Davebones
Senior Member
- Location
- North Of Tampa Fl
Looked for a small attic fan listed for a attic . Just can't seem to find anything for such a small attic area .
Using bath fan to vent attic space .
- Thread starter Davebones
- Start date
- Status
- Occupation
- Licensed Electrician
Buy the smallest fan you can and choke down the exhaust maybe.
Also what's wrong with moving some extra air?
Also what's wrong with moving some extra air?
- Location
- Massachusetts
delectric123
Senior Member
- Location
- South Dakota
Just saying what I've seen from experience with fantech fans, including the fact that fantech recommends the discharge duct from the fan to be one size larger. I was talking about having a fan in the basement that has to push air all the way through the house up through the attic and out, but I guess that's not what the OP is talking about.
- Location
- Massachusetts
The fan can't push more air than it sucks. It does not make air.
- Occupation
- Licensed Electrician
Interesting. I have hooked up many of these fans but only done the ducting on two of them. None were closer to the discharge end; most were in crawlspaces FWIW.
- Occupation
- Licensed Electrician
Not even if you apply some string theory?
From my ordinary engineering mind it seems a fan would blow more CFM through 100' of duct than it would draw through it. Maybe not.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
A lot would depend on the design of the fan and shroud and how well it coupled to the tube.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
This is the wrong professional forum to go too deep into air handling but from a basics view if you have a fan inline in a duct with 100 cfm going out the exhaust, then you will have 100 cfm coming in the inlet as well. Velocity could be different on one side then the other if duct size changes. Some fan designs may be better for moving air in a specific direction through the fan, but cfm is still same on both sides of the fan.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
CFM is the same only to the extent that the pressure is the same. 
What will definitely be the same is the number of molecules.
What will definitely be the same is the number of molecules.
- Location
- Massachusetts
Not sure about string therory but fans don't suck atmospheric pressure pushes the air into the duct.
Two reasons
Two reasons
Hey Bob,
It's been a while. I agree with you and add there are two other reasons why direct venting into an attic is non compliant according to UMC and IRC. Methane exhaust and any gas furnace equipment operational in attics do not permit interior air venting directly into the attic unless the manufacturer's equipment is approved by the AHJ.
Two reasons
Hey Bob,
It's been a while. I agree with you and add there are two other reasons why direct venting into an attic is non compliant according to UMC and IRC. Methane exhaust and any gas furnace equipment operational in attics do not permit interior air venting directly into the attic unless the manufacturer's equipment is approved by the AHJ.
- Occupation
- Licensed Electrician
Right. And I am thinking that more air can be pushed into the duct if the fan is at the start of the duct, in an open room, than if it is at the end of a 100' of duct.
If it turns out that I don't like the answer to my notion then I can just apply some string theory, create a new dimension and everything will work out, just like those physics guys do.
- Occupation
- Licensed Electrician
I get that. The basic question I have is does location of the fan in relation to the duct make any difference in the amount of CFM that can go in/out?
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
yes. Fantech inlines mounted near end of run (exhaust side) will provide best cfm the fan can. is there an inlet into the space located on opposite end of where exhaust is? if so simply mount fan as close to exhaust as possible and dont use an inlet duct, the air will flow in from opposite side, across the space and then out.
whats the purpose of this venting?
mgookin
Senior Member
- Location
- Fort Myers, FL
- Occupation
- Retired inspector, plans examiner & building official
How many cfm's you get with the fan at one end or the other probably is not critical in this matter. It sounds like you need to move very little air and will end up with a bigger fan than you need.
What I like to do with fans on my own installations is install the fan as close to discharge as practical to reduce noise in the environment. My bath exhaust fans all have the fan at the outlet creating nothing more than a moving air sound at the inlet (near me). We do the same with machine exhaust fans in our factory - put the fan at the outlet so we don't have that motor at our machines. The installer had to work a little harder but that's what he's paid for!
What I like to do with fans on my own installations is install the fan as close to discharge as practical to reduce noise in the environment. My bath exhaust fans all have the fan at the outlet creating nothing more than a moving air sound at the inlet (near me). We do the same with machine exhaust fans in our factory - put the fan at the outlet so we don't have that motor at our machines. The installer had to work a little harder but that's what he's paid for!
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
do not confuse cfm with air velocity. If you reduce duct size but still have same cfm (at same temp and pressure) the smaller duct will have to have higher air velocity through it. Now temp or pressure will naturally have to vary some in that circumstance as well making things more complicated, but if temps and pressures remain constant in each area of the system - what comes in has to equal what goes out. If you bring in more then what is going out you have to have temp or pressure changes going on somewhere,
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
agreed, i was just noting max efficiency to match ratings. if its for attic air replacement (move stale air) then its not good to have attic inlet locate at or near attic exhaust, need to have inlet as far away as possible so the fan can draw fresh air across the whole space.
if its for temp control then a temp switch is best. if its for humidity control then a programmable timer or humidity switch is probably better.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
attic ventilation is to prevent moisture problems, who cares if the air is stale otherwise, it is not an occupied space.
When temp is high the air holds more moisture, when it cools it will condense, ventilation allows that moist air to be exhausted before it cools enough to condense.
If you have stale air it is likely because you have a moisture problem.
When temp is high the air holds more moisture, when it cools it will condense, ventilation allows that moist air to be exhausted before it cools enough to condense.
If you have stale air it is likely because you have a moisture problem.
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
stale = not fresh. if that means the attic air is substantially different than outside ambient, or something else, i call it stale. moving "fresh" air, which will likely be not much different than the attic air (actually, if the outside air is cooler than it is actually closer to dew point than the "hot" air in attic as long as the hot air didnt absorb a whole bunch of water), helps keep attic air "fresh". the best the fan can do is allow attic to match outside ambient conditions. if the space is super sensitive to moisture than a dehumidifier may be required.
in some cases, eg; if its a hot dry day and all of a sudden is pours rain, probably best to not suck in the outside air until the rain has gone. thus a humidty sensing switch in attic will likely allow some dwell time before outside moisture moves into the attic space, and by this time (depending on how it rains) the moisture outside has subsided.
not all places have humidity issues. in some dry places in AZ,TX,CA,NV etc, humidity is rarely an issue, thus an attic fan to reduce attic air temp is the purpose, which helps reduce temp diffs across insulating boundaries = less energy needed to cool the living space. in my home venting attic is a tricky proposition, my roofline is insulated, not my ceilings, so i rarely want to pull in hot outside air because my attic stays much cooler than outside ambient, etc. so in this regard, my attic is always "stale".
in some cases, eg; if its a hot dry day and all of a sudden is pours rain, probably best to not suck in the outside air until the rain has gone. thus a humidty sensing switch in attic will likely allow some dwell time before outside moisture moves into the attic space, and by this time (depending on how it rains) the moisture outside has subsided.
not all places have humidity issues. in some dry places in AZ,TX,CA,NV etc, humidity is rarely an issue, thus an attic fan to reduce attic air temp is the purpose, which helps reduce temp diffs across insulating boundaries = less energy needed to cool the living space. in my home venting attic is a tricky proposition, my roofline is insulated, not my ceilings, so i rarely want to pull in hot outside air because my attic stays much cooler than outside ambient, etc. so in this regard, my attic is always "stale".
Last edited:
- Status