Modern heating systems are designed for efficient and sustainable operation. However, in some situations, the main heat source alone is no longer sufficient when outside temperatures drop sharply. Then a second one has to step in. The bivalence point is decisive for this.

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What is the bivalence point?

The bivalence point defines the outside temperature at which a heat pump no longer provides sufficient heating output and a second heating source must step in. This is due to the physical limits of the heat pump, as its efficiency decreases as the temperature drops. The bivalence point is therefore an important factor for designing a heating system.

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Why is the bivalence point important?

The bivalence point determines at which outside temperature a second heating source must support a heat pump. It directly influences energy consumption and therefore heating costs. If the bivalence point is set too high, the proportion of additional heating increases, which increases operating costs. An optimally set bivalence point ensures that a heat pump works efficiently for as long as possible. Depending on building insulation and heating load, the ideal bivalence point may differ.

What types of bivalence points are there?

Which type of bivalence point is relevant for a heat pump depends on the selected operating mode. Heat pumps can work in different ways, e.g. alone (monovalent operation) or in combination with another heat source (dual-mode operation).

There are three types of bivalence points in total:

• Mono-energetic operation: The heat pump works efficiently up to the bivalent point, after which an additional electric heater such as a heating element takes over. This mode of operation is often used with air-water heat pumps.
• Dual mode parallel operation: The heat pump covers the primary heat requirement up to the bivalent point, after which a condensing boiler (oil, gas or wood) switches on.
• Bivalent alternative mode: The heat pump only works up to the bivalent point, after which another heating source such as a gas or oil boiler takes over completely. This solution is typical for hybrid heating systems and ensures a clear separation of operating modes.

The choice of the bivalence point not only influences the operating mode of your heat pump, but also on heating costs.

How does the bivalence point influence heating costs?

The bivalence point directly influences heating costs, as it determines how often the additional heating comes in. For example, is the bivalence point at -5 °C, does the heat pump take over approximately 90% of heating demand, while the additional heater 10% delivers. On the other hand, is the bivalence point already at 0 °C, is the proportion of additional heating increasing 30% or more, which significantly increases costs.

Below, we show the effects of the bivalence point using an example. The heating costs are based on an average single-family home with an air-water heat pump and an electricity price of 0.30 €/kWh.

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A low bivalence point therefore means higher heat pump efficiency and lower heating costs. An excessively high bivalence point, on the other hand, leads to rising costs, as the more expensive additional heating system is activated more frequently.

How is the bivalence point calculated?

The bivalence point is determined based on the heating load of the building and the efficiency of the heat pump. The heating load depends on the insulation, the window surfaces and the construction method. The heat pump performance curve shows how much the heating output decreases when the outside temperature drops. The COP value indicates how efficiently the heat pump is working — the higher it is, the lower the bivalence point can be. The type of additional heating also plays a role.

Step-by-step calculation:

1. Determine the heating load of the building For example, your house requires 10 kW heating output at -10 °C outside temperature.
2. Check the heat pump performance curve For example, your heat pump delivers 9.5 kW is still 9.5 kW at 0 °C, but with 5 °C only 7.5 kW.
3. Find the temperature at which the heating load is greater than the heat pump output At -5 °C, for example, the building requires 10 kW, but the heat pump only produces 7.5 kW. The difference of 2.5 kW must be taken over by the additional heater.
4. Outcome: The bivalence point is -5 °C because at this temperature the heat pump can no longer cover all heat requirements.

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You can determine the bivalence point by determining at which temperature the required heat load is higher than the heat pump output. In this example, it is -5°C.

With a stronger heat pump or better insulation, the bivalence point could be reduced.

How can you optimize the bivalence point?

Here's how you can lower the bivalence point and reduce heating costs:

• Select a more powerful heat pump: A heat pump with higher heating capacity can deliver more heat even at low temperatures.
• Ensuring good building insulation: Better insulation reduces the heat load and moves the bivalence point downwards.
• Optimize the heating flow temperature: A lower flow temperature increases the heat pump's COP value and reduces power consumption.
• Choose a hybrid solution with efficient additional heating: A gas condensing boiler or a wood stove can reduce the energy consumption of the additional heater.
• Use an outside temperature sensor: Precise control based on the actual outside temperature enables finer adjustment of the bivalence point.
• Adjust operating mode: In bivalent parallel operation, the bivalence point can be set lower, as the heat pump and additional heater work simultaneously.

What does the bivalence point mean in practice?

The bivalence point determines at which outside temperature an additional heater supports your heat pump. An optimally selected bivalent point reduces heating costs and improves efficiency. With a well-coordinated heat pump, low flow temperatures and a low heat load, you can optimize the bivalence point. As a result, your heat pump works alone for longer and the additional heating is needed less frequently.

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