Cost – Biomass versus conventional boilers. Biomass boilers start at about £7,000 for a 12kW domestic version – which is sufficient to provide heat and hot water for a 4-bed house.A comparable sized gas boiler will only cost £2500 to install. A bigger biomass boiler with an auto-feed hopper may cost closer to £12,000.ost – Biomass versus conventional boilers. Biomass boilers start at about £7,000 for a 12kW domestic version – which is sufficient to provide heat and hot water for a 4-bed house.