How a buffer storage tank works

The principle of operation of a buffer storage tank is based on the high heat capacity of water. For example, 1 liter of water cooled by 1°C can heat 1 m³ of air by 4°C.

The buffer storage tank is designed to collect and store heat for later use in heating or domestic hot water systems. It is used in systems where the heat generation peak does not coincide with the heat consumption peak. In addition, it can be installed in systems where a low-power heat source provides long-term heating and a high-power consumer requires rapid heat delivery.

Buffer storage tanks are installed:

with a solid fuel boiler, which generates heat only during combustion, while the heat stored in the tank can be used throughout the day.
with a heat pump to produce heat during nighttime low tariffs or sunny days and store it for evening or night use.
with a solar collector to store the heat collected during the sunny day for evening use.

Hot water is lighter than cold water, so the hottest water remains at the top of the buffer tank, while cooler water stays at the bottom.

Therefore, low-temperature sources (heat pumps and solar collectors) are usually connected to the lower connections of the buffer storage tank, and high-temperature sources (gas, electric, or solid fuel boilers) are connected to the upper connections.

Buffer storage tanks are often used in bivalent systems with two heat sources. In such systems, the low-temperature source is used as the primary, and the high-temperature one serves as a backup or for covering peak loads.

For example, in a heating system, a dual-source configuration allows the heat pump to cover 60–70% of the heating season, while the gas or solid fuel boiler operates during the coldest days when the heat pump’s capacity is insufficient. This mode of operation is shown in the animation.

The heat pump is connected to the lower part of the buffer tank. When it turns on, it draws cold water from the lower connection and delivers warm water to the middle inlet. Because warm water is lighter, it rises to the top, creating upward thermal flow inside the tank.

Once the temperature in the buffer tank reaches the set value, the control system switches off the heat pump and puts it into standby mode.

The thermal insulation of the tank keeps the water hot for several hours or even a full day, making it possible to use the heat stored in the evening during the night or the next morning. At the moment of heat demand, the tank is full of hot water.

The return line of the heating system is connected to the bottom of the tank, and the supply line to the top. The control system monitors indoor temperature and, when it drops below the setpoint, activates the pump that feeds cold water into the bottom of the tank, pushing hot water from the top into the heating supply line.

As soon as the water temperature in the heating system reaches the desired value, the circulation pump is turned off.

When the water temperature in the buffer tank drops below the setpoint, the heat pump is switched back on.

If the heat pump cannot bring the tank to the required temperature, the control system starts the gas boiler.

The time the heating system can operate using stored heat depends on the system's output and the tank’s volume. Therefore, when selecting a buffer storage tank, it is important to determine which factor is more critical: providing heat to a system of a specific capacity for a set duration, or accumulating heat from a given heat source over a certain period.