Venting into return duct?

Is it safe/smart to vent hot air into an air return for a furnace/ac? (or pull air from a return?) What one would be better? Thanks.

Im just throwing this out there I could be absolutely wrong. I'd say you would be fine if you sent it back into the return because its sucking air not sending air, it doesn't matter whether hot or cold. Also I would assume that it would make your whole house smell if it doesn't already do. Unless you use a carbon filter or some sort of odor mask.

Thanks for you quick reply. I think if i pull air from another room and through sealed hoods and into the return it should be okay. With not much air getting out the the sealed room and a carbon filter on all the time.

No problem just trying to give a little back to the community. Sounds solid. I have heard that it could be bad to have forced air hitting your hps bulb could be wrong.

Anyone else have any ideas?

I would take into account carbon monoxide. I dont know hvac systems well enough to say es or know but if you somehow pushed carbon monoxide rich air out of your furnace it could kill you.

Mrblaze212 said:

Is it safe/smart to vent hot air into an air return for a furnace/ac? (or pull air from a return?) What one would be better? Thanks.

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I would not advise doing that.....you are compromising the HVAC system. Your furnace has certain needs as far as return air is concerned....those needs are taken into consideration when the system is sized and designed. As far as the Carbon Monoxide is concerned...that is exhausted thru the flu pipe that vents out your roof and into the atsmophere....so thats prob not an issue...but if you were using that flu to exhaust into the atsmophere then I would have some concerns with carbon monoxide because the flu could be undersized to handle the furnace needs and whatever your blowing into it. Then there is odor....even with a carbon filter....some odor does get past the carbon filter so if your piping your air into the system....its gonna be blowing out every heat register in the house. I'm curious.....what size fan are you using....what is the CFM out put of the fan?

My second story closet is cooled by trapped indoor air inside duct system primarily housed in basement/crawlspace at 50*-60*f. By blocking return intake grill and cutting into return from closet a box fan pulls through return to unit then to supply ductingand eventualy air from home via floor registers. A cieling mounted fan in closet removes heat creating a vaccume assisting fans. So far 80-85* home has a 65* closet with 2 400's,a flourescent Veg area and an upstairs location. If hvac unit is used return must be clear. Your carbon monoxide threat lies in heatexchanger regardless of your actions and as described would prob save lives before harming. Hvac guru in the past been awhile but designing ventilation for multiple restaurants in malls taught me a bit. Best bet use for fresh intake dump outside.

herbbilly said:

My second story closet is cooled by trapped indoor air inside duct system primarily housed in basement/crawlspace at 50*-60*f. By blocking return intake grill and cutting into return from closet a box fan pulls through return to unit then to supply ductingand eventualy air from home via floor registers. A cieling mounted fan in closet removes heat creating a vaccume assisting fans. So far 80-85* home has a 65* closet with 2 400's,a flourescent Veg area and an upstairs location. If hvac unit is used return must be clear. Your carbon monoxide threat lies in heatexchanger regardless of your actions and as described would prob save lives before harming. Hvac guru in the past been awhile but designing ventilation for multiple restaurants in malls taught me a bit. Best bet use for fresh intake dump outside.

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By doing that...whats the implication to the HVAC system while it is in operation??

this will throw the whole hvac system into a tailspin. your system is designed to pull a certain amount of air through the return, and you will be multiplying that by 10x, maybe more. The ac system will be unable to cool the hot air, and will be working non-stop to try to do so, and will burn out. the sensors will likely shut down the system if your lucky. the air will be so hot your house will be 100 degrees, and stink. like crazy.

these are the kind of things that new growers do that get them caught, period.

why aren't you just pulling from the outside, through the lights, than back outside? I do this even in the dead of winter in the north, my 1000's don't get any condensation, but my 400's do, so they just recycle the air in the main bloom room and serve as heat in the winter, than they get hooked back up to the main venting system.

I would NEVER, pull air out of Grow room, and vent it back into HVAC/FAU system... Like others said: Smell, #1 concern...
However, I have and do, pull air out of the return air duct.
My return air duct, is a plenum that only sucks air from house when heater/FAU kicks on. Pulling ambient air from the return air to give your grows "Fresh" air... shouldn't be a problem...
Good Luck...

Sand4x105 said:

My return air duct, is a plenum that only sucks air from house when heater/FAU kicks on. Pulling ambient air from the return air to give your grows "Fresh" air... shouldn't be a problem...
Good Luck...

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this was along the lines of my thought process. If your heater/fan isn't moving air (they cycle on/off) then you shouldn't be trying to push air into it, the air will be pushed back out the vents of the house before being forced through the heater.
Do you have to dump it into the vent? can you just dump the hot air in a different room(using the heat)?

I'm a contractor and have experiencewith Hvac.


First off, Hvac is not as easy as just air in and air out, and not all systems are designed the same. I don't know what you have as far as system static pressure, ductsizing, CFM, or heating and cooling loads so there is no way for anyone on line to tell you if this would work for your situation. I will give you some very basic info about Hvac and you can make your own decision from there.


Hvac systems run with static pressure throughout the system. The majority of the static pressure is taken up by the supply side of your system in order to have proper CFM and air speeds for your registers. The return side is (usually) designed with the least amount of pressure within the confinements of the duct sizing and space. Adding air to a return is fine in some situations. Some questions would be:


are you intending on using your system's air handler to remove the hot air or will you have an inline fan or booster fan to force air into your return? If you are going to be using your Hvac system's air handler in on mode, you may not be able to get enough pressure from the return because of the lower pressure in your return duct (compared to the higher pressure from your lighting duct system). The path of least resistance will be the air from the return grill, not the air from your lights. You could force more air from the lights using dampers, but this will effect your Hvac's efficiency and I wouldn't recommend doing it that way. An inline fan would work just fine, although the extra draw from your lights might make "worm" or "cold" areas of your home. The extra air from the lights means that less air is coming in from other areas of your home. Do you know how much air you need to move in order to deal with the BTU's from your lights? Are you in an area that requires cooling, or do you only have heat? If you have an AC unit, can it handle the extra load directly?


