I'm curious to hear what experiences you all may have with these simple and relatively inexpensive devices out in the real world.
Here are some pictures and a basic definition of the type of fans I'm talking about:
4.7.12 Alternative Systems - Ventilation Cooling (2013 CA Energy Efficiency Standards)
The CEC also maintains a list of models approved for sale in California. Does this list adequately represent the state of the art?
Or are there other emerging whole-house ventilation technologies that home professionals ought to know about?
For this discussion, let's try to not confuse an old school "Attic Fan" (which isn't designed to ventilate a conditioned space) with a "Whole House Fan" (which is).
Don't hold back!
Here are some facts.
the 30 inch fan from Home Depot (not one I would recommend) is rated at 6000 CFM.
The area of that blade is pi r squared or about 700 sq inches. That is about 4.5 square feet.
The rule of thumb for net free air attic vents is 1 square foot per 750 to 1000 CFM. That means it should have 6 to 8 square feet of net free area. CA Title 24 is now recommending 1 square foot for 375 CFM!
The latest innovations in whole house fans offer better ceiling interfaces and better insulation and better sound deadening.
I am curious about your comment about "unintended consequences". Can you offer some examples?
6,000 CFM at what static pressure? CFM ratings are worthless unless static pressure is also listed.
When outdoor conditions permit you could bring in outdoor air through the HVAC system to cool the space and/or run the compressor. Same way a commercial RTU economizer works. The issue is cost/payback time, it just isn't worth it for residential. Same goes for whole house fans in homes with central AC, payback time is too long.
The 6000 CFM is at 0.1 inwg
Have you ever seen data on static pressure in an attic rated versus the net free vent area?
About payback, how are you calculating it and why do you say it is too long?
In my home my HVAC costs me about 70cents and hour and my whole house fan costs about 5 cents an hour.
I need to run the HVAC for at least 2 hours to cool upstairs while the whole house fan does it in 15 minutes.
The cost of my whole house fan is $750. If I need to use it 6 months every night then I am saving about $1.30 per night or 180 nights times $1.30 or $234 per year. That pays off in about 3 years.
The system is guaranteed for 15 years so I am happy.
About bringing air in to the HVAC system that will work but the cfm is just 1500 in most home HVACs.
You live in a VERY different climate than I do. Whole house fan would be used 2 months per year (one in spring/one in fall). AC costs 10 cents per hour (2KW @ 5 cents per KWH).
Your outdoor air must be quite cool/low humidity with relatively warm indoor temps for your 70 cent per hour AC to take 2hrs to cool it off, yet a 6000CFM fan does it in 15 minutes. If your AC is the typical 1500cfm (your number) it shouldn't take 8 times as long to cool your house than a 6000CFM fan if the entering air temp is the same. If it's 50-60f at night, how hot is it getting in the day?
WOW I am jealous. Your energy is 5c/KWH?? Edison in SoCal is at 12 cents up.
My 2nd story gets over 85 during the day (100 degree days are common).
My HVAC cools all the rooms at once and very slowly at 1500 CFM.
My whole house fan just cools my master bedroom so it takes very little time to move the roughly 20 by 20 by 10 foot (4000 cubic feet) of air in the master with my 4,750 CFM whole house fan.
Fact is, I can cool the entire upper floor (1400 SF x 10 ft ceiling or 14,000 cubic feet) in about 3 minutes.
How do you figure AC is 2 KW??
OG&E actually increased our off peak rate to 6 cents per KWH August 1st. On peak (2-7pm weekdays) averages about 19 cents per KWH. AC power varies based on outdoor temp (all AC's do), 2KW is at 95 degrees. At 80 it's about 1.6KW. I have a whole house power meter that reports live energy use, read the screen before startup and 1 minute after. The photo below is with the AC and about 800W of other stuff running, 90 degrees outdoors.
I, too, have been very interested in whole house fans but living in the upper Midwest and with folks who have allergies, I have been reluctant to invest in something that I can only use a couple of months out of the year AND will bring in fresh, unfiltered (read allergen filled) air. I would very much like to reduce my A/C bills and see a whole house fan as a great option but I'm not yet convinced that window screens that trap allergens will do the trick.
You see the whole house fan as a great option because why? You have given a couple reasons not to install one, but no concrete reason to install one.
You would like to reduce your AC cost from what to what?
For folks with AC, or with combustion appliances (ie risk backdraft CO/flame rollout on water heater), it is a very very very rare case that justifies installing a whole house fan. Really WH fans are for folks without combustion appliances, who live in dry climates with cool nights and can get by without AC.
