combustion, i.e. boiler with power output below 250 kW, depending on national legislation. However, the only EU- adopted regulation is standard EN 303-5 that set relatively weak demands on boilers that are newly introduced to the market [2]. Modern boilers easily comply with the requirements mentioned in this standard. However, the boilers are mostly operated by persons with minimal knowledge about the operation principle. Furthermore, most of the boilers are “controlled” only in the way of on-off control systems with aim to sustain desired temperature of outlet heating water [3]. Especially during the unstable regimes can be increased CO and hydrocarbons emissions whereas the efficiency significantly drops. This might be taken as a minor problem due to low power output compared to large boilers unless it comes to the total number of the boilers installed. According to [4], the total yearly sale of small-scale biomass boilers in the EU is in the range of several tens of thousands, dating back to 2002. With such a number of the boilers, improperly controlled or uncontrolled biomass combustion may have significant negative impact on local air quality that need to be solved. Some of the national legislations in Europe have included emission limits also for small scale boilers to allow monitoring of the local pollution sources. However, practical application is quite complicated and limited due to the fact, that the operators of such boilers are private persons without special knowledge.