1 kW power x every 10 sq. M.m.
However, it should be understood that this scheme was created back in Soviet times and was suitable for small buildings, with a warm attic, low ceilings and high-quality thermal insulation of walls and windows. Alas, now very few houses correspond to this requirement. This technique does not take into account innovative building technologies, therefore it can not withstand other climatic conditions, rather than were created in the USSR.
In order to make a qualitative power calculation it is also necessary to take into account a list of some conditions:
- Climatic - each region has its own specific weather.
- Overall - the area of the dwelling.
- Intra-climate - home insulation.
- Thermal - heat loss in a building.
- Quantitative - the necessary amount of heat to heat water.
It is noteworthy that in those houses where ventilation is forced, the calculation takes into account the amount of energy needed for heating the air. For calculations it is convenient to use a special program:
Important! When purchasing a gas boiler, the capacity calculation should be made with a margin of 20% in case a sharp temperature drop takes you by surprise. 
Does it make sense to buy an expensive and powerful boiler?
Modern heating technologies have automated systems that control moderate gas consumption. This saves you from unnecessary waste. But do not think that the calculation of the power of the gas boiler is not needed, simply a heating system with good technical characteristics and gas consumption parameters.
The correct choice of heating equipment depends on the duration of its robots. If the thermal power without the bases begins to increase, then:
- you will need to buy-re-buy new system elements( the previous ones will break);
- performance will naturally decrease;
- malfunctions and fast wear of accessories is provided;
- the presence of condensation in the chimney also does not bring anything good, etc.etc.
Standard gas boilers for
houses If a new gas boiler is to be installed in a typical residential house, the calculation of the capacity will not be difficult. There is a formula that takes into account the size of ceilings( no more than 3 meters).
MK = S * UMK / 10,
where MK is the boiler output in kW;
S - total area of the premises in sq.m;
UMK is the specific power of the boiler, for every 10 sq. M.m.
The last result is set according to climatic conditions and looks like this on average:
- Southern regions( warm) - 0,6 - 0,9 kW,
- Average band - 1,0 - 1,2 kW,
- Moscow region -1,2 - 1,5 kW,
- North - 1.5 - 2.0 kW.
Important! The formula shows how to calculate the power of the boiler only for heating the house. If you have a dual-circuit boiler, then you should add another 25% to your calculations. 
How is height of ceilings taken into account?
To make the most honest calculation for private houses with an individual project, you need to use the following formula:
MK = Qt * Kzap,
where MK is the boiler output;
Qt - building heat loss in the forecast, kW;
Kzap - safety factor, 15-20%.
The main indicator in this formula is the heat loss forecast, determined by the formula.
Qт = V * Рt * k / 860, where V is the volume of the room, m3;
Pt - difference of external and internal temperatures in C °;
k - coefficient of dispersion, depending on the thermal insulation of the room and the type of building.
For small buildings with good thermal insulation, the calculated heating capacity can be small, but not the fact that you can find the required low-power boiler on sale. In this case, you can buy a gas boiler, the calculation of the power which meets the large requirements, but do not run it at full strength. It is important only to remember that it is better to solve the problem of automatic regulation of heating automatic systems, so it is worth to spend on "smart" electronics.
It is noteworthy that some manufacturers prudently approached the question of counting and equipped in their devices a special function-calculator, which allows you to make the necessary data for calculation and determine the boiler's power.
Examples of such calculations can be seen in the table below. 
When should you bring in specialists?
The buildings of high complexity require careful design and calculation of the equipment's power, and only engineers who can take into account in the project and different ceiling heights, and the presence or absence of a warm floor, and the presence of a swimming pool, can cope with this task.etc.
Good luck with your calculations and profitable amounts on the calculator!
