Planning aid for heat pumps

Determination of the total heat demand

For dimensioning the heat pump output it is important, to determine the total heat demand. This calculation must be according to the current valid standards of the respective country, for example, in Germany it is the DIN EN 12831 for the heating requirements of the building and the DIN 4708 for the domestic water heat demand. However, for the calculation of an offer it is sufficient, to determine an approximated heat demand firstly. According to the year and type of construction of the building it is possible, to determine the specific heat demand:

Passive house: 0,015 kW/m²
New construction according to EnEV: 0,04 kW/m²
New construction with standard insulation: 0,06 kW/m²
Renovated old building with insulation or new building without: 0,08 kW/m²
Old building without insulation: 0,12 kW/m²

Rule of thumb for approximated determination of the heat demand

living space [m²] × specific heat demand [kW/m²] = building heat demand [kW]

Low-energy house with 160 m² living space × specific heat demand 0,06 kWh/m² =
160 m² × 0,06 kW/m² = 9,6 kW (building heat demand at -16 °C).

Please note: The heat demand does not indicate the heat work of the heat pump.

Heat demand for domestic hot water preparation

The daily energy requirement for DHW preparation is mostly only small. However, the hot water is tapped f.e. during showering within a short time, and therefore it has to be reheated quickly. Therefore it is important to plan also the required energy for this. For a bath with about 150 liters, the energy consumption is about 6 kWh, and for a shower about 2 kWh.

As a rule of thumb, you should provide an additional heating power of at least 0,25 kW per person.

Calculation of electricity power-off times of energy suppliers

Some energy suppliers providing a cheaper heat pump energy price, which are often associated with power-off times. This electricity cut can last f.e. 3 x 2 hours per day. For calculating the additional power output to bridge this power-off times we use the following formula:

LZ = total heat demand [kW] × 24 h ÷ (24 h - power-off times)

The total heat demand of above example and with 5 residents is 10,85 kW, the power-off period is 6 hours.
LZ = 10,85 kW × 24 h ÷ (24 h - 6 h) = 14,47 kW

In order to have enough heat for disposal every time of the day, the heat pump operation with a heating buffer tank is highly recommended. The minimum buffer volume sould have enough space to bridge power-off times or other higher energy requirements (f.e. approx. 50 ltr per kW heating pump output).

Flow temperature of a heating system

The lower the flow temperature, the higher is the coefficient of power of a heat pump and the more economical will be the heating system. Heat pumps work Heat pumps therefore work most efficiently with surface heating systems just as underfloor or wall heating systems. In old buildings or existing buildings with radiator heating system mostly high flow temperatures are required. In this case, attempts should be made by remedial measures, such as enlarging the radiators or applying a thermal insulation to the outside building facade, to lower the required flow temperature.

Heat pump operation mode

To avoid an uneconomic heat pump operation, the flow temperature for heating is limited to 55°C and the maximum temperature for DHW is 60°C. If higher temperatures are needed or if the heat pump output is not sufficient for the determined total heat demand, a bivalent mode is required. For this purpose, either an electric heating element in the buffer tank can be used to cover the remaining heating power, or in times when increased energy demand exists (f.e. if there are very cold outside temperatures) there is one more possibility to cover this consumption peaks by a second heating generator.
Thus, three different modes of heat pump operatiion as described below are possible.

Monovalent operating mode

A monovalent operation is possible, if the heat pump's power is still enough also on the coldest day of the year to completely cover the total heat requirement. This is usually only possible in very new buildings according to EnEV with a small specific heat demand and a low flow-temperatures for the heating system.
Monovalent operating mode

Parallel or mono-energetic bivalent operating mode

If the heat pump can cover the heat demand up to about 90 - 95% of the annual heating work, it is usually sufficient to reheat the rest of the demand from the bivalence point by an additional electric heater in the buffer tank. This mode is called the monoenergetic operation mode. Due to the low purchasing costs, the installation of an electrical heating element is generally recommended.
When the heat pump works parallel with a second heat generator at the same time, it is called a bivalent parallel operation mode. If the heat demand is lower, the heat pump is usually the main heat source. From the bivalence point on the demand is covered simultaneously by the heat pump and the second heat generator. Although bivalent heating systems are more expensive, the purchase of the heat pump usually pays off after a few years, since the operation of the heat pump is cheaper than fossil fuels. The bivalent mono-energetic or parallel operating mode is therefore often used in existing buildings where a heating system is already present and the heat pump optimizes energy consumption as an additional, cost-effective heat source.
Monoenergetic operating mode

Alternative Bivalent operating mode,

In alternative bivalent operation mode the heat pump works at lower heat demand only. From the bivalence point on the heat demand is then covered by the second heat source. Here, for example, a wood heating can supply the heating completely in times with very high heat and temperature requirements and the heat pump is completely switched off on cold days. An advantage of the combination of heat pumps with a wood boiler is that the buffer tank is already present due to the wood heating. This safe costs in purchasing the heat pump.

Bivalent energetic mode