Attic Venting Idea - Will it Work?

Reply

  #1  
Old 06-26-18, 10:07 AM
J
Member
Thread Starter
Join Date: Mar 2014
Posts: 26
Received 0 Votes on 0 Posts
Attic Venting Idea - Will it Work?

My home has a basically rectangular shape. At each end there are two venting openings that are approximately 1' x 2'. A total of four give me 8 sq. ft. of venting. We live in a climate(Sacramento) that peaks the summer temperature at around 5 PM, so the air outside isn't really cool until around 10 PM, too late for early evening cooling with a whole house attic fan. So first I am going to try a gable mount attic fan to push out the hot attic air that accumulates through out the day. Normal summer temps here are low 90's to around 105 degrees peaking as I said, late in the day.

My idea is to mount a gable fan in one of the openings on the east side. The fan I plan to use is a QuietCool Pro 3013 CFM fan. But I was concerned that the fan would just end up sucking air from the nearest opening, the second of the four gable openings, located just about six feet away, instead of from the other two gables at the other end of the attic. I have two questions, the second depending on the answer to the first:

1. Will the nearby opening provide most of the air thus negating the pulling of the hot attic air from the openings at the other end? If the answer is no, no problem. If the answer is yes then read #2.

2. I would mount a gable vent panel with move-able slats facing out in the gable nearby. So when the fan is turned on the slats would be sucked in to a closed position thus forcing the exhaust air to come from the other end of the attic. Will this work?

If I do #2 and later install a whole house attic fan then the air forced into the attic will vent out through the three open gables and the move-able slatted one if needed. Will this work?

My home is 1850 sq. ft., single level. If I install a whole house fan it will be a 30" 6000 cfm MasterFlow. Is another alternative to just close off the gable next to the vent fan giving me 6 sq. ft. of vent?

I hope I haven't written this in too complicated a description. Any advice would be greatly appreciated.
 
Sponsored Links
  #2  
Old 06-26-18, 11:08 AM
H
Member
Join Date: Nov 2012
Location: USA
Posts: 1,748
Received 95 Votes on 84 Posts
You'd get a better result from placing a fan in the east wall, north window, blowing cooler outside air INTO the attic. An attic fan is going to end circulating air at outside temperature, anyway, so just blow outside air IN.

A fan is blowing OUT from a confined space like an attic, will have a "soda straw effect" and will likely "short circuit" and pull outside air from the nearby window.

A fan is blowing IN from the unconfined space outside the attic won't "short circuit".
A fan blowing IN also avoids negative pressure sucking up cool A/C air from downstairs.

I'd just get two small box fans that fit in the windows, and two "ThermoCube" switches, one that turns on at 78' and one that turns on at 120'. Each switch has 2 plugs. I setup 2 fans on an extension cord. The 78' degree switch plugs into the cord, the 120' switch plugs into one of the 78' switches outlets.
Then plug one fan into the 78' switch, the other into the 120' switch.
That way, one fan comes on at 78, but if the temperature gets up to 120, then the other fan kicks in.
 
  #3  
Old 06-26-18, 11:17 AM
J
Member
Thread Starter
Join Date: Mar 2014
Posts: 26
Received 0 Votes on 0 Posts
Thanks for the reply. Wouldn't your solution just blow the air out the nearest window just like you say it would "short circuit" suck the air from the nearest window? The result in either scenario is to leave the majority of the air throughout the rest of the attic in its existing "still" state.
 
  #4  
Old 06-26-18, 12:49 PM
czizzi's Avatar
Member
Join Date: Jul 2007
Location: USA
Posts: 7,388
Received 14 Votes on 12 Posts
You need a source of replacement air which usually come in the forum of soffit vents. Hot air rises, as it rises, it sucks cooler air in from the soffit vents to bring down the overall temperature of the attic space. If all you have are gable vents, and no soffit vents, -then there is no cooler air to mix with the warmer air to bring down the temperature.

So, $100,000 question - do you have soffit vents?
 
  #5  
Old 06-26-18, 01:36 PM
J
Member
Thread Starter
Join Date: Mar 2014
Posts: 26
Received 0 Votes on 0 Posts
There are six soffit vents along the back of the house and two in the front. Would not the four gable vents, each 1' x 2', be also fresh air vents? Or are they only considered exit vents?

