Hydro dwc temperature control and ventelation

This is one of my plants at day 20. I have replaced the 6-100w CFLs with a 400w Metal Halide 6000K lamp; I positioned the light 2’ above the plants. Temperature may be a problem if I’m not careful. I’ve got a 4" fan blowing across the plants and another 4" fan outside the grow box blowing into intake ports (3-3.5" holes at the bottom, on both sides of the grow box. If I leave the room door open I can keep the temp below 80*F.
TDS is 600ppm, pH 5.9. All 5 plants now have multiple roots extending into the reservoir.

Plan to do ScrOGing at 2’ above above the top of the pots. Was wondering if/when I should do sexing. The plants are feminized White Widow that I got from Robert. Is a male plant possible?

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At those temps I’d be a little worried the root zone is getting too hot, tons of aeration may compensate some but be careful. Your roots should be staying kinda bright white and firm, a little discoloration is normal and fine, especially with nutrients of organic origin but if they are getting too brown and mushy you got problems. You really need your root zone around 70F and no higher than 75F, no lower than about 60*F, you don’t want your roots freezing either.

I’m going try and modify my system to get the temp. First thing is to add more aeration. I only have one bubbler (cheap Wal-Mart 12" stone). Think I can get an air pump with two outlets so I can and more stones.
Something else I thought about is my reservoir has a black lid; will try to brighten it up with some duct tape or white poster board.

I saw one of Robert’s articles on hydroponics where you made comments about water temp so I’m getting the feeling this is a pretty big deal. What’s your opinion about water chillers? That may be the way I should go. I’ve looked at a couple that cost between $350-$450. Would be my most expensive “growing” cost yet.

Forgot to mention, I will be cooling 10 gal.

Keeping enough air flow through the grow box/room to keep air temps below 78F is a good place to start, that alone will make a huge difference. Below 75F is even better. And you shouldn’t need a chiller. Also for something like what you have, there may be cheaper alternatives that may be better suited for a smaller reservoir like yours. Check out my other responses and such for hints at what I have cooking and stay tuned for some new posts soon.

Pick up a can, maybe a pint of flat white primer house latex paint. It is pretty much non-toxic when dry. It should have a high titanium content in the latex which will reflect nearly as good as mylar and it won’t hurt anything if a little dried piece latex or titanium get in your reservoir. Scuff the top surface of the lid really well with sand paper and then paint a healthy coat or two on top side of the lid with the baskets removed. Let thoroughly dry before using and clean up rims of holes with a razor blade. I’ve seen original applications of latex paint done like this last the entire lifetime of the Rubbermaid tote container.

Adding another air pump or a bigger one with two outlets will help, you can never have too much air, and I kinda like running two pumps, not the most efficient electricity wise but it would be very rare that both pumps fail and so you wouldn’t end up with a dead stagnant reservoir/root zone in an emergency. Making sure you have a large enough and fine enough air stone helps as well, not all air stones use the really fine ceramic fine bubble stone material, or even more than one stone per pump in some cases.

Go to the big box home supply store. Get 2 6" ventilation duct booster fans. attach one to intake, and the other to out take port. Simple wiring will be required. You can run them all the time; or, get a thermostat.

Raise you ppm levels. When I had plants like that I ran 1200-1500 ppm; Especially now that you got a 400w lamp. The plants are light green almost yellow, because they are starving under the hot Sun. :slight_smile: At least raise ppm to 900 -1000.

I hope they are not too far gone, seeing the consistently light color of the leaves.

I thought of one other factor. Make sure that when you top off the reservoir; The nutrient solution level is not above, the bottom of the net pot. :slight_smile:

Oh, yeah, sexing, you should have all females with feminized seeds, very very rare that you won’t.

I’m attaching my Journal entry from yesterday. Painting the lid with primer sounds like a good idea but with the stage of my grow I’ll have to wait until after harvesting (if there is one)

Day 22 Modification made to lower temp with 400w MH lamp - controlled test. Added white poster board to surface of reservoir top. Reinstalled 400w MH lamp two feet above net pots. Increased nutrient to 960ppm by adding 20ml to each of Gro, Bloom, and Micro. At 5:30 PM the air temp is 67F, water temp is 70F, pH is 5.9, TDS is 960ppm. Next cycle is scheduled to start at 6:00 PM and end at 12:00 Noon tomorrow. Will monitor parameters during cycle.

