Air fans and efficiency

Snorkelwacker's post about attic venting reminded me of a question I wanted to pose to you posers.

I want to install an attic fan as house ventilation. My question is about CFM per watt basically. I know some designs are more effective at creating pressure (like a vacuum cleaner) and others are better at creating flow (like a squirrel-cage type). I'm basically looking for something that moves large CFM with the least amount of wattage. I would only use it with the windows open, so assume very little pressure is needed, just large amounts of airflow.

I want to use it as a middle ground; something I can use on modestly hot days instead of firing up the A/C, so it needs to be effective but not cost as much to run.

What style of fan will create the most airflow (assuming little or no pressure) for the least amount of electricity cost?

Attic fan styles are usually a factor of the installation location, not the CFM per watt.

If you need to mount it in a gable, a squirrel cage doesn't work very well (they draw from the center and exhaust 90* to the intake flow direction).

Any style fan with a bad set of bearings will burn more electricity. An optimized style with a non optimum fin or blade shape or ill-fitting housing will not move as much CFM's.

A solar powered vent (like the one I posted the install video on GPS's thread) doesn't burn ANY electricity that it doesn't create! It's electricity cost is zero. Therefore, it's CFM per watt is infinite!

SVreX wrote: Attic fan styles are usually a factor of the installation location, not the CFM per watt.

Its a single gable roof with four dormers. This is a picture of my house:

A solar powered vent (like the one I posted the install video on GPS's thread) doesn't burn ANY electricity that it doesn't create! It's electricity cost is zero. Therefore, it's CFM per watt is infinite!

Very true, however as you might be able to discern from my photo; solar supply for an exhaust fan is not a strong choice for the location. Not only do I live in a poor solar region (Pittsburgh, PA) but the property is heavily shaded. I may need peak flow during non-peak times, like warm nights or during overcast/rainy times.

Trust me... I am all about photo-electric panels, geothermal sourcing, and other friendly things. I'm gearing up to make my own biodiesel. The return on investment for solar electric is not feasible on this property. I have done the math. It would take me something like 225 years to recover the costs. I don't plan on living quite that long

I'm not doing this to limit attic heat, I'm using it as a house ventilation system. Instead of putting a 16" fan in a window, I want to flip a switch and have a big attic fan create cooling airflow, not just have a passive fan which activates when the sun hits it.

Curtis, you're looking for something like this: http://www.wholehousefan.com/

I am considering putting in a return air grille (with a manual damper) in the basement into the return air ductwork. One big one that would effectively cause the rest of the returns to be useless. The one big return will take the nice cool air from the basement and distribute it throughout the house. We keep the basement door open for the cat to make it down to the litter box so no worry about the supply air not making it back to the return.

Don't forget there are the Savonius turbine fans that run on wind power:

In reply to GameboyRMH:

That's not a whole house fan. It's a roof vent.

It won't do what he needs.

In reply to curtis73:

Even if a squirrel cage was more efficient (ie: created more CFM's per watt), you might be doing contortions to make it fit, or paying more for ductwork, etc.

Use the fan that fits the application, then worry about a green method of utilizing and/or creating the electricity.

Any whole house fan will be a LOT more green than running an AC unit, so you are on the right path.

My .02 on the cheap. Use an attic fan only to exhaust high heat from a baking roof, ridge vents will help if you have them, summer ambient air is better than baked roof attic air. Pull cool air from the basement and circulate through rooms of choice. Pa. basements are still relatively cool in summer. Remove return air door from furnace, run furnace fan w/ manual switch or if you wanna get crazy add separate tstat in duct work, conditioned rooms or basement to control fan. Even w/ furnace door removed there will still be make up air from returns. Damper off unused rooms.

I grew up in a house in NJ with a whole house fan.

It was a large through-the-wall unit in the attic. It pulled air from wherever we directed it, based on which windows we opened. If all windows were closed except the furthest, it pulled through the whole house. It worked well. As simple as it gets. Pretty much ran continuously from June through September.

If all the windows in the house were open, it didn't work at all.