If I use the QuietCool 3013 CFM attic fan to blow in outside air and block the adjacent gable vent(so I don't get the "short circuit" effect) that would leave my the soffits and the other two gable vents at the far end of the attic to exhaust the hot attic air. How do I calculate the amount of exit space I need for 3013 cfm? The two gables give me 4 sq. ft. and the soffits a nominal additional amount. This seems like it would work.
 
  #6  
Old 06-26-18, 01:57 PM
czizzi's Avatar
Member
Join Date: Jul 2007
Location: USA
Posts: 7,388
Received 14 Votes on 12 Posts
It is supposed to be a convection system. Hot air rises, draws cooler air in through the soffit vents. You can test if the soffit vents are working by seeing if they draw air in on a hot day. If you are a smoker you can hold a cigar/cigarette up to the vent to see if it is drawing air. Absent of that you could light a match and see if the smoke draws in when you blow it out.

I don't subscribe to the gable fan blowing "into" the attic space. The object is to pull the warm air out so that it is replaced with cooler air. I think if you create some negative pressure with a gable fan blowing out, they you will get replacement air in from all openings not just the other gable vents.

The whole house fan works when the heat of the attic moves down from the attic into the living area. You turn on the house fan, crack a window at the far end of the house and you suck all the warm air out and into the attic which blows through the gable vents. My first house had no AC and vertical slide windows. What I did was put a box fan in the attic opening facing up. When I came home at night I plugged the fan in, cracked a window and in 10 minutes had cooled the house down. When I moved in there were no gable vents. I cut my own in to create some movement of air.
 
  #7  
Old 06-26-18, 05:27 PM
H
Member
Join Date: Nov 2012
Location: USA
Posts: 1,748
Received 95 Votes on 84 Posts
Originally Posted by jborchel
Wouldn't your solution just blow the air out the nearest window just like you say it would "short circuit" suck the air from the nearest window?
Nope. You can feel the stream of air blowing FROM a box fan 10-20 or 30 feet away.
Move around to the back, and you only really feel the air being sucked into the fan when you're mabey 1-2 or 3 feet away.
 
  #8  
Old 06-26-18, 05:58 PM
J
Member
Thread Starter
Join Date: Mar 2014
Posts: 26
Received 0 Votes on 0 Posts
So now Hal_S says blow cool air into the attic and CZIZZI says "The object is to pull the warm air out so that it is replaced with cooler air". Who's right?
 
  #9  
Old 06-26-18, 06:48 PM
czizzi's Avatar
Member
Join Date: Jul 2007
Location: USA
Posts: 7,388
Received 14 Votes on 12 Posts
So if you want to vacate warm/hot air out of an attic, do you create negative pressure or positive pressure?

Blowing into the attic will not force air out, it will create positive pressure that may drive hot aid down further in the living space. Negative pressure will pull hot air out and passively force cooler air into the attic. Easy to experiment both ways. You will find that your attic vent fan is set up to blow one way, that is out. You would need to modify to blow in. Might be a reason for that.
 
  #10  
Old 06-26-18, 07:07 PM
H
Member
Join Date: Nov 2012
Location: USA
Posts: 1,748
Received 95 Votes on 84 Posts
Originally Posted by jborchel
So now Hal_S says blow cool air into the attic and CZIZZI says "The object is to pull the warm air out so that it is replaced with cooler air". Who's right?
Both are right. If you have 4 open windows, either method will exchange warm air for cool.

Distinction is subtle, fans are much more effective in one direction, same as how you can tow a car with a chain, but you can't PUSH a car with a chain. A fan will push air for quite a distance, but will only pull air from the immediate vicinity.
 