I don't see why you can't design something that would work. I wouldn't use my system's air handler to move the air; I would go with an inline duct fan, but still you will have some issues. For me, I would vent to outside (or the attic or to an air to air heat exchanger if in cold climates) rather than the return.


Sorry I can't give you an exact answer without looking over your system, but I hope that helps some. If you have more specific questions I might be able to help more.

seal off your hoods, put them on a seperate air channel with NO carbon filter, should only use one intake fan (because less heat is being put on the intake fan, making it last longer) on the side of your hoods then have it push air directly out of your house (shouldnt smell because hoods are sealed) now if the refresh cfms in your grow room are good enough the air will go through your original a/c house system and cool off the rest of your growroom heat. as long as you have negative pressure in your grow room and have it sealed off with a carbon filter on the exhaust the smell will be eliminated. the main thing you do with the hoods here is worry about condensation due to temp differences. be sure to use insulated material

Thanks for all your replies! I two 6 inch max fans, trying to cool 2 lights. I want to run a sealed room. However, my options are as follows. I have a old ass heat unit in my crawlspace. I could pull directly from the cold air return in the laundry room and through the laundry room down into the crawl space. (or the reverse of this, creating a sealed room) I have been avoiding putting a hole in the wall because the wall has this wooden planking that would be a bitch to repair or replace. (If i were to put a 6in hole in the adjasent wall) I also have two 4in dryer ducts that were used for a gas and electric dryer at one point, in the crawl space. I could rig it up to those possibly. I am just worried about the heat of the summer creating a minimal cooling affect on the lights. If you guys think that it is not possible to pull from a return without messing with the heat/ac unit. I can just put a hole in the wall and hope I can repair the planking at a later time. (also i have plaster walls, that is why i am trying to avoid putting a hole in the wall.) In the past I have just pulled from the laundry room down into the crawlspace. This was okay, however, I could not use co2. So I need to change things up!

So pulling out of a return is okay? Will a furnace use the return when the AC is running?

There is a chimney where that strait vent is going. Is that little 4in duct going into the wall useable?

Mrblaze212 said:

Thanks for all your replies! I two 6 inch max fans, trying to cool 2 lights. I want to run a sealed room. However, my options are as follows. I have a old ass heat unit in my crawlspace. I could pull directly from the cold air return in the laundry room and through the laundry room down into the crawl space. (or the reverse of this, creating a sealed room) I have been avoiding putting a hole in the wall because the wall has this wooden planking that would be a bitch to repair or replace. (If i were to put a 6in hole in the adjasent wall) I also have two 4in dryer ducts that were used for a gas and electric dryer at one point, in the crawl space. I could rig it up to those possibly. I am just worried about the heat of the summer creating a minimal cooling affect on the lights. If you guys think that it is not possible to pull from a return without messing with the heat/ac unit. I can just put a hole in the wall and hope I can repair the planking at a later time. (also i have plaster walls, that is why i am trying to avoid putting a hole in the wall.) In the past I have just pulled from the laundry room down into the crawlspace. This was okay, however, I could not use co2. So I need to change things up!

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If this is a laundry room, then using those two 4" vents you have for the dryers might work. They should lead to outside somewhere. I thought you were going to exhaust into the return. I wouldn't try to get supply air from the return without doing the math to make sure it would be effective while the system is running.

If you use the dryer vents then you don't have to mess with anything. You said you have 2 - 4" vents. I will assume they are flex ducting. Each 4" vent will be able to handle 50 CFM (cubic feet per min) of air with .25" WC (water column) per 100' at 573 FPM (feet per min). So run a 6" duct and fan into a 6"x4"x4" Y, and connect to both 4" vents. Buy a fan that will give you 100 CFM at .25" WC. This will make a negative room pressure that will help with the smell in the house. You may find you need a jumper from this room to the rest of the house for makeup air, but I doubt you will with only 100 CFM.

I don't know how many CFM you will need to cool your lights. If you need more, you would need to get a fan that can support a higher static pressure, and it will be louder. Fans will have different CFM ratings at different static pressures, and the more air you force through the duct the higher the static pressure will be.

If your 4" ducting is sheet metal then: 60 CFM per 4" duct = a total of 120 CFM at .25" WC per 100' and 688 fpm air flow.

I don't know if any of this will help but... there ya go.

Mrblaze212 said:

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There is a chimney where that strait vent is going. Is that little 4in duct going into the wall useable?

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The pics are too small for me to see. can you post bigger ones?

Ok So now you got a over 30 HVAC/R Tech, opinion here , ,Yes This can be done with No adverse effect on your furnance if done correctly, but need to know couple of things. this return duct you plan on tapping into is What size ,how mnay other retuns in your home. and what is the size of your furnance ( BTUs ) and also A/C size if you have A/C coil and condenser unit. you need to calculate how many cfm that duct is pulling before attempting ,because it might not be enough to due what you want.the other thing to remember is this will cycle on and off and will not be a constant exhaust ,this will be in the heating mode,you can however in A/C run the fan constant if you want to rewire and or change T/Stat. So there are several facts to consider here before doing if it is possable and that there is enough CFM ,there is no way to isolate this so it will be intergrated with your entire home. As far as the added heat load it will be minamal by time it mixes back with the other return air in your home.