Ductwork advice plz

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cymbaline

Well-Known Member
I currently have an exhaust running right into my attic. It works well enough but could be better.
Basically I'm exhausting cool air out of my room and into the attic.
The room reads anywhere from 70's-to mid 90's depending on where the thermostat is.
I went into the attic today and the exhaust was blowing air cooler than the attic ambient temps.
Felt like I was blowing money out of my vents..

So what I'm thinking is diverting my exhaust into the cold air return of my furnace.

I believe the furnace is a downdraft furnace being my house is on a slab.
So my cold air vent is in the ceiling.. Big ole duct runs across the attic and drops into the furnace.

How hard is it to drill into ductwork? I was thinking of drilling a 6" hole and dropping my flex pipe
into the hole and sealing. Or buying a "stack boot" that I would install into the duct and than attach my flex duct to.
I'm thinking the stack boot is the more proper way?
Besides the issue of drilling into metal ducting that has flex.
What issues would I have by connecting the exhaust into this air return?
I imagine it would just blow air down into the furnace and potentially even through the rest of the house ducts? I doubt the fan has the power to do that but would that be the worst case?
Besides the potential for smell leakage through the entire house as well..

extra possible relevant info..
There is no pilot light on the furnace, eletrical ignition and I don't even use the furnace. But lets say I do have to run the furnace while this exhaust is venting into the air return...
 
sixstring2112

sixstring2112

Well-Known Member
your whole house is gunna smell like pot.and not a good idea to mess with cold air returns. you will end up pulling air from outside the grow area, through it, and bring heat back to your plants and put moisture in your cold air returns. i dont like it.just let it blow in your attic. if it aint broke,dont f*** with it.
 
C

cymbaline

Well-Known Member
don't think smell will/would be an issue, it is being sucked through a can66. (hope not at least)
good point on moisture although my humidity levels are lower than I would like currently.

Would it matter if I added that where I plan on tieing into the ductboard, I could either go directly above a
giant register so the air would be blowing down into the house more than the return itself or offset it a few inches
from the register and the air could go either way.
Also the flex duct is insulated goes straight up and makes a 90 and than makes a gradual decline to the
point I'd like to tie in. Would the 90, length of ducting and the force of air help negate any amount of moisture
I could possibly dump into the ductboard? There would be less than 10 plants in this room at any given time also.
I don't feel any condensation at the end of my ducting as it is now. THat goes without saying when the attic hits 130 and up
it could change.
 
alexonfire

alexonfire

Well-Known Member
I was thinking about the same thing the other day about if people ever duct their exhaust into the cold air return. I guess if you had a carbon filter pulling air out of your grow room then it wouldnt spread the smell. I also dont know what will happen if you try to push air through the cold air duct when the furnace isnt on where will all the air go because the furnace only uses that duct when its on to suck air in? just my 2 cents
 
sixstring2112

sixstring2112

Well-Known Member
alexonfire said:
I was thinking about the same thing the other day about if people ever duct their exhaust into the cold air return. I guess if you had a carbon filter pulling air out of your grow room then it wouldnt spread the smell. I also dont know what will happen if you try to push air through the cold air duct when the furnace isnt on where will all the air go because the furnace only uses that duct when its on to suck air in? just my 2 cents
Click to expand...
In cymbalines case nothing, but if your furnace is older it could possibly blow out the pilot light on the furnace or if you were doing a big hydro grow you would be putting moisture in the duct and that is not good (mold).
 
sixstring2112

sixstring2112

Well-Known Member
cymbaline said:
don't think smell will/would be an issue, it is being sucked through a can66. (hope not at least)
good point on moisture although my humidity levels are lower than I would like currently.

Would it matter if I added that where I plan on tieing into the ductboard, I could either go directly above a
giant register so the air would be blowing down into the house more than the return itself or offset it a few inches
from the register and the air could go either way.
Also the flex duct is insulated goes straight up and makes a 90 and than makes a gradual decline to the
point I'd like to tie in. Would the 90, length of ducting and the force of air help negate any amount of moisture
I could possibly dump into the ductboard? There would be less than 10 plants in this room at any given time also.
I don't feel any condensation at the end of my ducting as it is now. THat goes without saying when the attic hits 130 and up
it could change.
Click to expand...
if your using insulated flex and you have a filter i dont see any harm. I would still just keep it in the attic if i were you, but if you are going to put it in the cold air return put it in the duct just after the vent that is already there(closer to your furnace). that way it will not come right back to your grow as hot air.you can start a hole in the duct by using a screwdriver, hold the tip against the duct in the middle of the hole you want to cut on a 45 degree angle and hit it with a hammer, when you have a big enough hole you can take a pair of red handle tin snips (reds make right hand turns, greens make lefts) and start cuting from your hole towards the edge and just cut it out. be carfull it is gunna be sharp as hell.you can buy a Collar that goes in the hole with tabs on it , stick it in and bend over your tabs and attach you flex. i would leave it in the attic just to cool itif your attic gets that hot. also if you have central air dont plan on it working as good anymore.
 
