Updated 05 Jan 2009

---

Introduction

A good extractor fan is often the best solution for a steamed-up kitchen or bathroom. Our guide explains what to look for.

Beware - grabbing the first likely fan from the hardware store may be entirely the wrong solution for your problem. You'll get a better result if you do some homework first. We help you work out what size, type and features will suit your needs.

What to consider

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.

What size fan?

As a rule of thumb 150mm models will suit bathrooms and kitchens. 100mm fans are usually only big enough to ventilate a toilet or laundry; and 120mm fans suit shower cubicles or smaller bathrooms. Really large kitchens may need two fans, or a combination of rangehood and fan.

Your fan should be big enough to work, but not so big that it causes draughts. In a large kitchen, it would have to be very big to be draughty. If you think your shower or cooking style creates more than the usual amount of steam, make sure you go for a large capacity fan.

Working it out

There are four steps to working out the size fan you need.

• Step one: measure the room. Say it's 2.0m long by 2.5m wide and 2.4m high - multiply all three dimensions to get the room volume: 12m³.
  
  
• Step two: decide how many air changes you want. Eight changes per hour is a good benchmark. That will give two complete changes of air in 15 minutes - long enough for a shower, plus a few minutes after, to remove remaining steam. Multiply the room volume by the air changes: 12 x 8 = 96m³/hr.
  
  
• Step three: Convert to litres per second, in case that's how the fan maker specifies its product. You divide the 96m³/hr by 3.6, which gives 27L/s for our example room.
  
  
• Step four: Buy a fan with a claimed hourly rate (in m³/hr) that's at least 20 percent above what you've calculated. Makers often quote the "free air movement" for the fan but don't allow for losses from grilles, passing down long ducts, and so on. Ask if there are figures with ducts and grilles. If you have no option but to exhaust through a gravity louvre, allow at least another 30 percent more. So for the sample room you may need around 32 to 36L/s (115 to 130m³/hr).