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Can Radiant Floor Heating Be a Primary Heat Source? Heat Loss, BTUs, and Sizing Guide
Yes. Electric radiant floor heating can be the primary heat source for a room, or for a whole home, when the heat it delivers meets or exceeds the room’s heat loss. Warmup systems provide 11 to 20 watts per square foot of installed heating capacity, which is about 37 to 68 BTU per hour per square foot. To confirm it will work, calculate your room’s heat loss in BTU/hr, convert it to watts (1 watt = 3.412 BTU/hr), and compare it with the output of your heated floor area. Radiant floors work best as primary heat in well-insulated rooms with low-resistance floor coverings such as tile or stone.
Floor heating as a primary heat source at a glance
| Question | Answer |
| Can floor heating be the primary heat source? | Yes, if its output covers the room’s heat loss |
| Warmup installed output | 11 to 20 W/sq ft (about 37 to 68 BTU/hr per sq ft); standard Warmup output is 13.1 W/sq ft |
| Watts to BTUs | 1 W = 3.412 BTU/hr |
| BTUs to watts | 1 BTU/hr = 0.293 W |
| What determines whether primary heat is possible | Heat loss, heated floor area, insulation, floor covering |
| Heat a warm floor delivers | About 28 BTU/hr per sq ft (84°F floor in a 70°F room) |
| Heat loss formula | Area × U-value × temperature difference, plus air leakage |
| Thermostat capacity | 15 amps |
| Best floor coverings | Tile, stone, and other low-resistance surfaces |
| Who confirms the design | The HVAC professional who performs the heat loss calculation, with Warmup design support |
Can floor heating be used as a primary heat source?
Yes. Warmup underfloor heating can be the only heat source in a room when it is sized to replace the heat the room loses on the coldest days. Warmup systems are designed to go under most types of flooring and deliver 11 to 20 watts per square foot to suit a range of heating needs.
Whether floor heating is enough on its own depends on four things:
- The room’s heat loss, measured in BTU per hour. This is how fast heat escapes through walls, windows, ceilings, floors, and air leaks.
- The heated floor area. Only the floor covered by heating mat or cable produces heat. Space under cabinets, tubs, and fixed furniture is normally left unheated.
- Insulation under the heater. Insulation sends heat up into the room instead of down into the subfloor or slab.
- The floor covering. Tile and stone pass heat easily. Thick carpet, pads, and thick wood slow down heat transfer.
If you are working on a primary heating project, contact Warmup and the design team will customize a solution for your space.
How many BTUs are in a watt?
1 watt equals 3.412 BTU per hour (often rounded to 3.41). To convert watts to BTU/hr, multiply by 3.412. To convert BTU/hr to watts, divide by 3.412, or multiply by 0.293.
Electric floor heating is rated in watts. Heat loss calculations and most furnaces, boilers, and heat pumps are rated in BTU per hour. Converting between them lets you compare a floor heating system directly with the heat a room needs.
| Watts | BTU per hour |
| 1 W | 3.41 BTU/hr |
| 100 W | 341 BTU/hr |
| 500 W | 1,706 BTU/hr |
| 1,000 W (1kW) | 3,412 BTU/hr |
| 1,500 W | 5,118 BTU/hr |
| 3,600 W (one 240V thermostat at 15 amps) | 12,283 BTU/hr |
Floor heating output in BTUs per square foot
| Installed output | BTU/hr per sq ft |
| 11 W/sq ft | 37.5 |
| 13.1 W/sq ft (standard Warmup output) | 44.7 |
| 15 W/sq ft | 51.2 |
| 20 W/sq ft | 68.2 |
What is heat loss, and how do I calculate it?
Heat loss is the rate at which a room loses heat to the outdoors, measured in BTU per hour. A heat loss calculation tells you how many BTUs you need to supply to keep the room warm and compensate for that loss. Once you know the number, you can size a floor heating system to match.
Heat escapes in two main ways:
- Conduction through walls, windows, doors, ceilings, and floors. For each surface: Heat loss (BTU/hr) = Area (sq ft) × U-value × Temperature difference (°F), where U-value = 1 ÷ R-value.
- Air leakage (infiltration) through gaps, vents, and doors. Drafty older homes lose much more heat this way.
The temperature difference is your target indoor temperature minus your area’s winter design temperature (the coldest typical outdoor temperature for your location). Add the heat loss of every surface plus air leakage to get the room total.
Rough heat loss ranges
Use these only for early planning. A proper calculation is needed for primary heat.
| Home type | Approximate heat loss |
| New construction, well insulated and air sealed | About 10 to 20 BTU/hr per sq ft |
| Average existing home | About 20 to 35 BTU/hr per sq ft |
| Older home, little insulation, drafty windows | About 35 to 50+ BTU/hr per sq ft |
Your climate, ceiling height, window area, and exterior wall count all change these numbers.
For primary heating, have an HVAC professional or energy auditor perform a room-by-room heat loss calculation, such as ACCA Manual J. Warmup can then design a system to match the result.
How do I know if floor heating can cover my room’s heat loss?
Compare the heat your heated floor can deliver with the room’s calculated heat loss. If the floor delivers at least as much heat as the room loses, floor heating can be the primary heat source.
