The radiator return temperature is the temperature of the water that has passed through your radiators and is returning to the boiler to be reheated.
Understanding Flow and Return Temperatures
In a central heating system, water is heated by the boiler and circulated through pipes to the radiators. This water delivers heat into the rooms via the radiators.
- Flow Temperature: This is the temperature of the water as it leaves the boiler and flows towards the radiators.
- Return Temperature: This is the temperature of the water after it has passed through the radiators and released some of its heat, as it returns to the boiler.
The difference between the flow temperature and the return temperature is often called the "delta T" or temperature drop. This difference indicates how much heat has been transferred from the water into the room.
Typical Example: 80/60 Setup
As referenced, most gas boilers are commonly set up using what is known as 80/60 flow and return temperatures.
- The flow temperature in this setup is 80°C, as the boiler heats the water to this temperature before circulation.
- The return temperature in this setup is typically 60°C, representing the water temperature after it has cooled down by releasing heat through the radiators.
| Temperature Type | Typical Temperature (Example: 80/60 Setup) | Description |
|---|---|---|
| Flow Temperature | 80°C | Temperature of water leaving the boiler for radiators. |
| Return Temperature | 60°C | Temperature of water returning to the boiler. |
Why is the Return Temperature Lower?
The water's primary job when circulating through the radiators is to transfer heat into the room. As heat is transferred from the water to the radiator surface and then to the air in the room, the water cools down. Therefore, the return temperature is always lower than the flow temperature (unless there is a system fault or the radiators are not effectively transferring heat).
Significance of Return Temperature
Monitoring and managing the return temperature is important for several reasons:
- System Efficiency: Modern condensing boilers are most efficient when the return water temperature is low enough (typically below 55°C). This allows the boiler to condense the water vapour in the flue gases, recovering latent heat that would otherwise be lost.
- System Performance: The difference between flow and return temperature gives an indication of how well the radiators are heating the space. A small difference might suggest radiators are undersized or the flow rate is too high. A very large difference might indicate a low flow rate.
- Boiler Operation: Boilers modulate their heat output based on the return water temperature to maintain the desired flow temperature and system efficiency.
Understanding the radiator return temperature helps in optimizing heating system performance and ensuring energy efficiency, particularly with condensing boilers.
