How long should a new system to last?
Industry experts suggest that a new unit should last between 12-15 years if properly maintained and with regular preventative service measures. All heating and air conditioning systems along with ventilation require regular and proper maintenance to maintain maximum efficiency. Frank Weglarz has a blog at http://frankweglarzhvac.blogspot.com/ with maintenance tips.
However, even if homeowners are vigilant and proactive in maintenance, many systems will only last a maximum of 20-25 years. If you live close to a sea coast, this number is reduced due to the outside conditions of the air. At the page http://www.whitepages.com/name/Frank-Weglarz you can read more.
A gas furnace should last from between 15-25 years. Heat pumps will last around 15 years also. If you have a tank less water heater, expect a 20 year life before a replacement is needed. With rapid technological improvements, thermostats will usually be replaced before their 35-year lifespan ends. Read about tank less water heaters at http://www.foxcarolina.com/story/29549491/frank-weglarz-explains-the-move-to-tankless-water-heaters.