Boiler body is insulated with technical aluminized wool KIM-AL30 (photo below) and so during combustion the body is completely cold. If the boiler body is insulated with a simple wool without aluminium foil or cheapest wool is used, it is impossible to achieve high quality insulation. In that case, boiler body will be warm during combustion. And although manufacturers are well aware of the fact, they still do it for higher profit margins.
It is estimated that heat lost due to defective boiler insulation could be used to heat up to 14-18m² area.