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Heat emission
When analysing and planning the installation of a floor heating system, the predicted heat emission should be calculated as thoroughly as possible. To do this, a formula1) calculating the heat emission of general heating systems within the temperature range needed for indoor heating is used.
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Heat emission per m²: |
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q = 8.92 x ÄT(1.1) |
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Taking this formula into consideration, the following table shows the results for different ?Ts:
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ÄT |
q |
ÄT |
q |
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1°C |
8.9 W/m² |
7°C |
75.9 W/m² |
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2°C |
19.1 W/m² |
8°C |
87.9 W/m² |
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3°C |
29.9 W/m² |
... |
... |
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4°C |
41.0 W/m² |
33°C |
417.6 W/m² |
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5°C |
52.4 W/m² |
34°C |
431.5 W/m² |
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6°C |
64.0 W/m² |
... |
... |
1) The heat emitting features of the floor coverings might differ a little. Emissions from the surface to the laminar boundary air layer that are located just above are different for a smooth material and for a rough carpet.
The above calculations show, that floor heating is much more efficient in keeping a constant indoor temperature than radiators.
In the following you will find a short example comparing floor heating (case 1) to radiators (case 2) that demonstrates the physical reasons:
In both cases, we would like to achieve an indoor temperature of TAir = 22°C.
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| Example: |
Case 1
Let us also assume that if we use floor heating, our floor temperature would be TFloor = 25°C to compensate for energy losses from our apartment at a certain given outdoor climate. Our ÄT = 3°C.
We can now calculate that for a 100 m² apartment, our floor emits
q = 29.9 W/m² x 100 m² = 2990 W.
The emitting area is the whole floor of AFloor = 100 m²
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Case 2
In case of radiators we have a high temperature, assume TRadiator = 55°C, which gives ÄT = 33°C and a small emitting area, ARadiator (assume for instance 5 radiators in a 100 m² apartment). As the energy losses are the same, we can assume that the radiators totally emit the same 2990 W as in case 1.
Through this we can calculate the emitting area to be:
ARadiator = 2990 W / (417.6 W/m²) = 7.16 m². |
What happens if an outdoor temperature drop makes the indoor air cool down by 1°C (for instance due to the sun setting in spring)?
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Case 1
The ?T immediately raises from 3°C to 4°C, resulting in the floor emitting q = 41 W/m² x 100 m² = 4100 W. The power increase is 1150 W.
We can see that the floor immediately increases it's heat emission, without the floor temperature needing to raise or react at all. This additional power of 1150 W is likely to limit the temperature drop to a few tenths of one centigrade. For each 0.1 C drop in the indoor air temperature, the floor increases by 115 W.
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Case 2.
The ÄT increases from 33°C to 34°C, giving us a total heat emission of q = 431.5 W/m² x 7.16 m² = 3090 W. The power increase is only 100 W. To a limited amount, the radiator thermostat can increase the radiator temperature by allowing a higher water flow. As the water temperature remains the same, this increase is highly limited. |
| Conclusion |
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Whereas, a floor heating system reacts with an increase of +115 W with a temperature drop of only 0.1° C, the inner air needs to drop 1.15° C for a radiator system to resist the drop to be equally as good. In this case, we can say that the floor heating is 11 - 12 times more efficient in keeping a required thermostat regulated temperature compared to the radiator system. The same applies if the indoor temperature rises.
In addition, floor heating systems start heating from the bottom (resulting in warm and comfortable feet), whereas the radiator heat goes up to the ceiling and first warms the air 1-2 m above our heads!
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