Newer boilers offer far greater efficiency than older boilers, but choosing between gas and oil boilers depends on a number of factors. All other factors being equal, gas-fueled boilers are typically less expensive to install and, if local gas connections are provided, are likely cheaper to run.
Coal-fired boilers were popular in the past, but environmental factors and coal's relatively low energy density caused most to be replaced by oil- and gas-fired boilers once the infrastructure for storing and transporting these fuels was built. Many of these older coal boilers were upgraded to burn oil and, in some cases, gas, which led to inefficient design and wasted energy. Similarly, wood-fueled boilers and furnaces have fallen out of favor, as of 2015, due to environmental and economical issues, although they are still used in regions where importing gas or oil is prohibitively expensive. A typical oil or gas boiler provides a significantly more efficient operation.
While gas-powered boilers are more popular in most of the United States and most of Europe, oil-fueled boilers allow users to safely store a larger amount of fuel, making them preferable in remote regions. In areas where gas must be imported and stored, oil may be a cheaper option. However, oil-powered boilers are more complicated, resulting in a higher upfront investment.