Flow Temperatures Heating Systems

Overview

Engineering reference data for Flow Temperatures Heating Systems in HVAC systems.

Key Formulas

Sensible Heat

$Q = \dot{m} c_p \Delta T$

Heat causing temperature change.

Latent Heat

$Q = \dot{m} h_{fg} \Delta\omega$

Heat causing moisture change.

COP (Cooling)

$COP = Q_c / W$

Coefficient of performance.

Variables

Symbol	Description	Unit
$Q$	Heat transfer	W
$\dot{m}$	Mass flow rate	kg/s
$c_p$	Specific heat of air	J/(kg·K)
$\Delta T$	Temperature difference	K

Flow Temperature vs. Outside Temperature Data

The following table shows the required boiler flow temperature based on the system's minimum outside design temperature and the current outside temperature, assuming a standard 20°C temperature drop across the heating system.

Key Definitions

• Temperature Drop: The decrease in water temperature as it passes through the heating elements (e.g., radiators). A common design value is 20°C.
• Outside Design Temperature: The lowest outdoor temperature a heating system is designed to handle effectively for a specific location.
• Boiler Flow Temperature: The temperature of the water leaving the boiler on its way to the heating emitters.

Design Considerations

• It is common practice to design a forced hot water system for a 20°C (36°F) temperature drop across the radiators at the minimum outside design temperature.
• To prevent "burned dust" and accidental burns, the maximum boiler flow temperature is often limited to 80°C (176°F).
• The system's required flow temperature is adjusted based on the current outside temperature to maintain efficiency and comfort, as shown in the table above.

Interactive Charts

References

• Original Source