Flow Temperature in Heat Pump Efficiency
Flow temperature is one of the most important factors influencing heat pump efficiency.
It describes the temperature of the heating water leaving the heat pump and flowing into the heat distribution system (such as underfloor heating or radiators). The higher this temperature, the harder the heat pump must work — and the lower the efficiency becomes.
Understanding flow temperature is essential when evaluating seasonal performance, operating costs, and system design.
What Is Flow Temperature?
Flow temperature (also called supply temperature) is the temperature of the water delivered from the heat pump to the heating system.
It is usually expressed in degrees Celsius (°C). Typical ranges include:
- Around 30–35°C for underfloor heating
- Around 40–45°C for low-temperature radiators
- 50°C or higher for conventional high-temperature radiator systems
The required flow temperature depends on the building’s heat demand and emitter design.
Why Flow Temperature Affects Efficiency
Heat pumps do not generate heat through combustion. They transfer heat from a low-temperature source (air, ground, or water) to a higher temperature level.
The larger the temperature difference between:
- Heat source temperature (e.g., outdoor air)
- Required flow temperature
the more electrical energy is required.
This temperature difference is called the temperature lift.
Higher temperature lift → Lower COP → Lower seasonal efficiency.
Flow Temperature and COP
Under standardized testing according to EN 14511, efficiency values are measured at defined combinations such as:
- A7/W35
- A2/W35
The “W35” represents a 35°C flow temperature.
If the same heat pump were tested at 55°C flow temperature instead of 35°C, the COP would be significantly lower.
This is why flow temperature is one of the strongest technical drivers of efficiency.
Flow Temperature and Seasonal Efficiency
Seasonal metrics such as SCOP are calculated according to EN 14825, which uses multiple temperature bins and part-load data.
Lower design flow temperatures generally result in:
- Higher SCOP values
- Higher seasonal space heating energy efficiency (ηs)
- Lower electricity consumption over the heating season
Therefore, system design directly influences seasonal performance.
Typical Flow Temperatures by Heat Emitter Type
Underfloor Heating
- Large surface area
- Low operating temperature
- Typically 30–35°C
- Very suitable for heat pumps
Low-Temperature Radiators
- Larger radiator surface than conventional systems
- Typically 40–45°C
- Moderate efficiency impact
Conventional Radiators
- Smaller heat emission area
- Often require 50–60°C
- Lower heat pump efficiency
The lower the required flow temperature, the better the seasonal efficiency.
What Determines Required Flow Temperature?
Flow temperature is influenced by:
- Building heat load
- Insulation level
- Emitter surface area
- Outdoor temperature
- Control strategy (weather-compensated control)
In well-insulated buildings, lower flow temperatures are often sufficient to maintain comfort.
Flow Temperature and System Optimization
Modern heat pump systems use weather-compensated control, which adjusts flow temperature based on outdoor temperature.
For example:
- Mild weather → lower flow temperature
- Cold weather → higher flow temperature
This dynamic adjustment improves seasonal efficiency and reduces unnecessary electrical consumption.
Common Misunderstanding
Higher flow temperature does not mean better comfort.
Comfort depends on:
- Correct heat load calculation
- Proper emitter sizing
- Stable room temperature control
Oversized flow temperatures increase electricity consumption without improving comfort.
Practical Takeaway
Flow temperature is one of the most influential system parameters in heat pump efficiency.
Lower flow temperature generally leads to:
- Higher COP
- Higher SCOP
- Lower operating costs
- Improved seasonal performance
When evaluating heat pump efficiency, always consider the required heating system temperature — not just the rated efficiency value.
