A very clear focus on environmental matters has developed around the world. When it comes time to replace old appliances, buy new automobiles and so on, a question many of us ask is - How much damage will it do to the environment? How can I minimize the impact? This is where energy efficient space heaters come in.
Of course the answer depends on what you are looking at. If you are looking to boost your heating with a small standalone unit, the vented heaters that are more energy efficient space heaters will take their intake gases from outside as well as deposit the exhaust fumes outside (that bit is a safety issue!).
The good news is that natural gas and propane space heaters - be it a hot water system or not - are more efficient than ever. In fact if you are replacing a unit that is, say, ten years old, you should realize a significant instant saving in fuel bills.
The same pressure from international protocols and local lobbyists that has brought gas heating into line has worked on other heating sources- oil fired home boilers for example - and also led to a resurgence of fuels that cause less greenhouse gasses - good old wood, for instance.
In other words, you can find energy wise units whatever fuel source is easiest for you. However, best of all is probably a solar system or a geothermal heat pump, if you really want to invest in energy efficiency.