Ceiling fan

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Ceiling fan 2758-5.jpg

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.


Lighting

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.


Balancing

Imbalance causes shaking at high speed, and noise.

Pair the blades

The first step to balancing 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.

Of course this can't be done with fans with odd numbers of blades. With these, just try to get as good a weight distribution as possible.

Adjust blade height

Step 2 for an unbalanced fan is to ensure all blades are at the same height. Height differences can cause significant unbalance. Rotating the fan by hand and comparing the blade tip height to a mark on the wall soon shows any that don't line up with the others.

Realigning 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, the wood is much weaker than the metal. It can be hard to judge the amount of bend applied, so can take several goes to get it right.

Another option is to use a washer to adjust the position of a blade. This can adjust a blade up or down by upto about half an inch. However it modifies the design trivially, which could affect the warranty on a new fan. A washer is placed between metal bracket and wooden fan blade at the bolt fixings, and this moves the outer blade end up or down, depending on whether the one central bolt or 2 outer bolts are washered. Aim to balance opposing blade weights when adding washers.

Balancing weights

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. This can't be done by just standing there with a crayon, humans move too much. Position a step ladder very close to the fan hub and slowly insert a crayon between the 2 until it just touches the fan hub. The crayon marks the hub on the lighter side, but not elsewhere. Watch where your head is positioned when doing this!

Now any remaining unbalance is corrected by adding or subtracting weight from the blade assemblies. This is done by adding extra washers to the blade mounting bolts. If necessary the bolts can also be lengthened.

In the unlikely event that this isn't enough, wood blades can be trimmed on the heavier side, and very small weights can be securely fixed on the lighter 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, resin glue & sand can be used on the outer end of the cane to add weight. Another option is to replace the blade fixing bolts with longer ones of the same diameter, adding 2 nuts or extra washers for weight.


Noise

Ceiling fan noise can be a significant issue in old buildings, which often have less rigid floor structures and typically no noise reduction measures built in.

Balance

Unbalance is the biggest source of noise, so achieving accurate balance makes the biggest difference. Usually this is enough to solve the problem. Further noise reduction measures include the following.

Rubber washers

Noise transmission may be reduced by using a penny washer and rubber tap washers on each mounting screw, such that the fan weight sits on a rubber disc at each mounting point, with another rubber disc above the mounting plate. 4 tap washers are used above the mounting plate altogether, so the fan can move up or down a millimetre or so. This reduces vibration transmission. The mounting screws are done up until there is no slack movement, but the washers aren't compressed, so the fan can move just a little.

Note this method is not suitable for nut & bolt fixings, which must be tightened firmly.

Rubber on Shade

Put a couple of rubber bands round the lightshade/s where the fixing screws touch them. This prevents the shade rattling and acts to some extent like a shock absorber for the fan hub.

Large fan

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 much reduce the risk of noise nuisance.

However this is rarely needed, as the above measures are very effective.

Cooling

The cooling effect of ceiling fans is sometimes misunderstood. Fans reduce the thermal resistance between air and objects, thus people and other bodies that are above ambient air temperature are cooled. They produce no cooling effect whatsoever on the air or walls in the room. 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.


Speed Control

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 with handheld remote controls are also available now.

Speed controls normally 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.

Built in cordpull speed controls are simply switch and capacitors. Separate wall mounted controls are normally electronic triac based, enabling them to work correctly with a range of fans.

Make a Speed Control

Its possible to make one's own wall mounted fan speed control using a 2- or 3-gang lightswitch and some capacitors & resistors. Measurement with a multimeter and maths are required.


Safety

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

Blades

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. Fans can often be converted to ceiling huggers by replacing the suspension rod with a shorter one. The fan must be properly balanced before conversion, as the drop rod acts to absorb much of the forces caused by imbalance, and the minimal drop rod of a ceiling hugger greatly increases stresses on an unblanced fan.

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 minor.

Mounting

Falling ceiling fans can cause injuries. They should be attached securely to ceiling joists, or a well fixed nogging. They should never be fixed to plasterboard or laths, neither of which are capable of reliably handling the forces fans produce long term.


Fire

A stalled fan can catch fire. Fans should incorporate an overheat cutout to prevent this.

Fans lacking this should be only mounted on a fireproof ceiling, such as plaster or plasterboard.


Custom Blades

Its 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. A lower speed than originally 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 possibly 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 much increases the blade injury potential. The main injury limitation mechanism is that the thin wood blades can snap off from the high momentum hub.

Its posible to use asymmetric blade systems if you're able to balance the forces effectively, but this is non-trivial. If you can write a little code to calculate the load forces and balance, its not hard.


Energy Efficiency

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 power 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.

Blade Types

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 your own using small efficient dc motors. The parts are not expensive.

When lifting pieces of flooring is practical, home made fans can have their motors put in the void between ceiling and floor, side fixing them to a joist. This must not be done with commercial fans, which use motors that rely on forced air cooling.


See Also