  #11  
Old 06-26-18, 07:14 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,514
Received 37 Votes on 34 Posts
Hi jb, not sure I can salvage this thread but will try. Some quick facts to remove some of the confusion.
1. Air is never pulled, it moves when a positive pressure, inside or outside pushes on it.
2. Longer story, but there is nor short circuit effect. All vents will experience any pressure change be it positive or negative.
3. Exhausting hot attic air can result in conditioned air from the house being pushed into the attic. Blowing air into the attic uses the same leaks to force warm air down into the house, essentially the same loss.
4. All vents have a NFA (net free area) rating so use that number and not the physical area.
5. The fan listed may be rated at 3,000 cfm, but that would be with little or no exhaust resistance. With limited exhaust vent area more pressure is required to allow replacement air to enter. Can be calculated but I suspect the fan mfgs have a chart.
6. The hole purpose of the fan is to remove unwanted heat from the attic, no winter issues, and that should be resolves by air sealing between house and attic along with excellent insulation. Yes it will get hot up there but who cares if the insulation is protecting the house below.

That will get us started.

Bud
 
  #12  
Old 06-26-18, 08:14 PM
J
Member
Thread Starter
Join Date: Mar 2014
Posts: 26
Received 0 Votes on 0 Posts
Bud, this house seems pretty well insulated. I've been crawling up there laying HDMI and ethernet cable so I know there is about 18" to 24" of blown insulation. The roof also has some type of foil coated layer under the plywood that is sold as helping. When the temperature outside is going to peak at about 105 around 5:00 PM the house gets into the low 80's around 2:30 PM, necessitating turning on the air until around 7:00 PM. I'm just trying to narrow the time by an hour or two on each end. I would think that the vent fan would do just that or they wouldn't be used. Since they are made to attach one way I will try that first as it will at least make the install a little easier.
 
  #13  
Old 06-26-18, 08:56 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,514
Received 37 Votes on 34 Posts
Exhaust fans are used when insulation and air sealing look like too big of a project and the fans are an easy fix, when in reality that fix comes at an operating cost. I do energy audits and with 18 to 24" of insulation up there the heat gain from a hot attic is very minimal. Yes you experience a heat gain, but it is not all coming from that attic. We can run a basis audit, there are simple online programs, that will point out where that heat is coming from. Do you have an unfinished basement, sorry didn't go back and review the entire thread? If you can see the electrical and plumbing from down there you can tell if those penetrations were sealed at some point. If not sealed then air leakage is contributing to your heat gain. Cool air inside the house leaks out the lower levels and allows warm air from the attic to be pushed in. Unfortunately, with that much insulation up there it becomes almost impossible to do more air sealing. And if the ceiling to leaks then the new fan fill be stealing conditioned air. I know, you probably feel yours is well sealed, but unless specifically done, it is not. Example, it would be common for a well built home to leak all of the inside air every 4 hours (summer stack effect isn't as bad as winter stack effect).

Time to turn in (east coast).

Bud
 
  #14  
Old 06-27-18, 06:43 AM
J
Member
Thread Starter
Join Date: Mar 2014
Posts: 26
Received 0 Votes on 0 Posts
It is a 1850 sq. ft. home in a single story built in 2003. It seems relatively well sealed as all walls and ceilings throughout home are drywall. There are many canned lights though which could contribute to leakage. I think the bottom line is am I willing to risk $200 and a lot of work on installing the fan to find out. Since I'm retired and have plenty of time I think it probably is.
 
  #15  
Old 06-27-18, 07:30 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,514
Received 37 Votes on 34 Posts
To avoid being too expensive most modern homes are built to code minimum levels. That's ok, but it is not super tight and there is a reason. Super tight home have eliminated many of the unintended air leaks and end up needing a fan running 24/7 to create enough air exchange to maintain a healthy home. Builders and the energy industry have come to a mid point where a home is considered well built but not so tight to need that fan running. That point is around one complete air exchange every 3 hours. You are in a cooling climate so probably 4 hours as natural pressures are less (unless the wind is blowing). I suspect powered attic ventilation will help reduce the attic temps and possibly help the house, but at the cost of added electricity for the fan and added energy costs for lost condition air from the house. Calculating these costs is certainly beyond my pocketbook so I rely on the experts who have tested hundreds of homes and come to the conclusion that attic fans are not money savers.

Citations available.

Good luck. Bud
 
  #16  
Old 06-27-18, 03:12 PM
H
Member
Join Date: Nov 2012
Location: USA
Posts: 1,748
Received 95 Votes on 84 Posts
I'll jump back in, just to mention two things,

First, that if you have cool nights using a fan to pull cool evening air through the house and attic should help.

