Buffer storage tank and heat pump: Does this make sense?

A buffer tank consists of a well-insulated steel or plastic container filled with heating water. It is located in the heating circuit between the heat generator (e.g., the heat pump) and consumers such as radiators or underfloor heating. It stores excess heat when the heat pump is supplying more than the house needs and releases it again when required. This allows it to “shift” energy to times when the heat pump is operating at low capacity or not at all.

BUFFER STORAGE TANK ≠ HOT WATER TANK

It is important to distinguish between heating buffer storage tanks and hot water tanks. The latter supplies drinking water and, for hygienic reasons, is designed as a closed tank with legionella protection. Heating water and drinking water must never be mixed – this is why buffer storage tanks are not drinking water storage tanks.

• Buffer storage tank = heating water (not drinkable).
• Hot water storage tank = drinking water (domestic water).

Whether a buffer tank is useful for a heat pump depends largely on the overall system. The decisive factors are not only the type of heat pump, but also the size of the heating system, the control behavior, and the individual operating conditions in the house.

TYPICAL APPLICATIONS

• Air-to-water heat pumps: These pumps require energy to defrost the evaporator. A buffer tank provides the necessary heat without radiators cooling down.
• Systems with radiators or small water volumes: Classic radiators have a low water content. Without a buffer, rapid heating can lead to frequent cycling of the heat pump.
• Hybrid and bivalent systems: Anyone who combines their heat pump with a photovoltaic system or a boiler needs the buffer tank as a hydraulic hub.
• Restricted periods & dynamic tariffs: With low night-time electricity tariffs or temporary shutdowns (energy supplier restricted periods), the storage tank compensates for the lack of heat supply. However, according to new regulations, energy suppliers must provide a minimum output of 4.2 kW, which means that storage tanks are less often absolutely necessary.

PRACTICAL EXAMPLES OF THE USE OF A BUFFER STORAGE TANK

• Altbau mit Radiatoren (120 m²): Eine Luft‑Wasser‑Wärmepumpe liefert 8 kW Heizleistung. Mit einem 600‑l‑Puffer (75 l/kW) reduziert sich das Takten erheblich; die Radiatoren bleiben warm, das Abtauprogramm läuft störungsfrei.
• Hybridanlage mit PV (200 m²): Die Wärmepumpe nutzt PV‑Überschuss für die Warmwasser­bereitung. Ein 500‑l‑Puffer speichert Solarwärme aus der PV‑Anlage oder einem Solarthermiemodul und speist sie in den Heizkreis ein.

WHEN CAN YOU DO WITHOUT A BUFFER TANK?

• Pure surface heating: Floor or wall heating systems have a large storage capacity; the screed stores heat. In modulating systems, the buffer tank can be omitted.

Direct integration of battery storage and PV: If you store PV electricity in a battery storage system and use it for heat pump operation, you can reduce the size of the buffer tank or do without it altogether.

The correct size of the buffer tank affects efficiency and comfort. A common rule of thumb is 50–100 liters per kW of heating capacity. The following applies:

Too small: The tank discharges quickly; the heat pump cycles more frequently, which increases wear and tear.
Too large: Higher acquisition costs and more heat loss. In addition, heating takes longer, which is impractical in the event of rapid load peaks.

EXAMPLE CALCULATION

A precise design takes into account the water volume of the heating circuits, the type of control (modulating or on/off), and additional heat sources (e.g., solar thermal energy).

The most sensible solution is therefore a combination of photovoltaics with a heat pump and battery storage. Solar energy produced during the day is first used in the buffer storage (warm) and any surplus is stored in the battery to supply the heat pump with electricity when needed. This maximizes self-consumption and reduces dependence on the grid.

Smart-Home-Integration

In the best case scenario, the heat pump is integrated into a smart home system and a central control unit connects the heat pump to the buffer storage tank, the battery storage unit, and the PV system. The software then decides when the heat pump runs, charges or discharges the buffer, taking into account weather forecasts and electricity prices. This increases the efficiency of the heat pump and the degree of self-sufficiency.

A buffer tank for heat pumps costs around €1,200–3,000, depending on its volume and features. Installation costs for piping, pumps, and controls must also be taken into account. Buffer tanks are eligible for federal subsidies for efficient buildings (BEG). If a heat pump with a buffer tank is installed, the basic subsidy is 30%. If a natural refrigerant (e.g., propane R290) is added, there is an additional 5%. In many cases, the subsidy amounts to 40–70%. An individual renovation roadmap (iSFP) increases the subsidy.

Even though buffer tanks are considered low-maintenance, the following points should be noted:

Ventilation: Air pockets impair heat transfer.
Check insulation: Damaged insulation leads to heat loss; replace the insulation layer in good time.
Check sensors and pumps: Calibrate temperature sensors and clean circulation pumps.
Legionella prevention for combination storage tanks: Perform regular thermal disinfection and maintain a hygiene layer.

A buffer tank is not a mandatory component of every heat pump. It is useful for air-to-water heat pumps, radiator systems, hybrid heating systems, or dynamic electricity tariffs. It may not be necessary for pure underfloor heating systems or modulated heat pumps. Careful dimensioning is required to avoid cycling and increase efficiency.