Conventional gas water heaters aren’t energy efficient; they waste a significant amount of gas. But they have a big advantage: they are inexpensive.
Common gas water heaters
The fact that common water heaters are inexpensive can make them a good choice for buildings with modest hot water needs - or as backup solar water heating systems.
New condensing gas water heaters and boilers
Newer gas water heaters and boilers may use the condensing technology, which makes them a lot more energy efficient. Condensing gas water heaters (and condensing boilers) can provide energy savings of 20% or more, compared to non-condensing units.
Unfortunately some of these these units (the American-style boilers) are a lot more expensive.
Top stainless-steel tank boilers prices can amount to $4,000 to $8,000, and are especially useful in homes with hydronic (hot-water) space heating.
These last type of boilers/heaters only become really advantageous for buildings with large hot-water needs.
Boilers Guide For Energy Efficiency
Electric Vs. Gas Water Heating
Condensing vs. Regular Gas Water Heaters
German Style Super-insulated Passive Houses
New Homes Insulation
Super-Air Sealing Guide