Trion humidifiers are sorted by model number. The model you need is based upon the size of your home or office and hoe tight your building envelope is. Older buildings are typicaly leaky when it comes to outside air leaking through cracks. Newer construction is typicaly much tighter and leaks less outside air. Parts for all the past and present models are located on a different link which can be found in the left hand task bar of this site. If you have any questions feel free to chat on-line or call us for free assistance.