Window fans are built specifically for use in open windows. They are designed to fit into a multitude of opening sizes, so as well as the fan unit itself there is often a extendable side panel section which can be adjusted according to the particular needs of the window opening it is being placed into. This then creates a sealed unit that only takes in or extracts air through the fan sections according the users desires. However not all fans have reversible modes, some fans may be extractor fans with only an exhaust mode, so check before purchase.
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The advantage of the twin unit over the single unit is not simply the extra power available by having two fans instead of one. Twin units are also more versatile because they can maximize the use of space of window openings that are oblong or rectangular, which is very common in homes and offices around the world (particularly in older buildings). A single fan by comparison is always going to be square (including the housing it is in), so if placed into a large rectangular window opening it will create a lot of wasted space to be sealed by the extendable panels.
Window fans have many different purposes, so choosing a particular fan depends on the job you need it to do. One of the most common applications for a these units are the extraction of moisture and smells from bathrooms and kitchens that do not have enough natural ventilation. In these cases, an extractor fan is all that is needed, so there is no need to go to the extra expense of purchasing a twin window fan with dual capabilities. However, a model that has multiple speed settings is a very useful feature in these instances, as the amount of moisture and/or odors that need extracting will vary from time to time depending on particular circumstances.
Another feature that will be very useful will be a window fan with an inbuilt thermostat. The thermostat can be adjusted and set. It can then automatically turn on the fan when the temperature in the room reaches a certain level. Window fans in certain rooms can also be wired into the light switch for the room in which it is placed. This enables the fan to come on as soon as the light is turned on (which should be when the room is in use) and is particular useful in small bathrooms with poor natural ventilation where moisture builds up quickly.
Other uses for fans include cooling down rooms which contain equipment, that give off a lot of heat, such as large numbers of computers and/or computer servers to give just one example. Whatever use you are looking for, there is plenty of choice between brands and models. An easy place to start looking for the ideal window fan is internet search engines and directories of businesses in your local area. Once you have an idea of what fans are suitable and in your budget, you can then approach some local businesses that can advise you further and hopefully supply and install the item for you.