C

cymbaline

Well-Known Member
Ok I appreciate the insight for sure.
I sure in the hell don't want to encourage mold growth in the cold air return and end up with Legionaires disease..

I don't have central air so no worries on that. Just a big ole window AC unit cooling the house down.
I was going to try and use my 6" hole saw instead of messing with the snips. Might end up having to use the snips
if I can't get enough downforce to drill the hole. (because yea that crap is sharp as razors)
Yea the plan was to originally let it vent into the attic but after running my air and feeling that coolness..
Just trying to save on energy costs.. Without creating hazards..
 
KitchenKhemist

KitchenKhemist

Active Member
cymbaline said:
Ok I appreciate the insight for sure.
I sure in the hell don't want to encourage mold growth in the cold air return and end up with Legionaires disease..

I don't have central air so no worries on that. Just a big ole window AC unit cooling the house down.
I was going to try and use my 6" hole saw instead of messing with the snips. Might end up having to use the snips
if I can't get enough downforce to drill the hole. (because yea that crap is sharp as razors)
Yea the plan was to originally let it vent into the attic but after running my air and feeling that coolness..
Just trying to save on energy costs.. Without creating hazards..
Click to expand...

Couple things to think about...

Just how cold is that air really? It may feel cold in that hot attic, but will you REALLY gain anything by pumping it into your vents? Especially in the return duct...lotsa room in that duct for a little air to move. Also, when your furnace is on, it WILL pull air from the grow room, so you lose that aspect of control with your exhaust. Finally, if it cools the attic at all, good! Less heat transfer from up there into the house...and if you wanna get technical, heat rises AND gravitates to cold, so in the summer, you'll still have the unnoticable cooling effect with far less work. I'd let it stay man.
 
C

cymbaline

Well-Known Member
KitchenKhemist said:
Couple things to think about...

Just how cold is that air really? It may feel cold in that hot attic, but will you REALLY gain anything by pumping it into your vents? Especially in the return duct...lotsa room in that duct for a little air to move. Also, when your furnace is on, it WILL pull air from the grow room, so you lose that aspect of control with your exhaust. Finally, if it cools the attic at all, good! Less heat transfer from up there into the house...and if you wanna get technical, heat rises AND gravitates to cold, so in the summer, you'll still have the unnoticable cooling effect with far less work. I'd let it stay man.
Click to expand...
Good points you have raised.
I thought about that actually; with the difference between the air coming out of the duct vs the attic ambient air. I am going to check the temp coming out of the ductwork to see if it's worth the work doing this.

I'm thinking I'm avoiding the real issue here and that is my fan is bigger than what the house can handle.
It's only a 6" fan with 409 cfm; yet even with 20~ ft of flex duct and a filter attached it still sucks doors open.
So in turn it's pulling the cold/ hot air out of the house. (it isn't a big house)
Plus I heat with wood and when that fan runs and you need to open the wood stove
door, it pulls so much air it causes issues with the stove draft.. Pulls smoke back into the room..
I can tell if I left the door open to the room or not when trying to start a fire.
( I don't think tieing into the return would fix that issue at all, slowing the CFM down probably would?)

I don't think the furnace would pull any air from the room if I put that damper into the collar I planned on.?
Than since I planned on installing the collar so close to the only cold air return in the house, air is going to be pulled
from the path of least resistance which would not be coming from my room but from the actual return.
(only 1 return in the entire house)
The room air is filtered so the furnace if it actually had that much CFM to pull the air, it wouldn't hurt anything there I don't believe..
Oh the house is insulated quite well so the heat transfer from the attic is very minimal. But why heat or cool the attic when
I don't need to and could possibly return good filtered air back into the house?
 
KitchenKhemist

KitchenKhemist

Active Member
cymbaline said:
Good points you have raised.
I thought about that actually; with the difference between the air coming out of the duct vs the attic ambient air. I am going to check the temp coming out of the ductwork to see if it's worth the work doing this.