Second, a digression about window fans and short circuiting,
Yes, it does happen, when my wife was learning how to adjust a farmhouse fireplace flue, I had a few situations when I had to open the windows to get the smoke out, and I've put a fan in one windows and watched the fan blow smoke out one windows only to have it sucked back in through the other.
 
  #17  
Old 06-27-18, 03:37 PM
J
Member
Thread Starter
Join Date: Mar 2014
Posts: 26
Received 0 Votes on 0 Posts
"I rely on the experts who have tested hundreds of homes and come to the conclusion that attic fans are not money savers."

I think you are probably right. It's 3:30 PM right now, 91 degrees outside going to a high of 92 in an hour or so, and 78 in the house. I just climbed the ladder and the ceiling in the living room which is the center of the attic is cool to the touch. So I assume there is no radiation of heat from the attic, thus negating the need for an attic vent fan.

I am going to install a whole house attic fan which will suck in the cool night to early morning air. This will set the inside temp at a much lower level than it is normally in the morning thus delaying, I hope, the need for the air conditioner by an hour or more. We'll see. There are many testimonials from the local NextDoor board who say it works this way for them.
 
  #18  
Old 06-29-18, 07:12 AM
D
Member
Join Date: Apr 2014
Location: us
Posts: 836
Received 23 Votes on 22 Posts
Let's start with the basics. In the summer with input from radiant sun energy attics can get a lot hotter that outside ambient air.

Even with good room ventilation, higher heat from attic raises room ceiling and other building material temperature. At night this exhibits a “Dutch oven” effect with hot ceilings raising the room temperature.

Whether the place is air-conditioned or not, heat load without attic ventilation is higher. Attic fans are relatively cheap, have low operating costs and long term payback.

Unfortunately the people who install attic ventilation often do not deal with the other issues. Air leakage from interior to attic and leaking AC ducts have to be dealt with. Adequate make up air has to be provided, etc, etc.

jborchel's experts who ignor the numbers cannot be expected to do a proper job on attic ventilation.
"I rely on the experts who have tested hundreds of homes and come to the conclusion that attic fans are not money savers."
 

Last edited by doughess; 06-29-18 at 08:53 AM.
  #19  
Old 06-29-18, 01:10 PM
J
Member
Thread Starter
Join Date: Mar 2014
Posts: 26
Received 0 Votes on 0 Posts
Doughess,

I hear what you are saying and one has to wonder why there are so many attic vent fans for sale if they don't work in the way they are intended. You did mention the attic heat radiating down into the interior through the ceiling and side walls. At peak temperatures late in the day, over 100 degrees, I got up on the ladder and could not feel anything but a cold ceiling. What am I missing here?
 
  #20  
Old 06-29-18, 03:06 PM
D
Member
Join Date: Apr 2014
Location: us
Posts: 836
Received 23 Votes on 22 Posts
Jborchel: First you post a quote that fans do not work (are not money savers),
"I rely on the experts who have tested hundreds of homes and come to the conclusion that attic fans are not money savers."
now you are saying the many for sale suggest they do work.

Attic fans exhaust hot air from attics. If you ceiling is cold to touch on sunny day then either it is insulation, ventilation or both. You should be happy
 
  #21  
Old 06-29-18, 04:32 PM
J
Member
Thread Starter
Join Date: Mar 2014
Posts: 26
Received 0 Votes on 0 Posts
I am happy. How could I not be. I don't have to work anymore and my ceilings are cold.

I was just trying to show that there were conflicting reports/examples/products that addressed the issue of cooling your attic in this thread. Logic tells me that if the industry sold so many examples of these fans then there must be some truth to them. Obviously they are for those older homes with less than adequate construction and insulation. These homes' ceilings are probably warm to the touch when it's 100 degrees outside.
 
  #22  
Old 06-29-18, 06:19 PM
czizzi's Avatar
Member
Join Date: Jul 2007
Location: USA
Posts: 7,388
Received 14 Votes on 12 Posts
If you have 24" of blown in insulation, there is a good possibility that the soffet vents were blocked off in the process unless baffles were installed prior to the insulation being added. You can easily check this situation by noting where the soffet vents are and see if the insulators left room for proper ventilation in the attic or if they blocked everything up between the rafters.
 
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: