Difference between revisions of "Ceiling fan"
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Revision as of 13:18, 14 May 2007
A Ceiling Fan is a serious comfort booster on hot summer days. They can also be used to move down warmer air from very high ceilings in winter.
The cheapest & easiest way to install a ceiling fan is to put it where a light fitting pre-exists. Consequently most domestic ceiling fans include a combined light fitting. These light fittings are frequently not fully satisfactory.
Single glass bowl fittings have limited power rating, typically insufficient for room lighting. They are also mostly of small size, making golfball filament bulbs the only usable bulb option.
Triple (or more) glass shade lights too often point straight at people in the room, which is an unpleasant form of lighting.
Imbalance causes shaking at high speed, and noise.
The first step to balancing is to ensure all blades are at the same height. Height differences can cause significant wobble. This is achieved if necessary by slightly bending the metal brackets the wood blades are attached to. Do not do this by pulling on the wood blades, as the wood is generally weaker than the metal.
Step 2 for an unbalanced fan is to remove the blades and weigh them, and group them into 2 pairs, the lighter pair and the heavier pair. Now reattach them with each pair of blades mounted opposing each other. This means that going round the fan in sequence the blades will be Heavier-Lighter-Heavier-Lighter.
If there is still unbalance after this, the imbalance position can be determined by holding a crayon very steadily very close to the hub as it rotates at max speed. The crayon will mark the hub on the lighter side, but not elsewhere. Be careful where your head is positioned when doing this!
Finally, any remaining unbalance can be corrected by trimming the wood blades on the heavy side, or by securely fixing very small weights on the light side. Care is needed when fixing weights, as the fixing will be subject to heavy vibration for years of use, and must remain totally secure. Flying metal weights are not recommended. For blades with a cane side where the cane is uppermost, woodglue & wood chips can be used on the cane to add weight.
Ceiling fan noise can be a significant issue in old buildings, which have less rigid floor structures and typically no noise reduction measures built in.
Unbalance is the biggest source of noise, so achieving accurate balance can make the biggest difference.
Noise transmission may be reduced slightly by using a penny washer and rubber tap washers on each mounting screw, such that the fan weight sits on the rubber discs.
Where near silent operation is important, choose a large fan and operate it at a low speed. Picking a fan a size larger than needed is a good way to reduce the risk of noise nuisance.
The cooling effect of ceiling fans is often misunderstood. Ceiling fans cool people and other bodies that are above ambient air temperature, but they produce no cooling effect on the air or walls in the room whatsoever. Leaving a fan running when not in the room achieves zero cooling effect. In fact the energy used by the fan is converted to heat and warms the room by a tiny amount.
Most domestic fans have 3 speeds selected by a pull cord on the fan. Fans are also available with a wall mounted speed control. Fans are probably available with handheld remote controls by now.
Speed controls traditionally switch the fan on at full power first, then as the switch moves further reduce the power. This is to avoid the possibility of a fan failing to start on low power and burning out.
It is possible to make one's own wall mounted fan speed control using a 2- or 3-gang lightswitch and some capacitors & resistors.
The magnitude of the risk posed by spinning unguarded ceiling fans is debated. The Mythbusters team has carried out tests, with results summarised here: Ceiling Fan Decapitation Test
Mount the fan high enough to avoid accidental impacts.
Where headroom is insufficient, a ceiling hugging fan may be used. These don't blow air as effectively, but they do avoid injury. Fans can often be converted to ceiling huggers by replacing the suspension rod with a shorter one.
Where headroom is insufficient for even a ceiling hugging fan, designs exist that can be used safely within touching distance, but these are rarely seen on sale. The simplest solution here is to use a small enclosed fan. These are noisier, but when temperatures become unpleasant, such issues may seem trivial.
Ceiling fans can still cause injuries, and should be attached securely to ceiling joists, not fixed to plasterboard or laths, neither of which are capable of reliably handling the forces fans produce.
A stalled fan can catch fire. Fans should incorporate an overheat cutout to prevent this.
Fans lacking this should be mounted on a fireproof ceiling, such as plaster or plasterboard.
It is possible to make your own custom blades if you are able to address the issues involved.
Blade sets can be made to any workable shape. Care is needed to avoid overloading the fan motor, blade sizing must be picked to give the same loading as the original design. With most fans loading margin is close to zero to minimise cost. Overload them and the motor fries. Probably the easiest way to measure loading is to measure fan speed on the lowest power setting. Lower speed means higher loading, and vice versa.
The blades must be robust enough to avoid any twisting in use, which would increase loading, leading to a positive feedback cycle and a flying ripped off blade section. This means no overly thin wooden sections.
If thin blade sections are wanted, metal backing can make this possible. However this approach should not be used on fans hung where there is any possibility of accidental touch, as it increases the blade injury potential. One of the injury limitation mechanisms is that the thin wood blades can snap off from the high momentum hub.
It is posible to use asymmetric blade systems if you're able to balance the forces effectively, but this is non-trivial.
Ceiling fans produce a good deal of comfort for modest energy use. Comfort per energy use is very roughly an order of magnitude better than air conditioning, though naturally the degree of comfort improvement is more limited than ac.
Energy use and energy efficiency both fall at reduced speeds for nearly all fans. Reduced speed is achieved by reducing current, reducing torque and increasing motor slip, with consequent increase in rotor dissipation. Energy reduction at lower speed is considerable, but not proportional to speed reduction.
Fans with a/c
When using a/c, simultaneous use of ceiling fans can reduce a/c run costs significantly by giving a lower cooling requirement for comfort.
There are 2 types of ceiling fan blades in use. The flat sheet blades most popular are not particularly efficient. Propellor blades are found on high efficiency fans, and roughly halve energy use for a given flow.
High Efficiency Fans
For offgrid electricity systems, high efficiency fans should generally be chosen. These have propellor blades and motors that remain energy efficient when speed is reduced. With these, fans using as little as 6w can give a gentle breeze. The downside of high efficiency fans is purchase cost. Where a whole house is to be equipped with such fans it may be tempting to make one's own using small efficient dc motors. The parts are not expensive.