fans are sufficient, dehumidification isn't usually needed during lights on. Using bluetooth weather sensors from Oregon Scientific, we charted our temps and RH and could see how it fluctuates from hour to hour.
Shortly after lights on, temps rise and the RH briefly spikes (plants warming up and transpiring.) Then the RH drops rapidly into the low 50s, which is acceptable, and close enough to ideal so as not to worry.
My big flower room may be extreme though, as we have 30 x 1000w DE lights, and 8000cfm exhaust, with 5000 cfm intake. So I can change the air every 30 seconds if everything is on (which it usually isn't this time of year.)
So, we finally get to the problem. Cold rainy days (dark photoperiod) and 100% humidity.
Now, you can run your fans to try and keep the RH down. It may work, but it probably won't, even with no intake fan going. Your carbon filters will clog up with all the moisture in the air, and won't last as long as a result, and any air you bring in is probably more humid than what you're trying to blow out.
So you need to probably shut the exhaust fans off, and use a dehumidifier, or maybe several.
To keep odor down, you can use a scrubber fan and filter to simply circulate room air. Or if you want to get fancy, use a couple motorized dampers on your exhaust duct (one Normally open, one Normally closed) to route the exhaust air back into your room during the dark period, then back outside on a timer when lights come on.
Anyway, plants transpire 24x7, but they do transpire faster in warm dry air than they do in cold humid air. During lights on, the fans will handle the humidity, but lights off belongs solely to the dehueys.
Let's say that your plants give off half their transpired water during the dark period (which is probably very high, but that's the idea) and that half is what we need to calculate.
So half of 76.8 pints for our first example is 38.4 pints, which doesn't sound so bad, certainly cheaper than an 80 pint unit. It's likely that 40 pint unit is going to run pretty hard 12 hours a day in flower, and if it ends up not being enough, you'll be able to tell in about a week.
I've had rooms take that long to dry out, just because moisture had saturated wooden beams and boxes in the room to the point of being almost dripping, so the dehuey might have a lot of catching up to do.
I do like Santa Fe dehueys (made by Thermostor, as are the Quest line dehueys) as they tend to be very efficient. The more pints removed per kwh, the better. Ideal Air dehueys suck, they rate their water removal at crazy high temps and RH, and they use 2-3x as much power as a Santa Fe per pint. Not worth it.
Dehueys also work best when mounted high up, as warm, humid air rises.
Don't forget to account for the condensate, or the water from the dehueys. Condensate pumps are handy, but the water is also quite clean and generally reuseable if desired.
I don't use dehueys in early flower or veg, but once flowers start to get bigger than a gumdrop it's time to start keeping them dryer.
I also use sulfur burners and a variety of chemicals to ward off PM and botrytis (black mold) so those can help reduce your need for dehumidification a bit (but not completely.)
Mnually operated dehueys are ideal, as we can turn them on and off with timers, or external humidistats or controllers as needed. Electronic controlled dehueys often don't restart once they are powered off.
Keep your dehuey filters clean, and try to prevent running them for 100% duty cycles. If they never shut off, you need another one. Or a bigger one.
Any questions?
Ok class dismissed!
