What's new
  • Please note members who been with us for more than 10 years have been upgraded to "Veteran" status and will receive exclusive benefits. If you wish to find out more about this or support IcMag and get same benefits, check this thread here.
  • Important notice: ICMag's T.O.U. has been updated. Please review it here. For your convenience, it is also available in the main forum menu, under 'Quick Links"!

How does indoor ventilation actually work?

G

grumpyavocado

A 30CFM fan should be capable of emptying a house in a matter of hours. I can't believe that this is the way it works so I must be missing something. I also read about people that left plants alone for a week and found them fine which isn't possible without air. So how much air does actually get pulled out and why?
 

kritios

Active member
It could empty a house in a matter of hours, you're right. Say 1800 sf house has 8 ft ceilings and therefore 14,400 cf. Your fan could move that volume in 8 hrs.



Practically I think what happens it doesn't change every air molecule in the house. Some air short-circuits: it leaks in near the fan and is immediately exhausted. on the other hand there will also be dead air spaces exist in the corners of the house farthest from the fan that are not exchanged.



Some of the same things happen exhausting a grow space: air circulation is oversized to make up for uneven flows across the space. This can be overcome with good design. Greenhouse grows use HAF circulation fans to move air within the space
 

packerfan79

Active member
Veteran
A 30CFM fan should be capable of emptying a house in a matter of hours. I can't believe that this is the way it works so I must be missing something. I also read about people that left plants alone for a week and found them fine which isn't possible without air. So how much air does actually get pulled out and why?

You want to exchange all the air in your grow space at least 1 time per minute. Ventilation, helps keep the heat from the lights down. A charcoal filter for smell will slow down the air exchange.

There are some really good threads with lots of knowledge about Ventilation here . It's in the grow room design, and equipment sub forum. It's the ventilation 101 thread.
 

Douglas.Curtis

Autistic Diplomat in Training
I've used about 3x the volume of the room exchanged every minute. So in a 100cubic foot space I would use a 300cfm fan. This air movement does not cool the room unless the incoming air is cool enough. Without access to cool air, you'll need air conditioning of some kind.

packerfan79 is quite correct, the equipment sub-forum has quite a bit of information on this.

Welcome to ICMag!! :D
 
G

grumpyavocado

Thank you for the answers and the welcome!
The 101 thread was very informative but didn't answer my question.
The fact that a bit of air leaks can contribute to explain how this works but I believe that the leak would be negligible with negative pressure inside the tent. Someone suggested that air would actually enter the house if it's not insulated, so I guess that there would be a light breeze across the house?

And other two questions came to my mind. Can exhausts be too powerful? For small setups(I'm thinking about a 2x2) could this be a problem if the air would move too fast?

Also is 2x the section of the exhaust fan an exact "magical" number? Would the smell leak if the intake was 3x to 4x?
 

Douglas.Curtis

Autistic Diplomat in Training
Thank you for the answers and the welcome!
The 101 thread was very informative but didn't answer my question.
The fact that a bit of air leaks can contribute to explain how this works but I believe that the leak would be negligible with negative pressure inside the tent. Someone suggested that air would actually enter the house if it's not insulated, so I guess that there would be a light breeze across the house?
Unless your fan is pushing the air from your house to the oustide, there will be no draw of air into your house. The fan will only push air around inside the room it's in. You have to use ducting to direct the air somewhere.

And other two questions came to my mind. Can exhausts be too powerful? For small setups(I'm thinking about a 2x2) could this be a problem if the air would move too fast?

Also is 2x the section of the exhaust fan an exact "magical" number? Would the smell leak if the intake was 3x to 4x?
Above 3 complete exchanges of air per minute, the air flow does get a bit rough on the plants. I would use a 4" fan for a 2x2, and at max flow it's almost too much air. Works great. :)

As long as your fan is pushing ALL of the tent air through a carbon scrubber, you should be just fine on smell. As you increase the size of your intake the negative pressure drops. Negative pressure is what keeps the smell inside the tent and going through the carbon scrubber.

Hope that helps :)
 
G

grumpyavocado

This helps for sure, thank you! But shouldn't the exhaust go outside the house? I read that it's better to not breathe the carbon particles that come out of the filter. And would there be enough air in a small room for a few days?
 