We always had the fan in the hall ceiling. Sucked air in like a whore will suck a golf ball through a garden hose.

mguar wrote: In reply to curtis73: Others have said it.. please believe it.. Attic fans are all about design and very little about power..Heat rises, cool air settles..*more later

I believe it! My application however is not just an attic "efficiency" fan, it will also be used as a fan that moves enough air to make a breeze to cool the whole house. (hopefully without costing as much to operate as A/C)

May I assume the reason you want an attic fan is for cooling purposes? If so first thing is reduce your heat gain.. What sort of (R value and type) insulation do you have in the attic?

That will be assessed when I cut a hole for attic access. So far, any of the holes I've cut in the ceiling for things like lights, electrical access, etc have been met with standard 4" batts of pink. I work in the Rental department at Home Depot, so when it gets to the point of fan installation I'll just bring home the insulation blower and 10 bags of paper insulation and do it right. The roof will also be due for replacement in the next 4-5 years and I'll add the goodies like ridge vents. I'm also going to do it with either steel or a composite like Ondura. That will also have much better self-venting with the isolation of the airspace.

* most basements are cool average earth temperature is 55degree. (below 6 feet) Assuming your basement is radon free. (check to confirm) draw cool air from the basement and pump it into the attic.. it will settle and eventually return to the basement cooling as it does.. if you have duct work in the house it should be relatively simple to achieve..

An excellent idea, however I have a lot of work to do on blocking water and humidity down there before I start using that air.

SVreX wrote: In reply to curtis73: Even if a squirrel cage was more efficient (ie: created more CFM's per watt), you might be doing contortions to make it fit, or paying more for ductwork, etc.

True, however the best space I have for a fan is directly on the ends of the attic. I could directly mount the mouth of the squirrel cage (or any fan) on the outside wall. Zero ductwork. The hole that I cut in the ceiling can be a manual register vent, or just a door that can be propped up.

Use the fan that fits the application, then worry about a green method of utilizing and/or creating the electricity.

I couldn't agree more. I wish I had more green options here. Wind - nope. Water - nope. Solar - very poor. Might have to build a diesel generator and make more biodiesel.

SVreX wrote: I grew up in a house in NJ with a whole house fan. It was a large through-the-wall unit in the attic. It pulled air from wherever we directed it, based on which windows we opened. If all windows were closed except the furthest, it pulled through the whole house. It worked well. As simple as it gets. Pretty much ran continuously from June through September. If all the windows in the house were open, it didn't work at all.

that's exactly what I want, but have it move enough CFM to make a palpable breeze with most of the windows open. We had the same basic thing in my house growing up, but it was a big fan in the window in Mom and Dad's room and my sister and I kept our windows open. I want that, but bigger and in the attic.

I have used a whole house fan with good results. Mounted in the ceiling of a hallway and in the center of the house. As was previously mentioned you control the airflow by selectivel oening windows. Also, you only have to crack the window open to get adequate air movement. 4"-5" is plenty.

What would be the best way to do this:

" most basements are cool average earth temperature is 55degree. (below 6 feet) Assuming your basement is radon free. (check to confirm) draw cool air from the basement and pump it into the attic.. it will settle and eventually return to the basement cooling as it does.. if you have duct work in the house it should be relatively simple to achieve.. "

Our house is 10 years old with central heat and ac. Are we talking about putting some kind of fan to pull the air up or push it up from the basement?

Sorry for the thread jack Curtis.

If you're looking for some hard data, try this. Go to Greenheck's website, registry, and download their CAPS program. Then you get to pick fans and sizes and whatnot. It will tell you everything you want to know about power usage and CFM.

http://www.greenheck.com/content/view/software_caps?ref=software

In reply to Rusted_Busted_Spit: If you wanted to pull basement air up into the house, re-read my previouse post. I would keep the windows closed if you're going to do this however. Otherwise all of the air that is being infiltrated back into the basement from the outside will be moist and you get more condensation in the basement. If you keep the windows shut, then after the initial part, the dehumidifier will only have to remove the moisture you release during breathing/showering/cooking...

In term of dragging air to the attic from the basement, I see no point. Just insulate between the attic and living spaces better. (R-30+)

curtis73 wrote: True, however the best space I have for a fan is directly on the ends of the attic. I could directly mount the mouth of the squirrel cage (or any fan) on the outside wall. Zero ductwork. The hole that I cut in the ceiling can be a manual register vent, or just a door that can be propped up.

