Heating and hot water supply are two separate systems, they do not contact each other. In the heating circuit, technical liquid circulates - water or antifreeze. And in the hot water supply system - running clean water, which after use is drained into the sewer.

A gas boiler can prepare hot water in two ways, depending on this, boilers are either double- or single-circuit.

Double-circuit boilers. An additional heat exchanger is built into such a boiler, which allows heating water in the flow heater mode. It works like an electric flow boiler: as soon as the owners open the tap, the gas boiler starts heating the water.

This scheme has its disadvantages:

You need to wait 10-15 seconds until hot water comes out of the tap. This time is spent switching the boiler to the water heating mode and "pumping" the section of the pipe from the boiler to the tap.
The boiler's capacity is enough for one tap, maximum two. If you open more at the same time, the boiler will not have time to prepare hot water for everyone - and it will come out warm.
It is impossible to install a water heated towel rail so that it works all year round. With a double-circuit boiler, the heated towel rail depends on the heating and will switch off as soon as the owners decide to turn off the heat.
But a double-circuit boiler takes up little space, it can even be hung in the kitchen, and it will fit perfectly into the interior.

Single-circuit boilers. They always work in one mode - for heating. And to heat hot water, an indirect heating boiler is installed in addition to the boiler - a tank with a built-in heat exchanger. It's like a kettle, only instead of electricity, the heating comes from the coolant.

A single-circuit boiler maintains a set water temperature in the tank, so there is always a supply of hot water in the house. The scheme is similar to the operation of a storage electric boiler. The tank volume is selected depending on the number of residents and taps. For example, if there are two bathrooms and a bathtub in the house, a tank of at least 150 liters is recommended.