While in the process of some extensive home remodeling, I thought I would add a whole-house attic exhaust fan. We enjoy a couple of months on the either side of summer where opening the windows and drawing air into our home is very comfortable.
The attic fan pulls in outside air and exhausts house air into the attic, which, in turn, helps to cool the attic. There is a louver assembly that uses gravity to keep the louvers closed, and when the fan is on, the airflow opens them. It saves energy over having to use the HVAC central system.
I picked up a three-speed fan at a local discount outlet for about $100. Installation was easy, and everything looked great. The fan was in place, the louvers were hung, and we were ready to test it. I pulled the cord, and the fan started, the louvers opened, and the air flowed. I walked out of the room to seek praise for my efforts when I heard a noise behind me. Running back down the hall, I found the fan had shut itself off. Then I noticed the pull cord, which had hung down from one side of the fan and through a hole in the louver surround. It had been sucked up through the louvers.
I pulled the louvers down and discovered the fan blades had caught the pull cord and cut the end off, thus shutting off the fan. I had turned the fan on to the lowest speed. It was pull once for high, pull again for medium speed, pull once more for low, and then one more tug to turn it off. It pulled the cord and the little weight at the end of it right up into the blades! Who designed this? Who tested it?
My solution was to completely remove the control switch and run wires down to a wall-mounted three-way rotary switch located out of harm's way. After this experience, I decided against putting one of these in the bathroom.
This entry was submitted by Bradley Miller and edited by Rob Spiegel.
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Ugh! Seems like a classic case of shortsighted design thinking and cutting corners. We had a number of ceiling fans in our former house and we pretty much universally had problems with all at some point in time with the various settings being off kilter. That said, the fans were not the same model and not even from the same manufacturer. With one, the settings for low, medium, and high speed were actually backwards. It wasn't anything that was dangerous or that we even took time (and expense) to fix. The bother was we had to create kludgy workarounds just to keep them going.
Classic case of solely using Analytical Problem Solving with no regard to Systems Engineering. In isolation, I'm sure the fan, the louvers, and the switch were perfectly compatible on paper and during assembly. But actually measuring the performance of the system as it would be used in the field was not addressed. To have it fail on first use is pretty silly. Even with modest Systems testing it should have been easily discovered in the lab. I'll definitely be using this story as an example in my Systems Analysis course...
I have a fan that works exactly like this. I was very surprised at first, but silly me, I figured there must be some logical reason it was designed that way. What once seemed over my head was probably just a dumb design.
I feel blessed to have two small, inexpensive ceiling fans with globe lights that have worked nearly perfectly for 12 years--they are going constantly in winter and summer--and many years before I inherited them when I bought the house. They are small, painted white (which does not fit the rest of the decor) and I at first considered replacing them but never got around to it. Now I'm afraid to. Everyone I know with great big more expensive ceiling fans by well-known national brands complains about some fault or other. The only problem we have is that the one whose light is switched on and off most tends to occasionally wobble slightly and vibrate, making a very annoying noise. I fixed that by inserting a large, thick rubber band around the neck of the globe which makes a thin cushion, before it's inserted into the housing and screws screwed in to secure it. I change it every 4 months or so.
William, your reference to the system engineering angle is well noted. What would your suggestions be to your students, given this example, in terms of helping avoid what looks to be an all-too common scenario?
@Beth -- By far, I have found the Number One important concept in System Engineering is to be mindful of the Environment... Not the environment as in Environmentalism, like clean air and water, but the System Environment. There are several views on design optimization:
1) Use the Best Ingredients (Input)
2) Use the Best Practices (Process)
3) Insure the Best Quality (Output)
All three of these are great, but taken separately, or even as a group, if the list does not include
4) Be mindful of the effects of things external to the system (Environment)
then we get things like the inflow of house air causing a current which entrains the pull cord which subsequently gets caught in the blades.
Testing a finished or prototype product in its actual usage Environment should identify a host of potential problems.
This comment is already too long, but it brings to mind the valuable lesson from Langley vs. the Wright Brothers. On paper, Sam Langley was an excellent engineer that could design the best powered flight machine using the best components, the best technology, and the most powerful engines. It took the Wright Brothers and their wind tunnels to simulate the flowing air environment of thousands of wings, kites, and control systems before they started testing on the beach -- a place that provided wind and a soft landing for anticipated crashes. Langley launched his machines from an aircraft carrier in the Potomac River... and continuously fished them out from the bottom.
I'm chuckling at the number of people who are confusing a ceiling fan with a whole house atic fan... The two are no where close to each other than the word FAN.