I'm thinking I'm avoiding the real issue here and that is my fan is bigger than what the house can handle.
It's only a 6" fan with 409 cfm; yet even with 20~ ft of flex duct and a filter attached it still sucks doors open.
So in turn it's pulling the cold/ hot air out of the house. (it isn't a big house)
Plus I heat with wood and when that fan runs and you need to open the wood stove
door, it pulls so much air it causes issues with the stove draft.. Pulls smoke back into the room..
I can tell if I left the door open to the room or not when trying to start a fire.
( I don't think tieing into the return would fix that issue at all, slowing the CFM down probably would?)

I don't think the furnace would pull any air from the room if I put that damper into the collar I planned on.?
Than since I planned on installing the collar so close to the only cold air return in the house, air is going to be pulled
from the path of least resistance which would not be coming from my room but from the actual return.
(only 1 return in the entire house)
The room air is filtered so the furnace if it actually had that much CFM to pull the air, it wouldn't hurt anything there I don't believe..
Oh the house is insulated quite well so the heat transfer from the attic is very minimal. But why heat or cool the attic when
I don't need to and could possibly return good filtered air back into the house?
Click to expand...

Yeah, sounds like you got a couple of problems there man. You can get a speed controller to knock that fan down a bit, but it will still pull smoke in from the woodburner if it's operating anywhere near what you want it to. I'd suggest a sealed room with that fan exhausting outdoors...roof or otherwise (dryer vent kits are handy if you can go low through a wall...and they have their own backdraft damper) and a relief-air pipe plumbed from the attic or crawlspace into the room.

Next thing...If you pipe your return ducting into that growroom, it WILL pull air from the room whenever the furnace/stove blower runs. I understand what you're thinking with the "path of least resistance," but it actually becomes easier for the air to enter the duct through 2 openings rather than one....therefore, less resistance (trust me...it's what I do). I'm not sure what kind of damper you are referring to, but unless you get a manual damper that you completely close during the cold months (even then, it won't seal completely), the furnace blower is gonna suck that air from your growspace. If you're carbon filtering the air before it hits the duct, that handles the smell problem, but you've still lost total control of your growroom air ventilation (think CO2 levels, temps, etc.)

I hope I'm not sounding like a knowitall prick...just trying to help. Plus, pulling a negative pressure in the house with a woodburner scares me man.

Just lookin out.
 
C

cymbaline

Well-Known Member
KitchenKhemist said:
Yeah, sounds like you got a couple of problems there man. You can get a speed controller to knock that fan down a bit, but it will still pull smoke in from the woodburner if it's operating anywhere near what you want it to. I'd suggest a sealed room with that fan exhausting outdoors...roof or otherwise (dryer vent kits are handy if you can go low through a wall...and they have their own backdraft damper) and a relief-air pipe plumbed from the attic or crawlspace into the room.

Next thing...If you pipe your return ducting into that growroom, it WILL pull air from the room whenever the furnace/stove blower runs. I understand what you're thinking with the "path of least resistance," but it actually becomes easier for the air to enter the duct through 2 openings rather than one....therefore, less resistance (trust me...it's what I do). I'm not sure what kind of damper you are referring to, but unless you get a manual damper that you completely close during the cold months (even then, it won't seal completely), the furnace blower is gonna suck that air from your growspace. If you're carbon filtering the air before it hits the duct, that handles the smell problem, but you've still lost total control of your growroom air ventilation (think CO2 levels, temps, etc.)

I hope I'm not sounding like a knowitall prick...just trying to help. Plus, pulling a negative pressure in the house with a woodburner scares me man.

Just lookin out.
Click to expand...
I do have a speed controller (speedster) and it sucks, the fan will start humming before you can even move
it to the medium settings. I've been dragging my feet on getting a variac but I think I need to stop procrastinating
on that.
I don't really want to add a wall vent in that area due to there isn't any reason for one to be there.. It would be the way to go I agree yet impossible to jedi mind trick someone with that.
I do have a bath exhaust near, that was never plumbed out of the attic, I might be able to pipe that through the roof/gable vent and place a y pipe to connect the two and exhaust out? Bath exhaust has a damper on it already.
That concerns me though since I don't want air coming back down through that and I'm not sure how
that fan would perform afterwards with one fan already blowing through the stack.
I would not be returning any kind of venting/ducting back into the garden,
the ductboard is actually in a hallway (2 doors away) where the tie in would happen if it does..
Good point on losing control of my environment and more to think about for future scaling.

I appreciate the insight, that's why I asked for the advice on here. I don't want to mickey mouse it;
just try and save some heating and cooling costs and try not to tear things up to bad that it sucks trying to fix it if I should sell/move down the road.
Either way you look at it, slowing that fan down is definitely one part of the solution
so I can stop the entire house negative pressure.
 
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