Tutorial Ventilation 101

[GrayZone]

I made a small makeshift box(60x60x130cm) for a couple of plants, light by 150w HPS. The temperature won't be a problem as that little lamp doesn't produce that much heat.

However, I still wanted to have some air exchange, so I installed a small 10cm inline fan to pull the air out and get some fresh air in via passive intake. As the box is not airtight,(there are holes where air can get in all over the place)) the ventilation won't work as it's supposed to, I get that. But on a test run, it seems that it doesn't work at all.

I left the lamp on for an hour, to see how hot it will get and it goes to about 28c, which is still fine. Then, to test the vent, I turned it on and hoped to see a drop in temperature. However, there was none, or at best it was negligible.
The fan's cm3/h is sufficient to exchange the air about 4 times/minute, so it works on paper.

Is it simply about the fact the box is not fully sealed/airtight which makes the ventilation totally inefficient?

 

[exoticrobotic]

Maybe you wont reduce radiant temperature from the light but if you make a large hole through which you can direct some 4 inch ducting you can direct this sucked in air wherever you want which will cool things a bit.

But air exchange is more about providing fresh air for plant growth, not cooling.

Little holes all over the place isn't really enough to provide air intake if you are actively extracting 4 x volume /minute. I know you want negative cabinet pressure but with too few intake holes It may place stress on your extraction fan's motor.

 

[GrayZone]

But air exchange is more about providing fresh air for plant growth, not cooling.

Little holes all over the place isn't really enough to provide air intake if you are actively extracting 4 x volume /minute. I know you want negative cabinet pressure but with too few intake holes It may place stress on your extraction fan's motor.

Well, I think it's about cooling too. Otherwise, you wouldn't be advised to put your exhaust fan at the top of the space(where the hot air ends up), you wouldn't have air-cooled hoods for HID, and if the air pulled in is 4-5c cooler than the air inside, there should be a drop in temps, creating a bit of a cooling effect, along with air-exchange effect.

Anyway, some air is getting it. I see smoke being pulled in at the intake, so there's that. Probably the grow box is too leaky so the fan can't create enough suction at the passive intake, as there are openings everywhere in the structure of the box.

 

[exoticrobotic]

Well, I think it's about cooling too.

Yes it is. You are right.

My tent runs up to 31C If i saw a steady 28C i'd be overjoyed.

The extraction pulls out a whole lot of air, more than i passively allow in (twice the hole area)

If i worry about how hot it gets i just raise the lights a bit. It has more of an effect than increasing airflow.

 

[exploziv]

Do you draw air from same space where you exhaust it? Then you should probably expect no cooling..

 

[GrayZone]

Do you draw air from same space where you exhaust it? Then you should probably expect no cooling..

Yes, and that's a valid point. However, with a 150w HPS that doesn't emit a ton of heat and the outside temp being 4-5c lower than the inside temp, it would make sense to see some drop in temp, at least to the point where outside and inside temps close that 4-5c gap.

I was experimenting with this, so just now I tried to increase the intake space. Got 1c drop, from 28c to 27c, so the intake size seems to be the problem. Let's see what happens if the intake size is even more increased.

My tent runs up to 31C

31c is not that bad, as long as your humidity isn't too high(hot and humid = happy times for microorganisms) and your day/night temp difference is not more than 10ish c.

 

[thaichimaster]

Hello.

I was not sure where to post mu question but here it is.
I am making small grow box 55cmx55cmx120cm and I am deciding between two filters, Phresh vs PrimaKlima Industry.
I know both of them are a bit overkill for the size od a growbox but I need to eliminate all od the smell.
Here are links they are the same price:

Phresh filter

PrimaKlima

And the vent would be something like THIS

Thank you guys.

 

[Ca++]

I was just having a crack at this(fan sizing), based on RH instead of temperature.

I first looked at what air holds. At 30c it was 30g/m3. I figured the air coming in would be 50% RH, so holding 15g/m2.
There will be some gain in RH within the tent. I figure a 2meter of canopy drinks about 6L a day, if pretty full, and 30C with LED. That is 350ml per hour, lights on. Sizing the fan for 400ml, should help at this point.
So, I have 15g of water per meter used, and 15g left. I can only gain 5g, and I'm at 67%. I'm looking at 2m tall tents here. So there is 2 m3 of air, per meter of canopy. So 10g / 10ml of water can be held before 67%. So my fan must extract them 2 m3 40x per hour. 80m3 fan.
Now RH is also reduced by warming the air.
If the inlet was 25c, each m3 holds about 3g of water less, at 50%. So warming the inlet from 25 to 30, needs 3g from the plants, to remain 50%. So I don't need to extract every 5g now, like the 80m3 before. I can extract at 40m3 if that 67% target remains. The 80m3 still sounds good, as now we might run at 58% if we get a 5c rise. With a full exchange every couple of minutes.

