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Radiators vs underfloor heating: what it does to your flow temperature and SCOP
You almost certainly do not need underfloor heating. What you need is enough emitter surface to deliver your heat loss at a low flow temperature. This works out what your existing radiators can do, and what changes if you upsize a few of them.
The rule behind the numbers
Radiator output falls roughly with the 1.3 power of the temperature difference between water and room. Drop from 75 °C to 45 °C and you keep about half the output — so you need roughly twice the surface. The table below is that curve, referenced to output at 75 °C, which is the figure printed in radiator catalogues.
| Flow temp | Output factor vs 75 °C | SCOP |
|---|---|---|
| 35 °C | 0.30 | 3.9 |
| 45 °C | 0.51 | 3.4 |
| 50 °C | 0.63 | 3.0 |
| 55 °C | 0.75 | 2.8 |
| 60 °C | 0.87 | 2.5 |
| 75 °C | 1.00 | — |
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Compare at least three installers. Ask each one for the heat-loss calculation and the design flow temperature in writing — that number decides your running cost more than the brand on the box.
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