Flow Temperatures Heating Systems

Reference data and engineering information about flow temperatures heating systems for hvac systems applications.

flowtemperaturesheatingsystems

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.

3 rows

Table: Boiler Flow Temperature (°C) for a forced hot water heating system designed with a 20°C temperature drop. Null values indicate the scenario is not defined for that combination.
Min. Outside Design Temp.(°C)	Outside Temp. -20°C(°C)	Outside Temp. -10°C(°C)	Outside Temp. 0°C(°C)	Outside Temp. 10°C(°C)	Outside Temp. 20°C(°C)
-20	80	66	51	37	37
-10	—	80	58	37	37
0	—	—	80	37	37

Source: engineeringtoolbox.com

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.