Subfloor & prep
Electric radiant floor heating: area, watts, circuit and running cost
Electric floor heat is sized by the open floor you stand on, not the room. Here is how to work out the heated area, the load on the circuit and the monthly bill for a bathroom and a kitchen.
What it is
Electric radiant floor heating is a heating cable, either loose or woven into a mat, laid on the subfloor and covered by the floor. Under tile and stone it is set in thinset or a thin pour of leveler; some systems are made for laminate, engineered wood and vinyl. It warms the floor and the people standing on it, and in a small bathroom it can heat the room. Hydronic systems, which run warm water through tubing, are a different design for whole houses and are not covered here.
The heated area
Heating cable cannot be cut. A mat or cable kit is bought for a fixed heated area, and the heated area is the open floor where people stand, not the whole room. It leaves out:
- a gap of 3 to 6 inches from walls,
- everything that stands on the floor: vanity, tub, shower base, toilet, kitchen cabinets and island,
- the area within a few inches of the toilet flange.
Buy the kit whose heated area is the size just below your figure, never above. A kit that is too big ends up with cable under a vanity or toilet, which makers forbid.
A bathroom worked through
An 8 × 5 ft bathroom
A 4 inch gap at the walls, 12 sq ft under the tub end, vanity and toilet, heating at 12 W per sq ft on 120 V.
- Inside the wall gap: about 32 sq ft; less fixtures 12 sq ft = 20 sq ft heated, about half the room
- Power: 20 × 12 = 237 W
- Current: 237 ÷ 120 = 1.98 A; with the 125% rule, 2.5 A, well within a 15 A circuit
A larger 9 × 7 ft bathroom heats about 41 sq ft at 489 W and 4.08 A.
A kitchen
A 16 × 12 ft kitchen on 240 V
With 40 sq ft under the cabinets and island.
- Heated area: about 134 sq ft
- Power: about 1,605 W
- Current at 240 V: 6.69 A, on its own 15 A circuit
The radiant floor heating calculator works out the area, load and circuit for any room. Kits over about 150 sq ft are usually supplied for 240 V, which halves the current.
The circuit
Electric floor heat needs a GFCI-protected circuit under the US National Electrical Code (NEC 424.44) and a floor-sensing thermostat, usually with the GFCI built in. A small bathroom mat can often share a bathroom circuit if the total load allows; larger floors need a dedicated circuit. In most places the connection must be made or inspected by a licensed electrician. Test the cable’s resistance before, during and after laying the floor, as the maker’s instructions say; a damaged cable under tile cannot be repaired easily.
Running cost
The cost depends on the heater’s watts, the hours it runs, the share of those hours the thermostat keeps it on, and the price of electricity. The US average residential price is about 18 cents per kWh, according to the US Energy Information Administration.
A 35 sq ft bathroom floor
420 W, on 4 hours a day, at $0.18 per kWh.
- Energy: 0.42 kW × 4 h = 1.68 kWh a day
- Cost: 1.68 × $0.18 = $0.30 a day, about $9.19 a month, $55 over a six-month heating season
The 134 sq ft kitchen on the same schedule is about $35 a month.
The radiant floor running cost calculator takes your own price, hours and duty cycle. A programmable thermostat that warms the floor for an hour or two morning and evening costs much less than one left on all day.
Thermostat settings
The thermostat’s floor sensor, a probe laid between two runs of cable, limits the floor temperature. Most makers cap it at about 82 to 85°F for wood and vinyl and allow more for tile. A floor at 80°F feels warm underfoot without heating the room much; heating a room from the floor alone needs higher output and more hours, and the running cost rises with both.
Insulation and warm-up
On a concrete slab, much of the heat goes down into the slab rather than up into the room. An insulating underlayment board under the heater cuts the warm-up time from hours to well under an hour and lowers the running cost noticeably. On a wood subfloor over a heated space, it matters less.
Under the tile
Mats are pressed into a layer of thinset and tiled over; loose cable is fixed with strips or a membrane and covered with self-leveler. The leveler must be at least 1/8 inch over the cable. The self-leveling compound calculator counts the bags, and the leveler guide explains the depth. The tile goes on as usual; the floor tile calculator counts it for the whole room, heated or not. Tile and stone conduct heat best; wood and carpet insulate, and the heater must be one rated for them.
The subfloor preparation guide covers the floor under the heater, and the tile guide the floor above it.