Types of fan

Wall / window models
Most of these come with a kit that includes fasteners, a short piece of duct for wall mounting, internal and external grilles. Check that the kit has everything you need - and if you have to mount the fan in a window, make sure the kit includes any special parts for this.

To prevent draughts, look for models with automatic shutters that open when the fan is switched on and close when it is switched off. The alternative is external gravity louvres - but these really affect the amount of air that is removed, because the air has to push the louvres up out of the way. Fixed louvres reduce airflow by approximately 30 percent; gravity louvres reduce it by 50 percent or more.

Gravity louvres can also rattle in the wind.

If your fan is exposed to prevailing weather, you should consider cowls or louvres that offer extra protection against wind and rain. This is worth doing even if it does reduce your airflow.

Ducted models
There's plenty to choose from.

Some ducted models are similar to wall/window ones, and can be fitted to ducts that pass up to a vent in the roof. But the roof vent can be expensive to install - and there's the risk that the vent will be a potential leak spot.

"Inline" ducted models have the fan in the ducting line. They draw air from a grille in the room, through the duct, and into a fan located in the roof space. The air is then vented through a second length of duct to a grille under the eaves or a louvre on a suitable wall. To reduce noise, the fan unit can be mounted well above the ceiling.

Ducted models work best with short ducts. If you're doing it yourself, cut the supplied flexible ducts to the minimum length needed, make sure they're are not crimped, and support them at regular intervals. If you have no option but long ducts, choose a higher output fan. Condensation in the ducts can be a problem if your ceiling space is very cold.

Lights or heat lamps are often fitted to the inlet grille of ducted models. However, fitting a lamp can restrict the space for a decent-sized inlet grille, which then restricts airflow.

Where will the fan go?

A fan must be vented to the outside. New Zealand's building code prohibits venting directly into the ceiling cavity.

The fan will work best if it's:

• as near to the source of steam as possible
• as high as possible
• opposite where replacement air will come from (a door or a window).

For safety reasons, you can't have a mains-voltage fan where it can easily be touched from inside a bath or shower. But you can fit a low-voltage model which must be IPX7 rated for moisture resistance. Mains voltage fans may be fitted above 2.25 metres from the floor.

It's easiest to fit to a wall or window, but ducted models that use a grille in the ceiling to push the air out to an external grille can also be very effective. This type can also have heat lamps or lights fitted.

What extras do you need?

• Backflow flaps provide an alternative to gravity grilles or automatic shutters, especially for "inline" ducted models.
• An "on" indicator is usually a light. Fans tend to be quite noisy, so an "on" indicator is rather superfluous.
• "Off" timers are useful for allowing the fan to run for a few minutes to clear remaining steam after the bathroom has been vacated, especially if the fan is wired to turn on with the light switch. These usually cost from $50 to $100 - they're cheapest if you buy a model that comes with a timer.
• An "on" sensor detects steam or humidity and turns on automatically.
• Heat lamps not only make a bathroom more comfortable; they help reduce condensation.
• You may also have to buy other extras such as a different external grille, more ducting, or a roof vent ... it all depends on where and how the fan is best installed.