UPDATE: Checked status at 10:40 PM. Air temp 69* F, water temp 72* F, pH 5.9, TDS 960 ppm.

UPDATE: Checked status at 2:40 AM. Air temp 71* F, water temp 75* F, pH 5.8, TDS 960 ppm.

UPDATE: Checked status at 8:00 AM. Air temp 72* F, water temp 78* F, pH 5.8, TDS 960 ppm. NOTE: At some point during past 14 hours the air temp reached 75* F.

UPDATE: Checked status at Noon; completed the planned 18 hour cycle. Air temp 72* F, water temp 78* F, pH 5.9, TDS 960ppm.

Conclusion: Water temp continues to rise despite a relatively constant air temp. OK, so maybe I should cover net pots with something…alum foil? Will also install another bubbler and see what happens to water temp.

I’m surprised, usually the temps are the other way around with the reservoir slightly below the air temperature.

I have a couple of questions for you. What is the relative humidity? And is the reservoir on a bare concrete slab? If so, what is the temperature of the slab? This is what might be keeping the reservoir from cooling much from the drop in air temperature. All of your insulation and reflection techniques should be working to minimize the heat absorbed from the lighting itself. As simple as putting cardboard or something even more insulating under the reservoir might be the answer.

A thought about the air pump or pumps, air line hose is super cheap, the pumps themselves radiate a lot of heat, put the air pumps far from the reservoir as not to contribute heat to the reservoir, keeping the pump above the reservoir or getting anti-siphon valves is cheap enough as well, you can even put the air pump in a really cold location to pump very cold air directly into the reservoir.

I was trying to get a good look in the picture. I see it is likely not on the concrete slab. Might there be any other reason the reservoir is getting heat from below?

My grow box is free standing closet made from particle board. The floor of the closet sets about 2" above the house flooring. Since my last post I have made a couple of changes.

First, I’ve covered all the net pots/clay pebbles with Aluminum foil. I saw a post somewhere where someone covered their pots in a similar manner. It makes sense that the dark pebbles may heat up and warm the water I’m pumping thru the pots. I’ve also added a duel port air pump with 2 small cylinder shaped stones. The bubble action of this arrangement seems to be working better than single port air pump attached to a 12" stone. To your point I suspended the new air pump a little above the light. I started the current light cycle at 6:00 PM, the water temp was 72* F. I checked at 10:00 PM and the water temp was 73* F. Too early to declare a victory; the real test is what the temp is at Noon tomorrow, when the light turns off.

One of your other questions was about the relative humidity. I believe it has varied between 45% to 60%.
I’m guessing because my indoor/outdoor thermometer/hygrometer died a couple of hours ago.

Definitely; Radiant air temps are always higher than reservoir temps. Hmmm

On the other hand; I really would not worry too much about your issue here. Although we would like to see 65-70 degrees in the reservoir; 75 is in the OK parameters. Marijuana plants adapt. Point being; The reason you want temps in the high 60’s to low 70’s is because, there are more oxygen molecules, or smaller, or a combination of both. :slight_smile: at these temperatures. Without the use if Co2; I believe you are solid in all reports, aside from the hiumidity levels that MacG asked for. Peace

Yes, I tend to agree as long as you can get the water temp. around 75*F or below you should be ok. It isn’t so much the size of the oxygen/air bubbles, it is a chemistry thing and the shape of the water molecule that make it hold more oxygen at colder temperatures, kinda like the way ice gets bigger when frozen instead of getting smaller as it gets colder like most other things, shape of the H2O molecule, chemistry and geometry.

A really high humidity will mean the reservoir temperature will normally be closer to the air temperature because it can not lose temperature through evaporation, part of the reason I asked but your humidity should be low enough, all things being equal, the reservoir would normally be able to lose a significant amount of heat to the atmosphere around the reservoir.

Yeah I actually recommended Mylar or aluminum cut outs to also cover the hydroton, but not as much for temperature as for giving the roots more area to grow without being photo inhibited by the light. Either way covering the hydroton should help in both ways, temp and giving dryer areas for the roots to grow which will help with high temperature tolerance.

