How it is calculated
Radiant heat is not installed under cabinets, tubs or vanities, so that area is subtracted. Mats come in set sizes and are chosen no bigger than the free area. Cable length is the wattage divided by the cable output per foot. For hydronic heat, tubing 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 cabinets and fixturesCable length = area × W per sq ft ÷ W per ft of cableTubing = area ÷ spacing × 1.1 + 2 × distance to manifoldExample
A 60 sq ft bathroom has 10 sq ft under the tub and vanity. A 50 sq ft mat at 12 W per sq ft is 600 W and draws 5 A on 120 V. Heating 6 hours a day it uses 108 kWh a month, about $18 at $0.17 per kWh.
Tips from practice
- 12 W per sq ft is typical for bathroom floors; 15 W per sq ft for primary heat.
- Put the floor sensor between two cable runs so it can be replaced.
- Test the cable resistance before and after tiling and keep the record.
Questions and answers
How much does radiant floor heat cost to run?
Multiply watts by the hours the thermostat runs it and your rate. A 600 W mat for 6 hours a day uses 108 kWh a month.
120 V or 240 V?
Small bathrooms usually use 120 V. Larger floors use 240 V. Heating loads should stay under 80 % of the circuit rating; an electrician sizes the circuit and GFCI.
What spacing for PEX tubing?
Usually 6–12 in: 6 in along exterior walls and in bathrooms, 9–12 in elsewhere. Keep each loop under 250–300 ft for 1/2 in PEX.
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