When upgrading to modern heating systems like heat pumps or condensing boilers, radiator sizing is critical to ensure efficient heating. Radiators sized for older systems, such as traditional oil or gas non-condensing boilers, may not perform adequately with lower temperature systems. Here’s a detailed explanation, including formulas and tables, for calculating the appropriate radiator size for various heating systems.
1. Heat Loss Calculation
Start by determining the heat loss of the room. For example, assume a room has a heat loss of 500W. Radiators must be selected to compensate for this heat loss based on the system’s flow and return temperatures.
2. Radiator Output and Temperature Difference (ΔT)
Radiator outputs are typically based on a water-to-air temperature difference
ΔT 50°C in manufacturer's product catalogues.
Increasing the ΔT value results in a higher radiator output. Conventional gas or oil non-condensing boilers work with flow and return temperatures of 82/71°C, whereas condensing boilers operate at lower temperatures of 65/55°C to improve efficiency. Heat pumps typically operate at temperatures of 45/40°C for enhanced efficiency. Refer to the examples below for guidance on sizing radiators for different heat sources.
ΔT=(Tflow+Treturn)/2−Room Temperature
Where:
Tflow = Flow temperature of the system
Treturn = Return temperature of the system
Troom = Room temperature (typically 20°C)
For example, a traditional non-condensing boiler with 82/71°C flow and return temperatures results in:
ΔT=(82+71)/2−20=56.5°C
Condensing boilers, flow and return temperatures 65/55°C and room temperature 20°C:
ΔT=(65+55)/2−20=40°C
Heat pump technology, flow and return temperatures 45/40°C and room
temperature 20°C:
ΔT=(45+40)/2−20=22.5°C
3. System Comparisons
The decreased temperature variance results in a lower radiator output. Numerous radiator manufacturers provide details on radiator output using a ΔT value of 50°C. The radiator table presented below illustrates typical information found in a radiator manufacturer's catalog.
Radiator Height (mm) | Radiator Length (mm) | Heat Output at ΔT 50°C (Watts) | Heat Output at ΔT 40°C (Watts) |
450 | 400 | 424 | 317 |
450 | 700 | 758 | 566 |
450 | 1400 | 1536 | 1149 |
4. Conversion Factors
To adjust the radiator output for systems with ΔT values other than 50°C, manufacturers provide conversion factors. Conversion factor table below gives typical conversion factors for different ΔT values:
ΔT (°C) | Conversion Factor |
20°C | 0.304 |
30°C | 0.515 |
40°C | 0.748 |
50°C | 1.000 |
60°C | 1.268 |
5. Examples of Radiator Sizing for Different Systems
Traditional Non-Condensing Boilers (82/71°C):
Heat loss: 500W
ΔT: 60°C (approx)
Conversion factor: 1.268
Selected radiator: From Table B1, select a 424W radiator and multiply by the conversion factor:
424×1.268=537Watts
Note: By utilizing the conversion factor table, one must determine the factor for a temperature change of 56.5 degrees Celsius. Since the table is provided in 5-degree Celsius increments, the factor for a temperature change of 60 degrees Celsius should be used instead.
Condensing Boilers (65/55°C):
Heat loss: 500W
ΔT: 50°C
Conversion factor: 0.748
Selected radiator: From Table B1, select a 758W radiator and adjust:
758×0.748=566Watts
Heat Pumps (45/40°C):
Heat loss: 500W
ΔT: 20°C
Conversion factor: 0.304
Selected radiator: From Table B1, select a 1536W radiator and adjust:
1536×0.304=466Watts
The larger radiator is necessary for systems like heat pumps due to their lower operating temperatures, which affect the output. The conversion factor ensures the radiator meets the room's heating needs despite the lower water temperature.
6. Summary of Radiator Adjustments for Different Systems
System Type | Radiator Output at ΔT 50°C (W) | Adjusted Output (W) | Radiator Size (L x H - mm) |
Traditional Non-Condensing Boiler | 424 | 538 | 400 x 450 |
Condensing Boiler | 758 | 566 | 700 x 450 |
Heat Pump | 1536 | 466 | 1400 x 450 |
In summary, upgrading to a condensing boiler or heat pump from a traditional non-condensing boiler requires careful radiator sizing adjustments. Radiators sized for higher temperature systems will likely need to be larger or upgraded to maintain the same heat output under lower temperature conditions. The conversion factors and ΔT values allow precise calculation of the radiator size necessary to achieve efficient heating for any system.
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