First choice: turbo or atmospheric?
We are used to the fact that a standard gas boiler( atmospheric) has a chimney through which the combustion products are released, but if this does not happen, remember that this one is referred to as a chimney, that is, turbo. In addition to the fact that it does not have a chimney outlet, it emits combustion products( which naturally exist) through a special tube and a hole in the wall. Smoke and other exhaust gases are neutralized by means of a turbine, which is what the term "turbo" means. 
Therefore, if you designed a chimney for a private house, then you need to buy a chimney boiler and the proposed option is absolutely not suitable for you. But in the event that you are the owner of an apartment, then this view will be for you the most optimal option. 
Second choice of gas boiler: single or dual circuit?
A single-circuit boiler performs the process of heating water for radiators, while two circuits can heat water, both for radiators and for consumer use( shower, sink, etc.).It is also noteworthy that if a gas boiler is of small capacity, then it is capable of heating a small amount of water, for example, with a power of 7 kW, only 3 liters of water per minute can be heated. So that there is enough water for everyone, and it normally circulates, so that the boiler can heat 10 liters per minute and not less. The power for this capacity should be at least 20 kW.The dual-circuit boiler is also very convenient for both floor and wall installation.
Therefore, it is important to buy a large capacity boiler so that it can cope with the task of both radiator heating and hot water supply. Otherwise, you will need to spend more and buy a gas column. You should buy a system with maximum power, if you want it enough for comfortable use of hot water. 
Third choice: one or two heat exchangers?
Actually, two-circuit boilers are divided into two types depending on the heat exchangers mentioned. The first is a separate, and the second - a bithermal heat exchanger.
The second one is responsible for heating water for radiators and hot water for consumer service. It is sensitive to bad weather and bad water, for example, if the water does not match the system, the heat exchanger is clogged with scum, which reduces the capacity of the water heater. This heat exchanger also does not last long, as it often burns out. To help him in the performance of his functions can only powder from scale, but he can be ineffective in this problem.
The separate heat exchanger is divided into two. One warms the water for the radiators, and the second - for the bathroom and other maintenance. This type is less sensitive to bad water and longer life, which is important. 
Fourth choice: condensing boiler, yes or no?
A condensing boiler is one that uses a large amount of energy that is released during the condensation of a water vapor. At the slave's mind, this means that the exhaust gases, having a high temperature, are used by the condensing boiler, while the water that enters the boiler starts to bask in those most exhausted gases, and then heats up in the burner with gas.
Such a choice of gas boiler is expensive, but in the process of work they will save you 20% of your funds. Often, they are used to insulate the cold floor, because they even with the minimum indicators give the maximum efficiency.
Choose only what is beneficial to you and to everything fit with the mind and then you will be lucky!
