Capacities of Radiators in Varying Water and Room Temperatures

The radiator heat output varies according to water and room temperature changes.

Heat outputs at temperatures other than those 75/65 °C water inlet/outet temperature and 20 °C room temperature are calculated using “F” factors enabling you to find the performance of standard radiator (75/65 °C and 20 °C) at different room and water temperatures.

The “F” factors are given in Table 2 (please see the F Factors Sheet below).
The following are two examples demosnstrating usage of “F” factors:

Example 1:
The heat output for 600/22/1000 Termolux Classic radiator at (75/65 °C and 20 °C) is Qn=1672 watts (from Table 1.A.). What will be the heat output at 70/55 °C water inlet/outlet temperatures and 18 °C room temperature?

In Termolux “F” factor table (Table 2) the first vertical column shows the incoming water temperature; the second vertical column shows the room temperature and the horizontal rows show the outgoing water temperature. When those columns are intersected, the “F” factor is found.

The “F” factor at 70/55 °C and 18 °C is 1.18. The new heat output is calculated with the formula:

Q = Qn / F
Q = 1672 / 1.18
Q = 1417 watt

Q: Required heat output
Qn: Standard heat output (at 75/65 and 20 °C)
F: Capacity factor from the table

Example 2:
“F” factor table can also be used to choose a radiator for a site (room or area) whose heat requirement has already been calculated.

Let’s assume that the calculated heat requirement for a room is Q = 1500 watt. How can we select and calculate the heat output of a standard radiator at 70/55 °C water inlet/outlet temperatures and 18 °C room temperature?

The “F” value from Table 2 (please see the F Factors Sheet below) is 1.18.

Qn = Q x F
Qn = 1500 x 1.18
Qn = 1770 watt

Then we choose from the Table 1.A (at 75/65 and 20°C) a radiator with Qn = 1770 watt.

If we straight away choose a radiator from the catalogue with a heat output of 1500 watt instead of 1770 watt, the temperature of the room would not come to the desired standard.

The above example shows how a non-standard condition can be converted to a desired standard condition.