Hot water tanks have become a necessity for many people in North America today. The standard size hot water tank is a 40 gal us or 33.3 imperial gal. Other sizes include 50 gal us, 75 gal us. There are power vents and regular tanks. In 2010 the gov. made it mandatory for all new home construction to have hot water tanks to be power vented for extra safety reasons and more efficiency.
If you have a regular vented tank and needs to switch to power vent the venting is not allowed to tie into the existing chimney and therefore re-doing the vent system.