I have never seen substantiated claims of savings. But if one (not you specifically Gary) is the type to put helium in your car tires for all the gas savings, I doubt you'll be convinced.
I do not recommend them. My concern is it is a big hole in the ceiling plane.
When the fan is operating, that is great. It pushes the heat cloud gathered at the top of you conditioned space out. Perhaps pulling in cooler air from a basement or shaded area outside of a window. That is its purpose.
It is the rest of the time that concerns me. Do the dampers close air tight? What are their R-Value? Maybe an effective R 4 or so? I speculate. Perhaps there is a canopy or insulated housing that can cover the fan during times of non-operation. Air tight and insulation value. Important questions when considering equipment design.
The insulation value of your ceiling plane can be drastically reduced in direct proportion to areas of low or no insulation. For example: attic fan, attic access hatch or can light cut outs. If 70% of the attic is insulated to an R 48 level and 30% of the area is insulated to R 3, the net insulated value is less than R 48. The exact calculation can be found in Krigger and RESNET standards. Probably some other places as well.
This is a great thread on whole house fans. I’m a big proponent of whole house fans but recognize there are weaknesses when it comes to a compromising the thermal and pressure boundary when the fan is not in use. Many vendors offer a variety of solutions, however for my personal needs; none of the current offerings satisfy my needs within my budget.
I’ve owned a variety of whole house fans over the past 40 years. My grandfather had a 60 inch whole house fan installed in his home that would move significant amounts of air. Of course this was before the advent of air conditioning.
My wife refuses to own a home without a whole house fan, since she likes the fresh air and the breeze the fan creates throughout the house. My challenge, as we prepare to build a house and move to NC, is to construct an Energy Star home with a whole house fan.
In our present 6,000 square foot home we have a Dayton 2EAX4 42 inch 2-speed fan mounted on suspension springs that does a great job of bringing in fresh air. The fan pulls 8,400 CFM on low speed and 12,600 CFM on high speed. What I like about this fan is that it only turns about 270 RPM on low speed and is so quiet that we can sleep with it running. At about 4 air changes per minute on high speed, the fan really cools down the house and attic quickly. The fan induced breeze makes the Mrs. very happy. Fortunately, our home has significant square footage of soffit vents so we actually see near factory specifications on fan airflow. For any whole house fan, sufficient exhaust venting is extremely important to fan performance.
My question is what’s the best way to ensure a good thermal and air pressure boundary with a large 42” whole house fan? Well, someone posted this idea, and I like it! https://www.youtube.com/watch?v=c3cN0kPqGQI
Building an insulated and airtight box around the fan in the attic would work well if properly constructed. I’d use at least 2” of foam board and make sure it’s airtight. The only design change I would make is to add some sort of electro-mechanical device to open and close the box lid and not depend on the fan air pressure to open the lid which diminishes the fan’s performance. Anybody have ideas or knowledge of a commercial available designs?
Love my whole house fan! :)
Love them but...............
Get rid of the air plane propellers - spend the money and get a good one.
Need good fan, air tight with insulated doors. Ducted are quieter.
Need adequate ventilation, It's best to cut in an eyebrow or gable vent to make sure
Multiple speed control is a must
Attic plane must be reasonable air barrier
The owner must be willing to operate them properly
The owner must understand that they only work when it's cooler outside
Great cooling option in radiant heated houses
I have installed them for years with very happy clients including myself. In our climate there is almost always cooler air outside in the evening. They can be very effective with low watt draw, mine is 157w on high and 78w on low. You can run a whole house fan for hours for a fraction of the cost of A/C.
There are other benefits, I have used mine in the winter to pull warm air in from the outside. Bad range hood? no problem use the whole house fan. Don't laugh we all do it on occasion. I actually enjoy the white noise to drown out the kids. Typical use- come home an open up a few key windows (not all the way), use on high to push out the hot attic air (conductive benefit), then run it all night on low until I go to bed. We seldom use the A/C anymore. That makes sense to me.
Who they are not for:
Automation geeks (I know there are options, I have installed the nightbreeze)
People who want to cool off during the heat of the day
Warm day warm night climates
Dust / allergy sensitive people
Noise sensitive people
High particulate zones, bad out door air quality situations (e.g. right next to freeways)
Final note: I think the CEC standards for CFM are too much, by double to be more specific.
My 2 cents