Back to the drawing board! After all my modifying and theorizing at the end of today’s light cycle the water temp was 78F. I believe you guys were right; it’s got to be the air temp that’s causing the problem. I think I mentioned yesterday that my thermometer died. In fact it had been acting squirrelly for a couple of days. One day the Max Temp read 115 F. Anyway I’m confident my water temps are accurate; I’m using a photo lab quality analog dial thermometer. My air temp/hygrometer data I don’t think I can trust. One question I’ve always had with it is where is the best place to put the prob to get a meaningful reading?

I’m going to run the light cycle with the closet doors open (there goes my stealth grow box). Get as much circulation as possible until I come up with a better circulation system

I think you want to add an exhaust port and fan near the top of the grow box 4" - 6" would be plenty for that size grow box and you might not need much of an inlet port as leaks around seams, edges and doors might be enough to allow enough air to turn over to keep temps low, which would also be giving lots of fresh air with extra CO2 in it into the box and removing excess oxygenated air from the box with the hot air.

Unless you plan on building an enclosed light system which would have cold air pulled or pushed though a closed hood around the light, that would also dramatically reduce heat in the grow box and then you could use dry ice, “CO2 in a bag”, or yeast CO2 supplementation to your kinda sealed grow box. In this setup you want your air pumps outside the grow box pumping regular oxygen rich air into the reservoir instead of the co2 air from the “sealed grow box”.

Maybe think about one 90 degree bend and flat black paint to keep light from coming in through the duct work or an offset almost ‘S’ like duct shape.


If you use a charcoal filter on the exhaust duct, that might block enough light on its own.

Oh, as for where to put the air thermometer…

It depends on what you want the reading of, for maybe a general idea of the room and more how it will affect the reservoir, I’d say in the shade just udr the top level of the reservoir.

For some the temperature at the canopy may be very important, and so I’d place it in the shade at the level of the canopy.

In both instances and pretty much any other, you want somewhere that it isn’t going to be picking up radiating heat directly from the light or ballast and an area that gets a decent amount of moving air, as well as being shaded from direct light.

I was looking at the specs in your growbox more closely and yes, I think you need to upgrade to a 6" exhaust fan and 6" ducting, it will make a huge difference. You might be able to use a 4" but you need a minimum of about 160 CFM, to aprox 200 CFM to be sure it kinda has the horse power to pull enough air through the entire box, You might not need to use the full power of such a fan, which means the added expense of a variable controller, but it’s nice to have if you need it, the cheaper type of inline fan just wont do a good enough job, especially with any restricted air flow, i.e. filter or lots of ducting/baffling to get through, also especially if air inlets aren’t really very big.

Thanks for the VERY helpful suggestions. Based on those suggestions and using what I had available in my garage, I’ve got what I think is at least a good temporary fix. Last part first. I started light cycle at 6:00 PM, air temp 73*, water temp just under 73*. At 10:00 PM, air temp 73*, water temp 73*.

On your suggestions about exhaust, I have an exhaust fan in the grow box ceiling with 3" ducting to the outside. Problem is it’s only 50 cfm…way too small. Near the base I have 3 - 3.5" holes (intake ports) on both sides of the box. As you can see from the pic the grow box sits inside a slightly larger closet. There’s only 6"-7" clearance between closet wall and the box sides, pretty inefficient for air flow. I’ve placed high velocity “air mover” fans in the gap between the closet wall and box sides on both sides, forcing air through the intake ports. The fans are 3 speed and are rated at 4800 CFM each. How is that for overkill? But it does appear to be working.

To your other point, I may have wait until next grow to move the air pumps outside the grow box.
I now have the remote temp probe located behind the reservoir, 2"-3" below the lid of the reservoir in the shadow of the reservoir. That’s where I’m getting the 73* air temps. I originally had the probe just above the top of the light (which is 2’ above the plants) and the air temp there got up to 83*.

OK, maybe I can change out my pitiful 50 CFM with one closer to 200 CFM w/6" duct and get the air flow I need (without using the 4800 CFM blowers)

Sounds good. 83*F at the canopy should be fine, shouldn’t cause any problems.

Moving the air pumps outside the grow box is primarily or most significantly beneficial if you had the sealed box with the CO2 enrichment (oxygen at the roots and CO2 for the leaves/canopy), otherwise if your inside temps are good with lots of fresh air ventilation and temperature under control you won’t see any big benefit other than a small heat savings by having the air pumps outside the grow box.