That is to heat an average family house in 200 sq ft (eg, in the suburbs) is sufficient boiler capacity of 20 kW. These relatively low-powered units are available in wall-mounted. They can be hung, for example, in the kitchen, connecting to the chimney. However, it should take into account that the maximum power produced is now wall boilers does not exceed 35 kW. So, if the total area of the cottage 350 sq ft, will have to install two wall or boiler, or a "serious" outdoor boiler, which is necessary to allocate a separate room in the house – the boiler room well ventilated.
The boiler may be single circuit and double-circuit. The first option is for heating the coolant in the heating system. Two-circuit option can work simultaneously on the heating system, and provide residents of the house hot water. This unit provides up to 15-20 liters of hot water per minute, which is quite sufficient for simultaneous operation of shower and kitchen sink. But do not forget that the double-circuit boilers operate in the priority of DHW. That is, until open valves with hot water, the whole boiler was spent on the hot water and heating system at home do not get heat. To ensure reliable operation of most modern boilers are equipped with built-in "group security "that includes a pressure gauge, automatic air, a safety valve and expansion tank membrane (compensates for the increase in fluid when heated.) We should also consider electronic ignition with flame control, protection from a lack of gas pressure and the water from the formation of scale, freezing, and automatic diagnosis of the state apparatus. All of these necessary functions to help prevent occurrence of accidents and dangerous situations. According to specialists ARISTON (manufacturer of heating and water heating equipment), to reduce heating costs and keep the house comfortable temperature is extremely useful function such as a smooth change in heat capacity.