Douglas.Curtis

Autistic Diplomat in Training
This helps for sure, thank you! But shouldn't the exhaust go outside the house? I read that it's better to not breathe the carbon particles that come out of the filter. And would there be enough air in a small room for a few days?
Glad I could help.

It all depends on what type of space you have and how you're using it. Personally, I do not like dust, pollen, insects or other stuff on my plants so I filter the air I grow with. When I care about smell (I don't these days but that's a different story) the air into my grow is filtered, then routed from the base of my plants then up past the light(s) to the exit. The exit is a fan and carbon scrubber.

The carbon dust is a good point I haven't thought of in some time. Yes, carbon scrubbers give off a dusting of particles and no I don't think it's a good idea to breathe them. The dust filters some scrubbers have will catch a lot of it but not all. Another reason to use filtered air for your flowers. ;) Not sure on how I'd handle that in a room people breathe in, since my grows have almost always been in non-people areas. It's not a lot of dust, but it's there.

Venting outside is something you would have to decide. Again, it really depends on your situation. With a low wattage grow, the heating/cooling system in your house is usually enough to keep things cool. The larger the grow, the more heat you'll have to deal with.

How many watts of light are you planning on using?
 
G

grumpyavocado

I'm thinking about a 135W HLG board. The thing is that the setup should be able to withstand 4 days without care. This is why I have so many doubts about ventilation. I know that ideally you would check your plants everyday but outdoor you can easily get away with checking twice a week.
 

Fixer

Active member
Thank you for the answers and the welcome!
The 101 thread was very informative but didn't answer my question.
The fact that a bit of air leaks can contribute to explain how this works but I believe that the leak would be negligible with negative pressure inside the tent. Someone suggested that air would actually enter the house if it's not insulated, so I guess that there would be a light breeze across the house?

And other two questions came to my mind. Can exhausts be too powerful? For small setups(I'm thinking about a 2x2) could this be a problem if the air would move too fast?

Also is 2x the section of the exhaust fan an exact "magical" number? Would the smell leak if the intake was 3x to 4x?


Hello grumpyavocado, When you say house do you mean your dwelling or a tent? A lot of air gets in and out of houses due to leaks in the building envelope. Read about air sealing and ventilation at jlconline.com There's a balance you need to strike in your tent. You want to be able to move a lot of air sometimes to cool it or remove humidity. Negative pressure lets you draw most of the odor through a carbon filter. If you're legal and love the smell of weed you can run positive pressure and filter the air. :tiphat:
PS Grow in soil if you can't check in frequently. and you'll need a variable speed controller for your fan.
 
Last edited:
G

grumpyavocado

Hello grumpyavocado, When you say house do you mean your dwelling or a tent? A lot of air gets in and out of houses due to leaks in the building envelope. Read about air sealing and ventilation at jlconline.com There's a balance you need to strike in your tent. You want to be able to move a lot of air sometimes to cool it or remove humidity. Negative pressure lets you draw most of the odor through a carbon filter. If you're legal and love the smell of weed you can run positive pressure and filter the air. :tiphat:
PS Grow in soil if you can't check in frequently. and you'll need a variable speed controller for your fan.
Thank you for your answer! When I say house I mean the actual apartment where the tent is located. Do I really need a controller for the fan? How much are those?
 

Douglas.Curtis

Autistic Diplomat in Training
At 135w, and in an apartment, I would simply make sure you have enough airflow. You probably have more wattage in room lighting around the house than you'll be growing with. :) Ideally you would have people in the apartment creating a nice supply of CO2, and the thermostat set where it keeps your room temps where you want.

:)
 
G

grumpyavocado

At 135w, and in an apartment, I would simply make sure you have enough airflow. You probably have more wattage in room lighting around the house than you'll be growing with. :) Ideally you would have people in the apartment creating a nice supply of CO2, and the thermostat set where it keeps your room temps where you want.

:)
Unfortunately this is not the case. Temps shouldn't be a problem but the plant(or plants) wouln't have buddies around. Just me every few days. I'm asking all these questions because I understand that this is an atypical situation and I would like to know if it's feasible. Maybe I should have explained that better. My bad.:shucks:
 
Top