Looking for an environmentally friendly product? the combi wall is for you! this energy star ® certified boiler has an annual fuel utilization efficiency (afue) of up to 95% while homeowners with older gas furnaces may only be using as little as 60% of energy consumed heating is the largest energy expense in most homes accounting for 35-50% of annual energy bills in colder parts of the world. Perfect for small homes with 1 bathroom. We support our business members with.