negative pressure - WHAT DO VENTILATION EXPERTS THINK?

Greetings Fellow Growers

todays topic is VENTILATION METHODOLOGY.



Picture a Room built within a Room.

The size of either doesn't necessarily matter, this is just an example, whats more important is the Exhaust system. The Inner Room contains a Carbon Filter which exhausts the air inside the Inner Room. This air in extracted through ducting FROM the small room OUT into the larger room where a powerful fan is located. The Fan then extracts the Inner Room air OUTSIDE through the wall. Needless to Say the fan matches / surpasses the size of the Carbon Filter meaning that extraction is working perfectly.

Ok.

Now imagine this powerful fan running on a low speed all day and night, which gradually creates negative pressure inside the Inner Room. This negative pressure will pull air from the Outer Room into the Inner Room through a Light Baffle and over time will ALSO create negative pressure in the Outer room. All Air will be eventually exhausted OUTSIDE and the Outer Room will be FULLY SEALED - so if the fan inside the Inner Room runs 24/7 both rooms will eventually have negative pressure and my plants will eventually be starved of Air. Right?


So what I would like to know is WILL opening the main door to the Outer Room everyday bring in enough air to allow the fan inside the Inner Room to run 24/7? Ideally I would have negative pressure in both rooms for odour control WITHOUT harming my plants. Im hoping that opening the main door each day fills the Outer Room with fresh Air. Im also hoping the Light Baffle will allow this air to be pulled into the Inner Room and through the Carbon Filter and eventually OUTSIDE. Basically im worried about starving my plants of much needed air, because the open door will be the only source of fresh Air. Will that be enough? For the sake of your calculations the Inner Room is (max) 5w x 7l x 6h and the Outer Room is 10 x 8 x 8 approx.

Guys, feel free to suggest your own Ventilation methods for these rooms. I feel that I have to seal the main door to the Outer Room and plug the gap at the bottom in particular, keyholes etc because I need to contain the smell of a very DANK strain, but I do have a window inside the Outer Room, and I do have access to OUTSIDE AIR but its dirty/ unfiltered Fireplace Air so I cant really circulate it into the Rooms air (can I?) I'll probably use a Ozone Unit with a meter in the Outer Room, but I want to get this final part right in my head. Dont want to starve my plants of AIR !!!







Any suggestions will be appreciated fellas!

Really hope im not asking the question wrong....

Just wanna make sure my plants dont suffer from a lack of Air if I keep my fan exhausting all the air in the room 24/7. I'd like to know if opening the door at least once everyday will give the plants enough air to breathe?

any thoughts

I am a little confused at what your asking. I am not familiar with negative air and everything??

Here's how I see it. If you have a bedroom with a tent in it, the air is carbon filtered and than blown back into the bedroom. Therefor the odor in the bedroom should be nuetralized.

As for the airflow as long as you have an intake hole for air to come in than the fans will be bringing in new air and expelling the air in equal amounts. For every action there is an equal and opposite reaction so I don't see what the problem is. As long as there is a fan blowing air out of it the box there has to be fresh air coming into it that's why intake fans aren't necessary because by having exhaust new air is automatically pulled in and I would imagine that if there was no fresh air coming in the fan would not be able to run-it couldn't.

Think of a balloon if you pump air into it it expands and will eventually pop but if you did it to something that didn't expand the pressure would build until it exploded. If you suck all the air out of it it will implode. There will be plenty of air in the bedroom even if you exhaust the air outside the room air must be coming into the box in order to ventilate the air. The room would have to be air tight which isn't all that likely but say that it was air tight and you were trying to exhaust air outside it would not be able to exhaust air because there is no air to come in and exhast the fan would not be able to run because of the pressure built up.

Thanks Pipe Dream. I think you know what negative pressure is because you pretty much described it. Here is a crystal clear example:

negative pressure is like what happens with an extractor fan in a Bathroom. To be blunt, when you take a shit, the fan keeps all smell inside the bathroom by constantly pulling the air inside the bathroom outside. What this means is Air within the bathroom CANNOT LEAVE the room. Why? Because every crack or gap by the bathroom door is being flooded with Air pulled into the room by the extraction fan. And that pull is stronger than the air carrying the smell, so the direction of air flow is inwards not outwards. This keeps the air in the bathroom and not floating around your house! and thats how extraction fans in bathrooms work. negative pressure controls the direction air flows. but based on your post you already knew that but maybe you werent familiar with the term negative pressure. I wasnt until recent but thats what it is.

Pipe Dream,

I didnt know that a fan would stop working if there is no fresh air coming in. That explains part of what I was thinking about thanks. Basically i want to use as MANY methods of odour / environment control as I can to run an efficient and safe grow. I'll use a large fan matched with a larger filter as well as Ozone with a meter. I also want to create and maintain negative pressure in the room by keeping the fan on 24/7 even when the lights are not on (carbon filter > cool tube > fan > outside) refer to my bathroom anaolgy above lol.

Balacing all these methods is where im having a slight problem, because I ALSO plan on sealing and insulating the grow room too. This means sealing the windows, the sockets and under and around the door etc. It wont be 100% air tight but it will def restrict the fresh air coming in. Its a relief to know that the fan wont work if the air restriction is too much. I guess I can remove insulation to improve air flow if I need to. For instance if the air flow is too tight the fan gets louder because its working harder. I'll def need to tweek the system but just Knowing the fan stops working with no fresh air helps. thanks.

My main question was much much simpler tho!
- In a well insulated room (10 x 8 x can my plants THRIVE if I only open the door to them ONCE a day? Is that enough air for them to have all the oxygen they need in a room with no large intakes whatsoever? Im not sure how much air they need thats the problem. If you or anybody else could answer that question that would be great. Im sure it varies from strain to strain but an estimation would be perfect. Once I know that I can continue planing the ventilation and insulation of my grow. Im hoping that opening the door wide for an hour once a day will give them all the air they need until the next day or even longer. Thanks for the response Pipe Dream - Appreciate it!!

this is a maths question , pure and simple. volume of air moving thru area, amount of cubic meterage the fan extracts in a given hour and where its repalced from and vented too( vented too dont mean shit as long as u gotta good filter, opening the door once a day will probably be enuff but as long as u got neg ative pressure in ur inner room and a supply of fresh air cuming in should be ok, chances are ur not totally sealing ur outer room anyway.

oh, just remebered, if u dont have enuff fresh air going in u will find out really quickly what dampness can do to a room, mold on ur walls, carpets any soft furnishings, stale taste in ur mouth and mold on ur plant leaves. im a painter and decorator to trade and i can assure you that if uv got heat in room, coupled with moisture from your grow this will happen. and ur fan will run becoz the air is still present(but its oxygen/carbon dioxide content will be reduced but ur also lookin at heat problems). why not get sum for inch ducting and stick it all the way up ur chimney ( can u get up to the chimney top coz the best thing would be to drop 2 lengths down and draw in with one and vent out with the other

Either run CO2 or crack your window and put a stick in it.
You need to vent outside the tent to increase any pressure or your just pushing the air around inside the tent. You could put an intake on the bottom on one side. ANd the filter blowing the air towards the window or just inside that room. The intake can be just a vent. The outtake will draw new air in threw that vent giving it the name passive intake. Now in order to do that properly, You must have you outtake leaving the scene. Either you vent outside or to another room or the attic??
You might need an intake fan to control heat better.
If everything is sealed up. You must add co2 and a dehum and probly an AC. You need to get that air out of the room or you will get mildew and the plants wont transpire as much. Hense slower growth. Just run your outtake to a rual area. You have to get that air out of the main room or its going to be reused and flow back into the intake.

Thanks Dura.

Good to know about Mold/lack of fresh air. I've budgeted for a humidifier AND a dehumidifer both with thermostats so I'll have some control over the environment. I also budgeted for an AC Unit which will kick in to keep things cool and vent outside. Also venting outside will be the lights/carbon filter via a powerful insulated box type fan.

You see what I mean about inner and outer rooms right? The inner room will be wood frame and reflective sheeting/zippers etc. Thats where I'll grow. The Carbon Filter will be inside there and the Fan will be sitting in the main "outer room". Both the AC and the Carbon Filter will be vented outside from there.

Now im working out Air intake. If I trust the science, then NOT sealing the outer room door and NOT sealing the window edges means that the Fan will create negative air pressure and FORCE fresh air to be pulled into the Outer room and therefore into the Inner room. Also, and the odour in the Inner room wont be able to leave the Outer room because the direction of the air will be coming INTO the room to replace whats being exhausted by the fan. If I trust science i guess I'll do that BUT common sense is telling me to SEAL/INSULATE the door and window to prevent odour leaving - thats why im worried about Air Intake. Sealing the door will make me feel safer but it will also prevent fresh air being pulled in. I gues I'll have to experiement. Out of interest, I'll also be using Ozone in the Outer Room with a osc fan so any smell that escaping the Carbon Filter in the Inner Room will be zapped by the Ozone in the Outer room. That way, even if I dont completely seal everything, and even if Air inide the Outer room DID somehow manage to overcome negative pressure and flow through my house, it will be odourless. Trusting the Science is the way to go I think. Over sealign everything will cause problems for me. I just have to trust the gap under the door wont give me away. Asking questions on here helps me think these issue through. Thanks fellas.

I also considered A FRESH AIR INTAKE as well, like you suggested, but alas its impossible (I think ) for two reasons.

- ONE, the air in the chimney is filthy so I cant and shouldnt vent air into the room through it because it wont be fresh and it will prob kill the plants and me lol.
- TWO, even if I could filter the air coming through and clean it, the only place i can intake from will be directly next to where im exhausting so it wont be as effective.

Ideally I'd have ducting running down the chimney to the room like you said but I cant get onto my roof number one and number two im not sure of any method to push ducting UP a chimney. If I did I might consider it but even THEN im probably best advised to just exhaust out the Chimney and allow air intake to occur under the door and around the window using fresh air already inside my house. Plus opening the door wont hurt either.


Any other thoughts fellas? im on a roll lol. its making more sense, I think

MrSmilelyFace that is an ugly as fuck avatar LOL

Thanks for making it sound simple! Im definetely going to exhaust, because my lights WILL generate heat. The fan is powerful and will def keep the my cool tube/s cool as the air in the tent is pulled out. The fan will also be powerful enough to still pull plenty of air through whatever Carbon Filter I buy.

So in short I KNOW I can generate negative pressure in the room. My options for installing a standard grill type vent are limited. Ideally I'd use passive intake as you suggested. That way the air pulled by the fan will be replaced by air being pulled into the house. But installing a random Grill outside this room will be suspect. HOWEVER, there is a option to put a grill in the ceiling and have a passive intake comign from a room upstairs. Or I can put a vent on the Wall and have passive intake coming from the room next door. Guess that would be better than nothing right?

When you said "You might need an intake fan to control heat better" Do you mean in place of an exhaust or with my exhaust? I think you mean with, but im assuming that my powerful fan (10K CUFT) will be enough to cool my lights. What do you think on that?

Finally. I def agree with getting the air out. I'll have a very active exhaust and a passive intake. You mentioned CO2. Will I have to dial in the CO2 will a temporary fan switch off? im assuing my precious CO2 will simply be sucked up the chimney otherwise? Also I agree with the automatic humid/dehumid etc. Also planning to get an A/C too, venting out along with the Main Fan.

Let me know what you think. Thanks Everybody for helping me understand things clearer!

Jorge has a dvd called Ultimate grow (I think, have not watched it in awhile) where he does exactly what you are talking about (builds a room in a room I advise you to get it and watch it

here is the preview
http://www.youtube.com/watch?v=vcnRqT6phvw&feature=related

Dura

to answer your question in meters. the room is approx 24m3 and the Fan pulls 360m3 an hour. I can def pull enough air through a big filter and still create negative pressure. what im think now is - I'll still seal the door and window, but I'll put in a Grill either on an internal wall or in the Ceiling. Ideally I'd punch a hole in the outside wall and install a grill but that would raise suspicions

Soooo, I'l seal the door and windows BUT install an adequate sized passive intake, so fresh air is being brought into the room. Meanwhile, the Carbon Scrubber will scrub the air inside the tent and the Ozone will kill anything outside the tent. And failing that, the door to the whole room will be insulated and sealed. And even failing THAT the negative pressure between the intake and the exhaust means that Air wont be able to escape out into the house through any gaps it finds, as Air will be rushing to get INTO the room thanks to the Fan.