That's not right.

A squirrel cage intakes through the center, then exhausts at a 90 angle. If you install it so the exhaust is directly through the outside wall with NO ductwork, it will be sucking air at a 90 angle to the wall- directly from the eaves of the house (either front or rear). This is an inefficient air flow (since your desired flow will be from somewhere at the opposite side of the house, parallel with the ridge). Additionally, standard exhaust louvers are not sized to correspond with standard sizes for squirrel cages (because people don't use them generally for through the wall applications).

If you install a manual ceiling vent or door, it won't be long before someone turns on the unit and forgets to open the vent, which will depressurize the attic and suck all that blown-in insulation you shoved up there out and blow it all over the neighbor's yard. Or maybe burn up the fan motor.

I realize you can't stand doing things the "normal" way (it's just who you are), but it will go a lot better if you use components that were designed for the application.

You probably wouldn't use an electric defroster fan on the air intake of a car to do the job of a turbo. 'Nuff said.

curtis73 wrote: that's exactly what I want, but have it move enough CFM to make a palpable breeze with most of the windows open.

Unlikely it will work that way.

You will short cycle the flow, and the fan will draw all of the air from the nearest opening. Once there is enough free space (in open windows) to supply the needed CFM's, you will no longer be creating negative pressure in the rest of the house (therefore no flow).

Unless, of course, you size the unit to move MORE CFM's than the opening area of all the windows combined. Then you will suck the drapes off the wall off the windows nearest to the unit.

SVreX wrote: I realize you can't stand doing things the "normal" way (it's just who you are), but it will go a lot better if you use components that were designed for the application.

Normal is for normal people

I agree. the only problem is that the "normal" components are designed to increase the efficiency of the attic insulation. It doesn't do much for actively cooling the house.

I just want a big fan that can replace the air in the house continually. Kinda like mom and dad's old window fan.

This will do it.

On a serious note, the weekend house my family owns had one these mounted in a gable with vent grills in the rooms. Belt driven off a 1/4hp motor, it was quiet and moved a crap ton of air. It worked pretty good as long as it wasn't humid.

SVreX wrote: A squirrel cage intakes through the center, then exhausts at a 90* angle. If you install it so the exhaust is directly through the outside wall with NO ductwork, it will be sucking air at a 90* angle to the wall-

True, but that doesn't mean its an inefficient design. There is a reason they are widely used - maybe its not because of a CFM/watt reason, but I do know that old-school blade fans are not very efficient at moving air at a low cost.

curtis73 wrote: I agree. the only problem is that the "normal" components are designed to increase the efficiency of the attic insulation. It doesn't do much for actively cooling the house.

That's not true.

Attic fans and powered roof vents are designed to vent heat out of the attic.

Whole house fans are designed to actively move air through the house.

They are much larger, and almost always a through the wall design.

You are looking for a whole house fan. GPS was looking for a powered roof vent.

curtis73 wrote:

SVreX wrote: A squirrel cage intakes through the center, then exhausts at a 90* angle. If you install it so the exhaust is directly through the outside wall with NO ductwork, it will be sucking air at a 90* angle to the wall-

True, but that doesn't mean its an inefficient design. There is a reason they are widely used - maybe its not because of a CFM/watt reason, but I do know that old-school blade fans are not very efficient at moving air at a low cost.

Of course there is a reason they are used. It's because they are appropriate for the application.

And they are almost universally used inside enclosures and ducts.

There is also a reason they are NOT used to vent whole houses in through the wall applications.

A squirrel cage fan is not inefficient. The design you are suggesting IS.

SVreX wrote: A squirrel cage fan is not inefficient. The design you are suggesting IS.

I guess I'm not sure why. Let's say the squirrel cage fan would move 1500 cfm and draw 4 amps. Who cares if it draws air from the eaves or the ridge, or anywhere else for that matter. It will be theoretically replacing the entire volume of the attic's air three times every minute.

I guess I'm not understanding why my design is so inefficient. I want to move X cfms of air while expending as little wattage as I can. Why would the squirrel cage be such a bad idea? It moves air, it doesn't take as much wattage/cfm as an equally sized blade fan. It also doesn't require cutting and bracing two studs in a critical area in an 80-year-old house.

Do enlighten me