Best Reference Book for HVAC Age?

Hi Members!

Please give me your opinion on the best reference book/CD to determine the age of HVAC units.


Prestons Guide is pretty good.

Prestons is OK, but it lacks lot of info.

When all else fails post it to this board for a quick and accurate answer.

he was asking for a guide that provides the age of HVAC units…Prestons provides that info.