While I could get more specific, it's worth looking at the minimum fan and filter we can buy. A 100mm fan and filter, where the filter is just 250mm long, and the fan that most people want to sell us, manage about 175m3 per hour together, If we don't restrict it any further. Which we will, by closing the tent. As this is the smallest stuff typically offered, we can see it's enough for a meter tent, or perhaps a 1.2
What I'm looking at, is what if you have half a meter, or even 25%
I have found the 2.7a 9733 fan, will get 60m3 through that filter, with speed control, for £15. While being more immune to further loading. It's happy to exhaust that through grey sink waste pipe. Though it's a bit noisy. It wants boxing.
I'm looking for quieter options

I just thought I would share my view on the subject. Which puts RH as the deciding factor with LEDs, not Temp as it were with HID. Though the inlet temperature makes a huge difference. I thought a look at the relationships would be useful, as no simple maths based on room size, really says enough. I think sizing for a full exchange every 90 seconds is pleanty big enough, but sizing is about actual flow, not stickers on things. If you close up everything on a tent, then you have no flow, no matter the fan. It's all got to work together.

Fans and filter choices, with presure drop examples
https://www.icmag.com/threads/free-fan-save-money-and-reduce-noise-massively.18124953/

90 mins typing... I should of slept. Then it might of been right

 

[GMT]

I think you're spot on there. We do need a shift in thinking. Any temperature control we need with LEDs is now going to occur from moisture removal anyway, so the priority should shift from "can I cool this down" to "can I prevent moisture build up".

 

[Ca++]

I think you're spot on there. We do need a shift in thinking. Any temperature control we need with LEDs is now going to occur from moisture removal anyway, so the priority should shift from "can I cool this down" to "can I prevent moisture build up".

Yes, and while a cool inlet temp used to be great, it can now be a problem. The British design temp for a lounge is 23c. However, a 23c inlet probably won't reach 28c by the exhaust, just 90 seconds later. This is connected to my heat recovery thread, where exhaust heat is transferred to the inlet. As we don't typically want the heat loss that water vapour extraction brings. Obviously we could use a heater, but that might cost more than powering the lights. Heat recovery is almost free.

It seems 150w would take that 23 to 28 at 80m3 an hour. These are not calcs I have done before, so I have no expectations, to evaluate my calculations accuracy. This is quite useless info though. 150w would never do that in a tent, as so much just leaves through the walls. Another issue is power consumption actually getting water into water vapour. Heat is being lost in so many ways. Running hot would likely only occur due to weather conditions, that a fan can't do much about anyway. Our lighting heat gain is so low, now we use LED.

For me, it's all about extraction to keep RH correct now, or VPD. With temperature change a byproduct of this.

 

[GMT]

Outdoor temp has increased now to 9 C. That's my intake temp as the room has the window open for room moisture removal. To get my air to your intake temp id not only have to turn the fan off, but wrap the tent in blankets. Is the reduction in RH on a temperature curve or is it the same reduction from 20 to 25 as 10 to 15?

 

[flylowgethigh]

My tent makes its own humidity because the leaves are transpiring. It can run 20 points of rh higher than the lung room. I distribute the incoming air at the base of the plants and suck it out at the top of the tent. The clip fans make sure the air inside the plants on all the leaves get good circulation.

All that water that the roots take up has to go somewhere

 

[Ca++]

Outdoor temp has increased now to 9 C. That's my intake temp as the room has the window open for room moisture removal. To get my air to your intake temp id not only have to turn the fan off, but wrap the tent in blankets. Is the reduction in RH on a temperature curve or is it the same reduction from 20 to 25 as 10 to 15?

I used a chart here https://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html
The difference isn't linear, and may not plot as a curve either. Though within the band we are interested in, not a lot changes. At 10c you are pushing the envelope though.

You saw the heat recovery thread? https://www.icmag.com/threads/cold-and-dry-tent.18129851/
As much as 90% of your exhaust heat, can be transfered to your inlet. If just 50%, it is still real heat, growing real bud.


