Introduction: Flexibility Through Different Operating Modes

Heat pumps and heating systems in general can be operated in various ways to optimally meet requirements. In this article, we explain the most important operating modes:

• Monovalent: Heat pump as sole heat generator
• Bivalent: Heat pump + second heat generator
• Mono-energetic: One energy source, possibly two heat generators
• Hybrid: Intelligent combination of different systems

What Do the Terms Mean?

The technical terms derive from Latin and describe the number of heat generators:

Term	Meaning	Number of Heat Generators
Monovalent	"Single-value"	1
Bivalent	"Two-value"	2
Trivalent	"Three-value"	3

The "valency" describes the number of different heat generators in the system.

Monovalent Operation

In monovalent operation, the heat pump is the sole heat generator. It covers 100% of the heat demand – even on the coldest days.

Requirements

For monovalent operation to work, several conditions must be met:

Requirement	Explanation
Adequately sized	Heat pump must cover peak loads too
Well-insulated building	Low heat demand
Low-temperature heating system	Underfloor heating ideal
Suitable heat source	Air, ground or groundwater

How It Works

Outdoor temperature: -15°C to +20°C
         │
         ▼
    Heat pump ──────► 100% heat supply

The heat pump runs all year round and adjusts its output to demand (inverter technology).

Advantages of Monovalent Operation

Operation with only one heat generator offers clear advantages:

Advantage	Explanation
Simple system	Only one heat source
Maximum efficiency	No switching between systems
100% renewable	Completely CO2-free with green electricity
Low maintenance	Only one system to maintain

Disadvantages of Monovalent Operation

The simplicity also has its limits:

Disadvantage	Explanation
Higher investment	Heat pump must be larger sized
Efficiency in cold	Less efficient at very low temperatures
Building requirements	Not suitable for every old building

When Sensible?

• New builds with good insulation
• Well-renovated buildings
• Buildings with underfloor heating
• Ground-source or water-water heat pumps (stable heat source)

Bivalent Operation

In bivalent operation, the heat pump works together with a second heat generator (e.g. gas or oil boiler, electric heating).

Two Variants

Bivalent-Parallel

Both heat generators work simultaneously when demand is high.

                    ┌── Heat pump ──────┐
High demand ───────►│                   ├──► Heating
                    └── Backup heating ─┘

Bivalent-Alternative

Above a certain temperature (bivalence point), the second heat generator takes over completely.

Above -5°C:    Heat pump ──────────────► Heating
Below -5°C:    Backup heating ─────────► Heating
               (heat pump off)

The Bivalence Point

The bivalence point is the outdoor temperature at which:

• The heat pump reaches its performance limit
• The second heat generator kicks in

Typical values: -3°C to -8°C

Advantages of Bivalent Operation

The combination of two heat generators offers specific advantages:

Advantage	Explanation
Smaller heat pump	Does not need to be sized for extreme cases
Lower investment	Heat pump cheaper
Flexibility	Optimal efficiency depending on conditions
Supply security	Backup available

Disadvantages of Bivalent Operation

The flexibility also brings disadvantages:

Disadvantage	Explanation
Two systems	Higher maintenance effort
More complex control	Switching must work
Fossil fuels	Not CO2-neutral with gas/oil as backup

Advantages and Disadvantages by Variant

The two variants of bivalent operation each have specific characteristics:

Variant	Advantage	Disadvantage
Bivalent-Parallel	Heat pump runs longer (more efficiency)	Both systems active simultaneously
Bivalent-Alternative	Clear separation	Heat pump switched off in cold

Mono-energetic Operation

Not to be confused with monovalent!

Mono-energetic means: Only one energy source is used – even if several heat generators are present.

Example

Solar electricity ──────┬──► Heat pump
                        │
                        └──► Electric immersion heater

Both heat generators use electrical power – the system is bivalent (two heat generators) but mono-energetic (one energy source).

Advantage

With green electricity, the entire system can be operated 100% CO2-neutral!

Hybrid Heat Pumps

The hybrid heat pump is the intelligent combination of heat pump and conventional heat generator.

Structure

Often both heat generators are installed in one compact unit:

• Heat pump (primary)
• Condensing boiler or electric heating (secondary)
• Intelligent control (integrated)

How It Works

Hybrid operation automatically switches between modes:

Depending on outdoor temperature and heat demand, the system automatically selects the optimal mode:

Situation	Operating Mode
Normal (mild)	Heat pump only
Increased demand	Both in parallel
Extreme cold	Heat pump + boiler
Very extreme cold	Boiler only

The Intelligent Control Decides

The control automatically optimises according to:

• Outdoor temperature
• Current heat demand
• Economy (electricity vs. gas price)
• Efficiency of the heat pump

Advantages of Hybrid Heat Pump

The intelligent combination offers numerous advantages:

Advantage	Explanation
Optimally matched	Components fit perfectly together
Compact	Often one device instead of two
Intelligent	Automatic optimisation
Economical	Always uses the cheapest heat source
Future-proof	Heat pump share can be increased later

Disadvantages of Hybrid Heat Pump

Despite the many positive features, there are also limitations:

Disadvantage	Explanation
Not 100% renewable	Fossil fuels with gas backup
More complex system	More components
Manufacturer dependency	Often only with certain combinations

When Sensible?

• Renovation of existing buildings – existing boiler is supplemented
• Old buildings with high heat demand
• Transition period to fully renewable heating
• When gas connection is available

Overview: Which Operating Mode for Whom?

The following overview shows which operating mode is best suited to which building situation:

Operating Mode	Ideal for	Not suitable for
Monovalent	New builds, renovated buildings	Unrenovated old buildings
Bivalent-Parallel	Old buildings with moderate demand	—
Bivalent-Alternative	Very cold regions	Mild climate zones
Hybrid	Renovations, existing buildings	New builds (oversized)

Decision Guide

Questions for Orientation

1. How well insulated is the building?

   • Good → Monovalent possible
   • Poor → Bivalent/hybrid sensible

2. Which heating system is installed?

   • Underfloor heating → Monovalent ideal
   • Old radiators → Possibly bivalent

3. Is there a gas connection?

   • Yes → Check hybrid option
   • No → Electric backup or monovalent

4. How important is 100% renewable?

   • Very important → Monovalent or mono-energetic
   • Less important → All options open

Conclusion

Continue reading: In the next article Heat Pump Types and the Dream Team with Solar Systems, you will learn everything about air-water, ground-source and the optimal combination with photovoltaics.

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The Complete Article Series "Heat Pumps"

1. The Anti-Refrigerator: How Does a Heat Pump Work? – Fundamentals
2. The Components: Heat Exchanger, Compressor and Expansion Valve – Components
3. Heat Pump Key Figures and Sizing – COP, SPF and more
4. Operating Modes: Monovalent, Bivalent and Hybrid – You are here
5. Heat Pump Types and the Dream Team with Solar Systems – Air-water, ground-source & solar

Sources

• Heizung.de: Monovalent, Bivalent – Operating Modes
• AroundHome: Hybrid Heating
• Vaillant: Monovalent
• SBZ-Monteur: Hybrid Heating
• Enpal: Monovalent Heat Pump
• Grünes Haus: Bivalent Heat Pump
• Bosch: Hybrid Heat Pump