Major manufacturers, such as Honeywell and Whirlpool, allow owners to access heater manuals online by using model numbers. Manuals for especially old models, however, might not be available.
Digitized manuals are generally easy to find for newer furnaces and boilers made by large manufacturers, and using the unit's model number makes it easy to search. If the desired manual can't be found on the manufacturer's website, other sites may have scanned versions, although some websites charge money for the service. Manuals are often available as PDF files, which may require a special program to read if the user's Web browser used doesn't recognize the file format.