I have wrapped many a tent in blankets. In the HID days, I always did it. I had to keep the summer heat inside, where it could be extracted from. I couldn't let it escape into the room, as then it would hang around doing more damage. Insulating the tent and duct, made a big difference. Today, we are talking about keeping that heat though, with the opposite ideology. We want the heat to escape the extract ducting, and blowing the cold at it, is a great way of doing that. Though if you don't actually extract from the room anyway, then this isn't immediately viable.

 

[GMT]

When it's pissing down with rain in the UK, what's the Rh coming into the room from outside? I'm assuming that's 100% or is 100% only at the bottom of a bucket of water? On that table, is the maximum content of water in the air, 100% relative humidity? I've never measured humidity so I don't know base numbers. But it's 9 degrees and raining. There's absolutely no way to get into the right vpd section unless I can somehow get my leaf temperatures above the air temperature. The only way to do that would be to install a microwave with the door removed (which obviously no one is dumb enough to do). I would need to be running dehumidifiers constantly and keep the heating on to hit vpd. Which I simply can't afford. Ironically, according to the table and chart showing the max water content of air, the greater the temperature, and the temperature differential between air coming in and tent air, the less the need to exhaust for moisture's sake. So there is a cross over point at which you need to be venting for temp, venting for moisture, and needing to vent less to preserve one, the other or both.

One chart to bind them all is needed. We need the precious chart.

 

[Ca++]

9C isn't on the list, but I'm going to call it 9 grams. That is it's maximum water holding potential. 100% RH.
If you warmed that air to 30C, it could hold 30 grams. As it only holds 9, it is at about 30% capacity. Or 30% RH, we say.

Either way is was 9g of water in a cubic meter of air, but at 9C it was just about to condense into mist and fall out. While at 30C the air was a bit dry. Same water content though. It was just relatively different, due to temperature. As the air warmed, it seemed relatively drier. Still it was 9g though. Just the holding capacity of the air increased.

It does make sense, but it's also a bit awkward.

So even if a 9C fog was coming in, it would be 33% RH after warming to 30%. A 66% deficit, compared to what it could hold. As that is where RH and VPD meet. RH is how much of maximum is held, and VPD is how much of maximum, is missing. It's what's missing, that matters to a plant. As that is space the plant can loose water into. RH doesn't actually matter, as the plant has no need for water in the air. It needs water missing from the air. It is really two sides of the same coin though. The graph slope you spoke of, is that coin. With RH a measure of what's under the slope, and VPD a measure of what is over it.
I have stuck with RH as my measure, as it's what £3 meters display. VPD is technically correct, but inseparable from RH anyway.

 

[GMT]

Vpd has to be more than the other side to rh as rh doesn't give a shit about the temperature of objects within the air, only the air itself. Vpd seems to depend largely on the differential between the air temperature and the leaf temperature.
But going on what you say, the rh in the UK is normally 100%. As it's normally raining.

 

[Ca++]

Vpd has to be more than the other side to rh as rh doesn't give a shit about the temperature of objects within the air, only the air itself. Vpd seems to depend largely on the differential between the air temperature and the leaf temperature.
But going on what you say, the rh in the UK is normally 100%. As it's normally raining.

You make a good point about vpd. Leaf temp differential, leads to differing potential that can aid moisture movement. As can other conditions. We speak of this now, when we didn't in the RH days. However, it existed then, and we still don't measure for it now. We tell our climate controller the vpd we want, and it just sniffs the air for temp and RH.

D is for deficit, not differential

Vapour Pressure Deficit, or VPD, is the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated.

Humidity is the concentration of water vapor present in the air.

Same curve, but the other side of the line.

 

[GMT]

Ah no I wasnt using the D from VPD, I was using the vpd calculator thing https://vpdchart.com/#C,39,15,14,0
As you move the leaf temp, it adjusts where you fall in the chart. So the air temp compared to the leaf temp is the differential D I was refering to.

 

[flylowgethigh]

Before you can try to control the temp and humidity, you have to measure it. I have used this system with 3 remote sensors for a few years. It has enough range that it can be located on the other end of the house and that allows me to be able to monitor how environmental tools are working, especially during the night. I put one sensor at the filter box to measure the lung room conditions, one in the tent down by the soil level, and one up higher at the canopy height.

I like being able to see how things like a Huey or dehuey, wall banger ac unit, space heater, etc… affect the steady state conditions without having to get up and go look during the night.

The AMZN link won’t post here. Oria 3 sensor remote.

 

[growsjoe1]

@flylowgethigh I know the sensors are not waterproof. How well do they handle the high humidity of vegging plants?

You've had them for several years. Have you ever had a failure?

Oria 3 sensor remote.