If you are in usa the cost per kwh- a useful unit of energy (kilowatt per hour) has natural gas at 7 cents electric at 20 cents so gas is cheaper in energy costs for winter heating of a furnace and hot water heating compared to electric..
The green options:
Air source or ground source heat pumps for heating and summer cooling are high capital costs but low running costs. Also heat pumps for water heating have a very low energy cost per kwh of water heated.
Solar thermal panells for heating of stored hot water have a high capital cost and very low energy costs.
Biomass boilers for winter heating via a furnace and hot water heating may be a good option in off gas areas compared to oil fired or electric but expect higher maintenance costs compared to gas, in some areas there may be a government grant for biomass which is sustainable wood chip.
See attached links for options.
Whatever's cheapest.
None of those options are what I would call "green." They are all fossil fuels (including electricity, which is often derived from fossil-fuel-fired power plants). Even nuclear-sourced electricity isn't that fabulous an option - dig into the hassles of dealing with spent fuel rods and you'll see why.
So my recommendations are these:
1) Use whatever is cheapest. While you are saving as much money as possible,
2) Insulate your place. Insulation means needing the boiler less.
3) Upgrade the boiler to the highest possible efficiency. On top of needing it less, you'll need less fuel for it.
4) Convert to renewable energy. When you don't need much, the more diffuse resources of sunlight and wind are better able to provide for your needs. Retain the high-efficiency, smaller boiler to carry you through extended sunless periods.
Id go with gas it has higher btu than electricity an more efficient. An it burns cleanner than oil an coal.
Not sure bout coal but oil is bad for sooting up if not cleaned regularly an properly. (Unfortunately i have lots of experience from cleaning them.)
If you are using a boiler that heats up the water, I would go with a natural gas fired. Clean, cheap, and safe. There are a lot of alternatives that have been developed in the last few years. Check with a reputable hv/ac contractor and get some options and prices.
In my opinion, gas will be the ideal fuel for boiler, for it's easier to install and manage; moreover its cleaner and more efficient as compared with other kinds of fuel. As far as safety being concerned, the boiler should be located away from or outside the household.
Yip
Anthracite coal. It has a high heat content, burns with a blue smokeless flame and makes very little ash. It is also less expensive than any other kind of fuel. A stoker can be installed with your boiler so you will not have to shovel coal. Anthracite coal is beyond a doubt the best fuel for home heating that there is.
Your question assumes that everybody agrees what is "better". But they don't.
Most people probably think cheapest is best, so for most people, gas would be best.
However, you might have a cheap source of coal or gas.
Some people think that fossil fuel boilers pollute and have a risk of carbon monoxide poisoning, so they prefer electricity.
If you are planet-friendly, live in the right place, and can afford it, the best thing is really good insulation and solar heating.
If you live near a fast-flowing stream you could generate your own electricity.
Gas will likely be cheap and plentiful for the next 20 years or so.
Coal? - Forget it unless you're talking about an industrial installation with scrubbers.
Oil is likely to get expensive.
If you heat with electricity, and live in a temperate area, look into a heat pump.
Air source or ground source heat pumps for heating and summer cooling are high capital costs but low running costs. Also heat pumps for water heating have a very low energy cost per kwh of water heated.
Solar thermal panells for heating of stored hot water have a high capital cost and very low energy costs.
Biomass boilers for winter heating via a furnace and hot water heating may be a good option in off gas areas compared to oil fired or electric but expect higher maintenance costs. it is cleaner on your appliance and less polluting.