Hi
I think these extractor fans are not a good solution anyway. Theyre not particularly effective, theyre noisy and annoying, and the bearings are guaranteed to deteriorate making them very noisy. And they simply throw your heat away.
Alternatives are:
HRV, which costs very little more, works like a fan but recovers most of the heat.
A window lock enabling it to be locked 1/4" open - good, effective and free for most of the year, but not so good for the coldest few months.
Dehumidifier - a bit more to install, much more efficient, doesnt ventilate or lose any heat.
Finally the simplest option of all, an adjustable vent. Easy, cheap, simple, no run cost, and effective year round.
Anything but a fan, they are just the worst option.
Regards, NT