I do have to admit, I've never seen a whole house fan with a pull cord, let alone 3 speeds. Every one I've ever seen was wired down to a wall switch with a 30 minute mechanical timer with a hold position.
Thinking about putting one of these in myself actually, for the exact same reason, they move a ton of air, probably in the 10-20,000 CFM range.
Since I happen to have the technology available, I was planning on using a small 230v VFD and running it variable speed vs. on/off or 3 step control.
I'm sure that the pull chain attic fan was a regular ceiling fan motor mounted in a frame for installation between joists. It was NOT what you describe because it would have cost more if done correctly.
I have seen this same thing happen several times with Pedistal fans. (having had to replace the switches, if the motor did not burn up) (Pedistal fans are the larger fans more for industrial use, that stand about 6' tall, or like you see in many exercise places to move the air around.)
I have seen it suck up the cords when someone attaches a light string onto the chain, it sucks in the string and then the chain. Normally, the fan has a wire surround guard that the blocks the chain from going into the fan. Air flow is insufficient to lift up the chain and suck it between the wire mesh, but add on a light string and there you go. I even had someone add on the string, had this happen, removed the string afterwards and stated it was never there. He had just thrown the string in the nearby trash can, and when I looked, there it was.
I have also seen this happen when the fan was placed into a Boxed-in frame. This helped to create more air flow immediately around the motor housing (which the chain come out of) and sucked in the chain.
And then there are those who do not test what they sell, as others have advised. Seems everywhere you turn, you find something that could have been made more user friendly with only minor changes. But with mass produced off shores junk being imported, actual testing is very limited. And any part that is heavier add a fraction of a cent to cost...
Good point, Jmiller. It sounds like he was taking a window fan and using it as an adict fan. Certainly a fan mounted in the attic. Certainly you wouldn't need three speeds for a fan way up in the attic.
I'm willing to bet that the probelm is that many of these consumer items are never tested in their environment. I'm sure that the goal was to produce a fan that would sell and thus make money. In fact, I've had experience with products designed for specialized markets that were poorly tested prior to release and thus design flaws weren't discovered until they were in the field, when real people used them for real, and costs to correct the flaws is extremely high compared to the design phase.
I have a number of fans in my home. Four of them are ceiling fans with pull chains. All of the came with the first pull starting at the highest speed. One of them had to have the switch replaced, and it ended up with the slowest speed as the first speed. I have never had a problem with that. I also have a whole house fan with ceiling louvers. It is controlled by a wall switch that is temperature sensitive. You can adjust the switch so the fan will shut off when it cools down the room.
I have four other ceiling fans that are controlled by rotating wall switches. They are variable speed, and they all start at the high speed settings.
If the original story had a fan strong enough to pull the chain up, and into the fan, it seems to me the easiest way to solve that problem whould had been to add a heavier weight to the bottom of the pull cord.
In my experience one of the biggest failures of engineers is not going down to the lab and putting there hands on the finished product. In so many companies you write some kind of test request and someone else does the test. Engineers should be right in there helping with or definitely monitoring any test results.
Another failure of young engineers is to not understand how their components or systems affect the next guy down the line.
Further, how many times do companies actually take the time to do a good design review and/or design failure mode analysis.
I think this may also be an example of the failures that can occur without proper project management. Sometimes the system just isn't set up to allow someone to take into account how everything goes together. It seems like we bog our project managers down with so many presentations to upper management and cost analysis and other non-productive stuff that they don't have time to do simple design reviews. Kind of sad. Good example of what not to do, though.
Interesting that it appears the entire ceiling fan market has issues. I wonder at what price point does the american market place start to pay more for higher quality. Will we pay more for a higher quality toothbrush. probably. Will we pay more for a higher quality car, absolutely. But when it comes to that $50-$125 price range it appears we will pay for what we get and not complain. Until we start to vote with our dollars and communicate to businesses that we will pay for qualiy, we are doomed to stay where we are.
The rationale for the first position of a 3 position switch or pull of chain is that it gets the motor running at maximum torque. As the motor ages, this is needed. Starting the motor in the slowest position, lowest torque, may stall it and cause higher currents,etc. So starting at high speed is sensible and the user can then pull twice or turn switch to low position after the fan gets moving.
I have never seen a fan built for any residential, or commercial application for that matter that would not start on the lowest setting. If the motor has lost that much of it's available output power, something is drastically wrong, and will most likely catch fire or trip on thermal overload.
The only load the fan must overcome is the initial friction of the bearings. Reverse flow isn't something one worries about in residential applications.
Interesting since EVERY fan in my home (ceiling fans, stand mounted and range exhaust) uses the "Off/High/Medium/Low" sequence. The range hood is a variable speed unit that the pot is wired to go slower the farther you turn it. All three window air conditioners in my home and garage use that paradigm as well.
