Carbon filter- blow air into or pull air out?

Am I suppose to blow air in or pull air out? Everything I’ve read online says to pull air out. I even contacted Phresh filters and they said I should be blowing air into it. What are your thoughts? I’m going to be getting a tent with 1400 cubic feet and I want to exhaust the air and minimize the smell completely. Any info will help!

A2Michigan said:

Am I suppose to blow air in or pull air out? Everything I’ve read online says to pull air out. I even contacted Phresh filters and they said I should be blowing air into it. What are your thoughts? I’m going to be getting a tent with 1400 cubic feet and I want to exhaust the air and minimize the smell completely. Any info will help!

Click to expand...

Fig1 and Fig2.
The carbon filters outer sock catches all the particulate dust and crap. Be sure to use 1.

If you want/need the air inside the tent to be virtually odor free.
Gotta run a scrubber.

The air has to be going out for the filter to be effective. Once the air goes across the filter the odor is gone.

A2Michigan said:

Am I suppose to blow air in or pull air out? Everything I’ve read online says to pull air out. I even contacted Phresh filters and they said I should be blowing air into it. What are your thoughts? I’m going to be getting a tent with 1400 cubic feet and I want to exhaust the air and minimize the smell completely. Any info will help!

Click to expand...

Blow air in.

What you're dealing with are two types of pressure: positive and negative.

Blowing air into the filter creates positive pressure and pushes air through the filter.

Sucking air out of the filter creates negative pressure like a vacuum cleaner.

In filtration cases you always want positive pressure for two reasons:

1. It makes the filter more efficient.
2. It enables the bypass valve to function better should the filter become clogged for any reason.

With negative pressure, the filters bypass valve will never function properly due to there never being any positive pressure to open it.

What's more is that with negative pressure as the filter becomes clogged and the bypass valve fails to open your fan will begin working harder and shorten its life. In extreme cases it can overheat and burn up. (Like putting your hand over the intake hose of a vacuum cleaner.)

Once the air goes across the filter the odor is gone.

TintEastwood said:

Fig1 and Fig2.
The carbon filters outer sock catches all the particulate dust and crap. Be sure to use 1.
View attachment 4349053

Click to expand...

I use figure 1 but it then goes through a sealed hood to remove the heat from the light before being vented outside. I think that's how most people growing in tents do it. You filter the air and remove the heat from the light at the same time.

I don’t know what kind of fan you have but many have mounting brackets hole in them. If you’re not going to use the bracket (most people don’t to reduce noise) tape over the holes. Depending on your configuration, the holes can allow odor to bypass the filter.

I don't have any experience with these filters so I can not say what is best for a bypass valve. As far as pushing air in or sucking it out of a filter it should not matter one way or the other. All that matters is the pressure dropped across the filter. With blowing into the filter you have more turbulence from the fan which may maximize the area of the filter being equally used. But then looking at it another way, the fan may blow air in a particular pattern and have a dead spot out the center. The ducting makes a difference also. Hard to say in all cases. One thing to think of is the fan might be quieter if blown through the filter. As far as the filter getting clogged and the fan working harder, the fan gets unloaded if there is a restriction so it is easier on the motor. Because the motor is unloaded it spins faster, it also draws less current. But then if the fan is designed to use the airflow to cool the motor the lower airflow will reduce the cooling.

So do I have much helpful to add? Does not seem like it. Other than there is no pat answer, it depends on each piece of equipment and how they are being used with each other. I can see advantages to each of the four examples in the diagrams. The use of a sock to catch bigger crap can be used in all methods although you would have to have something to hold it open. The fan outside the room reduces any heat that the fan housing will produce inside the room. If you are dumping the air outside it would seem that you would need to do example #1 or #2 otherwise you would have a housing around the filter and then run the air outside.

Those can filters are designed to be at the far end of the tubing and have air pulled into them thru the pre-filter. I've never seen one with a built in flow valve and I'm sure mine doesn't have one even tho it's still sealed up in the box it came in. Will be needing it in a month or so if these kush girls are as stinky as they should be.

My buddy had his set up blowing air in so it wasn't filtered and he lives in a dusty area so it plugged up the insides. Gave that and other stuff to me in exchange for pot and when I tried to use the filter the proper way it had no flow at all. At least the filter on the outside was nice and clean.

Only way to fix that would be to drill the pop rivets out then clean and replace the charcoal. I think I'll dump it out and turn the filter into a kief tumbler for making dry sift. It's a no-name POS anyway and the filter I bought is way better. And brand new.

It does not matter mate. Sod positive pressure, your place will stink. Positive pushes your air out and not through the filter. Negative pressure puts everything through the filter. I.E. no smell.

welshsmoker said:

It does not matter mate. Sod positive pressure, your place will stink. Positive pushes your air out and not through the filter. Negative pressure puts everything through the filter. I.E. no smell.

Click to expand...

Why would positive pressure not go through the filter?

printer said:

Why would positive pressure not go through the filter?

Click to expand...

Positive pressure in the grow space will push air out cracks etc before it can go thru the filter and stink up the house. Some leave the fan on steady while the lights are on which is OK but makes it harder to control other aspects of the environment in there like humidity and temp.

My exhaust fan is on controllers so only runs when the temp or RH gets high enough to trigger it. Negative pressure when the fan runs, static pressure when it's off. No stink is getting pushed out so just smells up the grow room until the fan goes again.

Filtered air leaks have less probable cause.

printer said:

Why would positive pressure not go through the filter?

Click to expand...

Not a problem in a sealed room or lab, but everyone in the hood will know who's will soon be rollin in green.
If you run a slight negative in the room, example 625cfm filtered intake from outside the room directly into the bottom of the tents to (displace heat) and have a ceiling mounted 650cfm filter pushing hot clean smelling air out of the room, any air leaks will be coming into the room avoiding detection.

printer said:

Why would positive pressure not go through the filter?

Click to expand...

As oldusersays, do not do positive pressure, you might as well not bother using a filter mate.

welshsmoker said:

As oldusersays, do not do positive pressure, you might as well not bother using a filter mate.

Click to expand...

Well I say it works fine on every single "Cannabis" carbon filter I've seen. The oversized grille to my fan has a pleated furnace filter,and the duct work is fitted legitimately, not with scotch tape elmers glue and cardboard as seen on these forums.

If it didn't work fine for you, i can picture your grow now.. A black sun of electrical tape around a punctured hole inna wall with a tube ran through it...

“ Where is the fan usually mounted?


It is normally mounted in close proximity to the downstream side of the lint filter. The fan speed will also have a faster response time with a variable frequency drive equipped application.”


From Energetics Corporation. Carbon filter probably the same advice.

LinguaPeel said:

Well I say it works fine on every single "Cannabis" carbon filter I've seen. The oversized grille to my fan has a pleated furnace filter,and the duct work is fitted legitimately, not with scotch tape elmers glue and cardboard as seen on these forums.

If it didn't work fine for you, i can picture your grow now.. A black sun of electrical tape around a punctured hole inna wall with a tube ran through it...

Click to expand...

This is why I don't bother much with this site much. My room is a sealed room not some poxy tent with tape. If you think positive pressure is good you crack on son.

LinguaPeel said:

Well I say it works fine on every single "Cannabis" carbon filter I've seen. The oversized grille to my fan has a pleated furnace filter,and the duct work is fitted legitimately, not with scotch tape elmers glue and cardboard as seen on these forums.

If it didn't work fine for you, i can picture your grow now.. A black sun of electrical tape around a punctured hole inna wall with a tube ran through it...

Click to expand...

Are you talking about direction of air flow thru the filter of having the room pressurized?

Asking for a friend.