Installed a few (ahem) years ago and I've never changed a filter. Help! It vents straight outside, and doesn't have a removable carbon filter that I can see. What am I supposed to change? Too embarrassed to go into a shop and ask, or ask anyone in RL!!
Can't find the instruction manual if we ever had one, it's one of those things that I'm probably just supposed to know, being mid-40s. 
Thanks!