When designing an air-to-water heat-pump system, one temperature matters for more than comfort or weather compensation:
the bivalent temperature.
The EU Ecodesign definition of bivalent temperature describes it as the outdoor temperature at which the heat pump’s declared heating capacity equals the heating part-load requirement. Below this point, supplementary heating capacity may be required to meet the complete load.
In practical system design, this means that above the bivalent point, the heat pump can normally meet the calculated heating demand by itself.
Below it, the building may require more heat than the heat pump can provide under those conditions. A supplementary heat source may then be needed to cover the difference.
This is why the bivalent point matters when designing systems that combine a heat pump with:
- an electric backup heater;
- an existing boiler;
- another auxiliary heat source;
- or an integrated additional heating system.
However, the bivalent temperature is not the same as the lowest temperature at which a heat pump can operate.
Understanding that distinction is essential for correct system design.
What Is Heat Pump Bivalent Temperature?
The bivalent temperature — usually written Tbiv — represents the outdoor temperature at which the heat pump’s available heating capacity matches the building’s heating requirement.
Below that point, supplementary heating capacity may be required to satisfy the full load.
The concept can be expressed simply:
heat-pump capacity = building heat demand
at the bivalent point.
If the outdoor temperature falls further, two things generally happen:
- the building loses heat faster and therefore needs more heating;
- the heat pump’s available capacity may change as operating conditions become more demanding.
Eventually, the two curves cross.
That crossing point is the thermal basis of the bivalent temperature.
Start With the Building Heat Loss
The bivalent point cannot be determined correctly from heat-pump capacity alone.
First, the designer needs to know how much heat the building requires as outdoor temperature changes.
A proper heat-loss calculation considers factors such as:
- wall, roof and floor construction;
- insulation levels;
- windows and doors;
- ventilation and air leakage;
- required indoor temperature;
- geographical design temperature.
This produces the building’s heating requirement under the selected winter design condition.
For more detail, see our guide to heat loss calculation for heat-pump sizing.
As the outdoor temperature rises, the building loses less heat. As it falls, the heating demand rises.
This gives us one side of the bivalent-point calculation.
Heat-Pump Capacity Also Changes With Operating Conditions
The other side is the heat pump.
An air-to-water heat pump does not necessarily provide exactly the same heating output at every outdoor temperature and water temperature.
Capacity depends on operating conditions including:
- outdoor air temperature;
- required flow temperature;
- compressor operating envelope;
- refrigerant-circuit design;
- defrost operation;
- control strategy.
This is why professional selection should use performance data at relevant operating conditions rather than only the nominal capacity printed in the model name.
For example, the behaviour at A7/W35 does not tell us everything about performance at A−7/W55.
The outdoor condition and required water temperature both matter.
A Simple Bivalent-Point Example
Consider a building whose heating demand increases as outdoor temperature falls.
The following figures are purely illustrative:
| Outdoor temperature | Building heat demand | Available heat-pump capacity | Result |
|---|---|---|---|
| +7°C | 3 kW | 8 kW | Heat pump alone |
| +2°C | 5 kW | 8 kW | Heat pump alone |
| −3°C | 7 kW | 7 kW | Approximate bivalent point |
| −7°C | 9 kW | 7 kW | 2 kW supplementary heat required |
| −10°C | 10 kW | 6.5 kW | 3.5 kW supplementary heat required |
At around −3°C, the available heat-pump capacity and building load are equal.
Below that temperature, the heat pump can continue supplying heat, but another source would need to provide the remaining load if the system is designed as a parallel bivalent system.
The actual bivalent temperature must be calculated from the real building load and verified performance data for the selected heat pump.
Bivalent Temperature Is Not the Same as Operating Limit
This distinction is particularly important.
Bivalent temperature
The bivalent temperature is the point where the heat pump can no longer satisfy the complete heating load by itself.
The heat pump may still be fully capable of operating below this temperature.
Operating-limit temperature
The operating-limit temperature is the outdoor condition below which the manufacturer declares that the heat pump can no longer provide heating capacity.
EU Ecodesign rules define these as two separate parameters: Tbiv for the bivalent temperature and TOL for the operation-limit temperature.
Therefore:
backup heating starting does not mean the heat pump has stopped working.
It may simply mean that the building now requires more heat than the heat pump alone can provide.
Does Every Heat Pump Need Backup Heating?
No.
A heat pump can be designed as a monovalent system, meaning the heat pump is selected to satisfy the full calculated heating load at the project design condition without another space-heating source.
Other systems are deliberately designed as bivalent systems.
This may make sense when:
- an existing boiler is being retained;
- extreme winter temperatures occur only for a limited number of hours;
- increasing heat-pump capacity would create excessive oversizing during milder weather;
- electrical supply capacity is limited;
- the project has a specific economic or control strategy;
- another heating source is already available.
The Energy Saving Trust domestic heat-pump guidance distinguishes between monovalent systems and several forms of bivalent operation, including concurrent and changeover control.
Neither configuration is automatically superior.
The correct choice depends on climate, building demand, heat-pump performance, electricity supply, fuel availability and project objectives.
What Is a Bivalent Parallel System?
In a bivalent parallel or concurrent arrangement, the heat pump continues operating below the bivalent point.
The supplementary heater then provides only the additional heat required.
For example:
Building demand: 10 kW
Heat-pump output: 7 kW
Supplementary heat: approximately 3 kW
The two heat sources operate together.
This approach allows the heat pump to continue contributing useful heat even during colder conditions.
What Is Bivalent Changeover Operation?
A different strategy is bivalent changeover.
Below a defined outdoor condition, the heat pump stops and the secondary heat source takes over the heating load.
The secondary source might be:
- an existing gas boiler;
- another boiler;
- or another compatible heating appliance.
This arrangement can be relevant in hybrid retrofit projects.
However, the changeover point should not be chosen arbitrarily. It should reflect the technical performance of both systems, building load, energy costs, emissions objectives and applicable local requirements.
What About Electric Backup Heating?
Electric resistance heating is another form of supplementary heat.
An electric backup heater can provide additional capacity when the heat pump cannot meet the complete load.
However, resistance heating normally converts approximately one unit of electrical energy into one unit of heat, while a heat pump can move several units of heat for each unit of electricity under favourable conditions.
For this reason, electric backup heating should normally be considered supplementary capacity, not an excuse to undersize the heat pump without analysis.
Its expected operating hours should form part of the system-design and seasonal-energy assessment.
Why Not Simply Install a Much Larger Heat Pump?
It might seem logical to select a heat pump large enough to avoid supplementary heating under every conceivable condition.
But oversizing can create other problems.
For most of the heating season, outdoor temperatures are considerably milder than the winter design condition.
A very large heat pump may therefore spend much of the year operating below its ideal output range.
Potential consequences can include:
- more frequent cycling;
- reduced ability to modulate continuously;
- unnecessary equipment cost;
- greater electrical infrastructure requirements;
- less stable part-load operation.
Our guide to heat pump short cycling explains why minimum output and building demand must be considered together.
The objective is therefore not automatically to select the largest possible heat pump.
It is to select a system that performs appropriately across the expected operating range.
Bivalent Temperature and Minimum Modulation Are Connected
The lower-temperature end of the heating season is not the only design challenge.
During mild weather, building demand can become very small.
An inverter heat pump can reduce compressor output, but every system has a minimum modulation limit.
This means system selection involves both ends of the load curve:
Cold weather: Can the system provide enough heat?
Mild weather: Can it reduce output enough to operate stably?
This is why a slightly bivalent design can sometimes be more technically appropriate than selecting a much larger heat pump solely to cover a small number of extreme-weather hours.
However, that decision must be based on real load and performance data.
How Does Flow Temperature Affect the Bivalent Point?
Flow temperature can materially affect the result.
A heat pump generally faces more demanding operating conditions when it must produce higher-temperature water.
For example, a system using:
- 35°C underfloor-heating water;
- 50°C low-temperature radiators;
- or 60°C retrofit radiator temperatures
does not impose the same refrigeration-cycle conditions.
If higher water temperatures reduce the available capacity at low outdoor temperatures, the bivalent point may occur sooner.
This is why the calculation should use the flow temperature actually required by the building.
Our guide to high-temperature R290 heat pumps explains why maximum water temperature and optimum operating temperature are different concepts.
Radiator Sizing Also Matters
The heat pump may be capable of supplying the required heat, but the emitters must still transfer that heat into the rooms.
If existing radiators are too small at the selected flow temperature, simply increasing heat-pump capacity will not solve the complete problem.
The designer should therefore assess:
building heat loss + heat-pump capacity + radiator output + flow temperature
together.
See our guide to heat pump radiator sizing for the room-by-room emitter calculation.
What Does R290 Change?
R290 is the refrigerant used inside the heat pump’s sealed refrigeration circuit.
It does not determine the bivalent temperature by itself.
The bivalent point depends on the relationship between:
- the building heat load;
- the selected heat-pump model;
- its available capacity at the relevant outdoor conditions;
- required water temperature;
- and system-control strategy.
Modern R290 heat pumps can offer useful cold-weather and higher-water-temperature operating capability. However, the label R290 alone does not establish how much capacity a particular model will provide at a specific condition.
The model’s verified performance data must be used.
Tongyi’s R290 inverter air-to-water heat-pump range can form part of systems that include additional heating equipment where supplementary capacity is required.
The backup source should be controlled as part of the wider heating-system design.
How Should the Backup-Heater Temperature Be Set?
The answer should come from the system design rather than from a generic internet setting.
The installer should consider:
- the calculated building heat loss;
- heat-pump capacity at the relevant outdoor temperatures;
- required flow temperature;
- emitter capacity;
- expected defrost behaviour;
- supplementary heater capacity;
- electrical or fuel-supply limits;
- the chosen parallel or changeover strategy;
- manufacturer control parameters.
A setting such as “enable backup at −5°C” cannot be recommended universally.
The same outdoor temperature can represent very different heating loads in two different buildings.
Bivalent Point vs Economic Balance Point
Another term sometimes encountered is the economic balance point.
This is not necessarily the same as the thermal bivalent temperature.
The thermal bivalent point is based on capacity.
An economic changeover point considers the cost of operating competing heat sources.
For example, in a hybrid heat-pump and boiler installation, changing electricity and gas prices could alter the economically preferred heat source even though the physical heating demand has not changed.
Similarly, carbon or grid-control objectives may favour a different strategy.
Technical capacity and economic dispatch should therefore be treated as separate calculations.
What About Defrost?
Air-source heat pumps periodically need to remove frost from the outdoor heat exchanger under suitable cold and humid conditions.
During defrost, some energy is temporarily used to clear the coil rather than provide continuous space heating.
Manufacturers account for seasonal operating behaviour through their product testing and control design, but the heating system still needs sufficient thermal capacity and correct hydraulic design to maintain stable operation.
A bivalent calculation should therefore use appropriate manufacturer performance data rather than assuming that nominal catalogue output remains unchanged under every winter condition.
Electrical Supply Must Include Backup Heating
If electric supplementary heating is installed, it must be included in the electrical-load calculation.
For example, the electrical designer may need to consider simultaneous operation of:
- the heat-pump compressor;
- circulation pumps;
- an electric backup heater;
- a domestic-hot-water immersion or booster;
- other major building loads.
This is particularly important when deciding between single-phase and three-phase installations.
Our guide to single-phase vs three-phase heat pumps explains the electrical assessment in more detail.
Practical Bivalent-System Design Sequence
A robust design normally follows this order:
- Calculate the building’s design heat loss.
- Establish the local outdoor design temperature.
- Determine the required heating-water temperature.
- Check radiator or underfloor-heating output.
- Review heat-pump capacity at relevant outdoor and water temperatures.
- Identify where heat-pump capacity and building demand intersect.
- Determine whether supplementary capacity is necessary.
- Select the supplementary heat source if required.
- Choose parallel, concurrent or changeover control.
- Verify electrical or fuel-supply requirements.
- Configure the controller according to manufacturer instructions.
- Commission and verify the complete system under operating conditions.
The bivalent temperature is therefore not an isolated setting.
It is the result of the whole heating-system design.
Common Bivalent-Temperature Mistakes
Confusing the bivalent point with the minimum operating temperature
The heat pump can often continue operating below its bivalent point.
Choosing a fixed temperature without calculating heat loss
A generic value such as −5°C cannot represent every building and climate.
Using nominal heat-pump capacity only
Capacity should be checked at the actual outdoor and water-temperature conditions.
Assuming R290 eliminates the need for system design
R290 can provide a broad operating envelope, but refrigerant choice alone does not determine building load or supplementary heating requirements.
Oversizing the heat pump simply to avoid backup heat
This can create undesirable part-load behaviour during milder weather.
Allowing the backup heater to run unnecessarily
Incorrect control configuration can increase auxiliary-energy use even when the heat pump could satisfy the demand alone.
Ignoring the backup heater in the electrical design
Electric supplementary heating can materially change maximum electrical demand.
Frequently Asked Questions
What does bivalent temperature mean for a heat pump?
It is the outdoor temperature at which the heat pump’s available heating capacity matches the building’s heating requirement. Below it, supplementary capacity may be needed.
Does the heat pump switch off at the bivalent temperature?
Not necessarily. In a parallel bivalent system, the heat pump continues operating while supplementary heat provides the remaining capacity.
Is bivalent temperature the same as minimum operating temperature?
No. The minimum operating or operation-limit temperature describes a different condition. A heat pump may continue producing useful heat well below the building’s bivalent point.
Does every heat pump need a backup heater?
No. Some systems are designed to meet the full design heating load using the heat pump alone. Others intentionally use supplementary heating under specific conditions.
Is −5°C a normal bivalent temperature?
There is no universal value. The correct point depends on building heat loss, climate, heat-pump performance, water temperature and system design.
Does R290 reduce the need for backup heating?
Not automatically. R290 heat pumps can be designed for strong cold-weather and high-temperature operation, but the requirement for backup heat must be calculated for the specific model and building.
Can an existing boiler be used as backup?
Potentially, yes. A compatible bivalent or hybrid system can use a boiler as a supplementary or changeover heat source if the system controls and local requirements allow it.
The Takeaway
The heat pump bivalent temperature is the point where building demand and available heat-pump capacity meet.
Below that temperature, supplementary heating may be required — but that does not necessarily mean the heat pump stops operating.
The correct bivalent point comes from the interaction between:
building heat loss + outdoor temperature + heat-pump capacity + water temperature + emitters + control strategy
For R290 heat-pump projects, a wide operating envelope can provide valuable design flexibility. However, the final decision must still be based on verified model data and a proper heating-system calculation.
For project-specific equipment information, explore Tongyi’s R290 inverter air-to-water heat-pump range.
