How it is calculated
Underfloor heating is not laid under fitted furniture or a bath, so that area is subtracted. Mats come in 0.5 m² steps and are chosen no bigger than the free area. Cable length is the power divided by the cable’s watts per metre. For a wet system the pipe length is the free area divided by the spacing, plus 10 % for bends and the run to the manifold.
Heated area = room area − area under furnitureCable length = area × W/m² ÷ cable W/mPipe = area ÷ spacing × 1.1 + 2 × distance to manifoldExample
A 6 m² bathroom has 1 m² under the bath and vanity. A 5 m² mat at 120 W/m² gives 600 W and draws 2.6 A. Heating 6 hours a day it uses 108 kWh a month, about 27 € at 0.25 € per kWh.
Tips from practice
- Under bathroom tiles 120–150 W/m² is usual; for primary heating 150–180 W/m².
- Put the floor sensor in a conduit between the cable runs so it can be replaced.
- Measure the cable resistance before and after covering it and keep the record.
Questions and answers
How much does underfloor heating cost to run?
Multiply the power by the hours the thermostat runs it and the price per kWh. A 600 W mat for 6 hours a day uses 108 kWh a month.
Does it need its own circuit?
Above 3.5 kW (16 A) it needs a dedicated circuit. An electrician should choose the breaker and RCD.
What pipe spacing for a wet system?
Usually 100–200 mm: 100–150 mm along outside walls and in bathrooms, 200 mm elsewhere. Keep each loop under 80–100 m.
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