Does that make sense people? Have I finally made sense of this? Thanks for the input!!!

i assure you that although you will create a negative pressure effect as desired, in no way shape or form will you manage to sustain a high enough 'vacuum' to 'starve' your plants of oxygen...
plants get the vast majority of their oxygen from h2o... not the air.
there is not a fan in existance that is powerful enough to draw enough air out of an enclosed space to create a vacuum. you will create negative pressure, but only because of restriction of airflow on the supply side of the system, just like how an internal combustion engine creates vacuum pressure on the intake nd if there was such a fan, im quite confident you would implode the structure from negative pressure long before you created a vacuum strong enough to harm your plants.

ps- i use negative pressure on all my indoor grows. it is highly efficient at removing heated air, far more so than using an a/c to cool a room. the only hassle ive ever encountered is damage to hinges and door hardware from the draw when opening and closing the doors, over time.

safety comes first said:

MrSmilelyFace that is an ugly as fuck avatar LOL

Thanks for making it sound simple! Im definetely going to exhaust, because my lights WILL generate heat. The fan is powerful and will def keep the my cool tube/s cool as the air in the tent is pulled out. The fan will also be powerful enough to still pull plenty of air through whatever Carbon Filter I buy.

So in short I KNOW I can generate negative pressure in the room. My options for installing a standard grill type vent are limited. Ideally I'd use passive intake as you suggested. That way the air pulled by the fan will be replaced by air being pulled into the house. But installing a random Grill outside this room will be suspect. HOWEVER, there is a option to put a grill in the ceiling and have a passive intake comign from a room upstairs. Or I can put a vent on the Wall and have passive intake coming from the room next door. Guess that would be better than nothing right?

When you said "You might need an intake fan to control heat better" Do you mean in place of an exhaust or with my exhaust? I think you mean with, but im assuming that my powerful fan (10K CUFT) will be enough to cool my lights. What do you think on that?

Finally. I def agree with getting the air out. I'll have a very active exhaust and a passive intake. You mentioned CO2. Will I have to dial in the CO2 will a temporary fan switch off? im assuing my precious CO2 will simply be sucked up the chimney otherwise? Also I agree with the automatic humid/dehumid etc. Also planning to get an A/C too, venting out along with the Main Fan.

Let me know what you think. Thanks Everybody for helping me understand things clearer!

Click to expand...

Do you have any vents already going into the room? From your furnace?
10k cubic feet per minute? wow
As long as your venting outside of the outter room you will be fine. The big fan will draw air into the room via. cracks. Now if your talking about the inside "tent" then if you have a zipper just keep it half or a qaurter open. That could be your passive intake. Or just put a vent on the grow tent. ASlong as your venting out of the main room you will be fine. BEcause air replaces air. And if your outtake is on new air will be drawn in. No matter what. An intake vent/fan will make your fans work less tho. Pulling the air in as the outtake takes it away. Insted of the outtake doing both jobs. Never run your intake when your outtake shuts off. It will cause stinky air to leak out of your room. Induced by pressure.
Yes that is my retarded stepbrother after taking a bong hit haha

safety comes first said:

Dura

to answer your question in meters. the room is approx 24m3 and the Fan pulls 360m3 an hour. I can def pull enough air through a big filter and still create negative pressure. what im think now is - I'll still seal the door and window, but I'll put in a Grill either on an internal wall or in the Ceiling. Ideally I'd punch a hole in the outside wall and install a grill but that would raise suspicions

Soooo, I'l seal the door and windows BUT install an adequate sized passive intake, so fresh air is being brought into the room. Meanwhile, the Carbon Scrubber will scrub the air inside the tent and the Ozone will kill anything outside the tent. And failing that, the door to the whole room will be insulated and sealed. And even failing THAT the negative pressure between the intake and the exhaust means that Air wont be able to escape out into the house through any gaps it finds, as Air will be rushing to get INTO the room thanks to the Fan.

Does that make sense people? Have I finally made sense of this? Thanks for the input!!!

Click to expand...

sounds good and logical buddy, i'd go for it...what i will s ay though is that sumtimes when u think u can smell ur grow in the house its frequently just the smell coming from ur clothes after youve been in the room watering/feeding, seems to stick on to every available surface like fuckin glue, i spray myself and clothes with a fabric spray after i come out the room, u may not notice it sumtimes becoz uv become used to it but others will.