The problem with fossil fuels is that they were formed many millions of years ago when carbon dioxide levels were much higher. Carbon Dioxide was absorbed when the plants or microorganisms were formed and locked underground allowing the Earth to cool to where we are now, which is a climate that suits the current species of plants and animals. When we burn fossil fuels to heat our homes, heat our water or produce electricity we release that stored CO2 back into the atmosphere. As we increase the atmospheric carbon dioxide concentration back to prehistoric levels it absorbs more heat from the Sun, producing the greenhouse effect.
When we burn renewable biomass like logs, wood pellets or chips, animal waste, oil from plants and crops, waste from food production etc. it basically becomes part of the natural carbon cycle. The Carbon Cycle is the natural process of uptake of carbon dioxide from the atmosphere by plants directly and animals indirectly and its return via decay. Burning these materials as fuel just speeds up the decay phase and produces no net increase or decrease in carbon in the atmosphere. While the equilibrium is balanced, the climate is stable and using biomass as fuel helps to sustain the balance.
Biomass was the first fuel we used to produce heat and has been used exclusively throughout most of human history. In the past, open fires produced heat for single rooms but the efficiency was low with much of the heat lost up the chimney or negated by the drafts. Nowadays, the Firex Biomass Wood Fired Hot Water Heaters supplied by Automatic Heating reach 82% efficiency so you can safeguard the planet and use a cheap, convenient fuel.