In fact, I can't think of a single instance where a fan I've delt with starts out on the lowest setting.
Tecsonics got it right, small motors are notorious for being difficult to start. Starting out at high speed insures that there is enough initial "kick" to get it moving instead of sitting "locked rotor" and burning up. It also gives immediate feedback to the user that the fan is actually "on".
To the original article, that does sound like a "never tested" assembly. The lack of a factory attached weight on the pull cord is a SERIOUS error!
OldGeek- Same in my house, even an over-the-range microwave with a software-controlled vent hood starts at the highest setting. OTOH, the blower motor in cars starts with the low setting. Perhaps because they are DC motors? I'm an EE but not a motor expert.
Kenish, you are right! I didn't think about blowers in automobiles!
Yes, DC motors have a steeper torque vs. speed curve than the multi-speed motors used in household fans and are much less prone to sticking. At reduced speed settings, the AC units are essentially constant torque devices where the torque is determined by the current in the windings. The current is controlled by tapped windings in the motor or by externally increasing the impedance. At lower speeds, these motors present an almost pure inductive load that shows little variation with speed. I once replaced the blower motor in a window a/c unit with a higher horsepower "universal" replacement. On turning it on, I found that I had no change between highest and lowest speed settings because the blower didn't load it enough to slow it down!
OldGeek, your last comment may be the most germane when it comes to uncovering the monkey business in this story. The fact that the pull cord could get sucked up into the airstream may be the real design flaw.
Yes Rob, I think you'll agree that regardless of the sequence of operation, the string would have been sucked into the fan as soon as it was put on "High".
I think OLD_CRUMUDGEON has a good point too. If mounted in a narrow hallway I could see the air velocity being more likely to cause the string to be sucked into the inlet than if mounted in a large room.
Still, for it to happen the string must not have had much if any weight on it!
Seems like a problem like the amount of weight on the fan pull cord is a hard thing for a company to miss. It's certainly not a matter of cutting corners. I guess it must be a matter of just not thinking.
I agree with one of the other bloggers. It seems that some of these comments are confusing ceiling fan operation with whole house central fan operation. One comment was that the ONLY thing the two devices have in common is the word "fan".
I have installed whole house fans, gable-end attic space fans AND consumer ceiling fans for my own homes, for nontwchnical relatives, and for friends over the years.
The whole house fans that I've installed are designed w/ a dedicated fan-duty squirrel cage motor, in the 1/2 to 1 hp power range. All of these fans were powered using a separate wall-mounted speed control switch w/ a horsepower rating.
Regarding "systems engineering"...... Having spent several decades doing systems engineering, both from a hardware point of view and a software point of view, I would offer that the manufacturers of these whole house fans really can't do a satisfactory, all-inclusive systems engineering test, since each attic space will have its own unique airflow equation. Some houses may have a peak vent, in which case the attic air will flow more easily; some houses will be designed w/ a truss system for the roof members, providing considerable flow resistande, others may have corner breaks, etc., while others may have soffet vents. ALL of these conditions will alter the flow rate of the air through an additionally provided (self-closing) louver device.
In fact, in our current house in FLA, we have a gable end attic fan w/ a gravity/flow operated louver. It has been in place for close to 20 years, and now it is evident that the rivets holding the louver blades to the frame are wearing, resulting in erratic operation of the louver when the fan is energized. However, this fan was installed to assist in "cooling" the attic space to aid the living space HVAC unit. Since the house has recently been re-roofed, and a full-length high-flow (passive) ridge vent installed, we hardly use the fan any more.
Almost all fans offered to both consumers and many commercial applications do indeed start with the highest speed setting because some of these fans are such junk that they wont start at the low speed setting, although they will run there once spinning. That is because the motor design is marginal and the phase shifting capacitor needed to make the motor run is a cheaply made device, with no capacitance to spare. So to avoid thye stall condition the fastest speed is what comes on first. That is useful because the inferior lubricant on many of these devices does tend to turn sticky and to restrict rotation. So the high speed start is definitely a "CYA" type of design choice.
All the ceiling fans in our house, some dating more than 20 years have split phase motors. And, to the best of my recollection all of them were manufactured either in Taiwan or mainland China, so I'm NOT talking American-made HUNTER or CASABLANCA (now defunct!) units. Yet, when I inspect the chamber where the direction switches & speed control swithes are housed, I see there is a multipole capacitor with the approx. values of 2 uf, 5 uf & 8 uf. So, one would assume that these are switched into the winding to provide the additional phase shift to change the fan motor's rotational speed.
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