- Calculate the room’s heat loss in BTU/hr.
- Measure the heated area. Count only floor that will be covered by mat or cable.
- Find the installed output. Multiply the heated area by the system’s watts per square foot, then by 3.412 for BTU/hr.
- Check delivered output. A warm floor delivers about 2 BTU/hr per square foot for each °F the floor surface is warmer than the room air. At a comfortable floor temperature of about 84°F in a 70°F room, that is about 28 BTU/hr per square foot.
- Compare. The delivered output should equal or exceed the room’s heat loss. If it falls short, improve insulation, increase the heated area, or add a supplemental heat source.
Why installed watts and delivered heat are different
Installed output is the most heat the cable can produce. Delivered output is the heat the floor actually releases into the room, and it is limited by how warm the floor surface is allowed to get. Floor temperature is capped for comfort and to protect floor coverings, so a system’s extra installed capacity mainly helps the floor warm up faster. It does not raise the floor’s steady-state output. This is why heated floor area matters more than wattage when floor heating is your only heat source.
Worked example: a 12 × 15 ft room
- Room size: 180 sq ft
- Heat loss: 180 sq ft × 22 BTU/hr per sq ft = 3,960 BTU/hr (1,161 W)
- Heated area: 150 sq ft (after leaving out fixed furniture and built-ins)
- Installed output: 150 sq ft × 13.1 W/sq ft = 1,965 W, or about 6,700 BTU/hr
- Delivered output: 150 sq ft × 28 BTU/hr per sq ft = 4,200 BTU/hr
- Result: 4,200 BTU/hr delivered is more than the 3,960 BTU/hr heat loss, so floor heating can be this room’s primary heat source.
- Electrical: 1,965 W at 240V is about 8.2 amps, within one 15-amp thermostat. At 120V, it would be 16.4 amps, which is too much for one thermostat. See our 120V vs. 240V floor heating guide.
What affects whether floor heating is enough?
Insulation under the heater
Insulation is the biggest factor you control. Without it, a large portion of the heat moves down into the subfloor or concrete slab instead of up into the room. Installing Warmup Insulation Boards under the heating system reduces downward heat loss, shortens heat-up time, and lowers running costs. Insulation is especially important on concrete slabs and over unheated spaces such as crawl spaces and garages.
Floor covering
Every floor covering resists heat flow, measured by its R-value. The lower the total R-value of the flooring and any underlay, the more heat reaches the room.
| Floor covering | Heat transfer | Primary heat suitability |
| Tile, stone, polished concrete | Excellent | Best choice |
| Luxury vinyl tiles (LVT) and planks (LVP) | Good | Good, within the flooring maker’s temperature limit |
| Laminate and engineered wood | Moderate | Good with a low-R underlay; check the flooring maker’s temperature limit |
| Solid hardwood | Moderate to low | Possible; thinner boards perform better |
| Carpet | Low | Limited; use a thin pad and low-pile carpet |
See what flooring can be used with underfloor heating for product-specific guidance.
The building itself
Window area, ceiling height, exterior walls, and air sealing all raise or lower heat loss. Large windows and high ceilings increase the heat a room needs. Air sealing and upgraded windows reduce it, often making it practical for floor heating to carry the full load.
Heated floor coverage
The more of the floor you heat, the more heat the room receives. For primary heat, plan to cover as much open floor as possible. Rooms with large fixed cabinets or built-ins leave less area to heat, which can limit delivered output.
Which Warmup system should I use for primary heat?
Choose your heating system based on your subfloor and floor covering. All Warmup systems can serve as primary heat when they are sized to the room’s heat loss.
| System | Best for |
| DCM-PRO System | Tile and stone over an uncoupling membrane; flexible cable spacing |
| In-Slab Cable System | New concrete slabs, basements, and large open areas |
| StickyMat System | Simple rectangular rooms under tile, stone, or vinyl |
| Foil Heater System | Floating laminate and engineered wood floors |
Pair the system with a programmable or smart thermostat, such as the Warmup 7iE or Terra, so you can set schedules and keep each room at the right temperature.
What electrical setup does primary floor heating need?
Primary heating usually covers more floor area than comfort heating, so the electrical load is larger. Most whole-room primary heating systems run on 240V, which halves the amperage compared with 120V. Each Warmup thermostat carries up to 15 amps. Larger loads need a relay, such as the Relay-25, or a contactor panel.
All wiring should be done by a licensed electrician. For more information on circuits, wire gauge, GFCI, and relays, see our Electrical Requirements for Warmup Floor Heating guide.
How much does it cost to run floor heating as a primary heat source?
Running cost = system kilowatts × hours of operation × your electricity rate. Because a thermostat cycles the heater on and off, the system rarely runs at full power for every hour it is switched on.
Example: A 1,965 W (1.965 kW) system that runs the equivalent of 8 full-power hours on a cold day uses about 15.7 kWh. At an example rate of $0.17 per kWh, that is about $2.67 per day. Your rate, climate, insulation, and thermostat settings will change this.
How can I lower floor heating running costs?
- Insulate under the heater.
- Use a programmable or smart thermostat with setback temperatures for nights and time away.
- Choose a low-resistance floor covering such as tile.
- Air seal the room and upgrade drafty windows.
Estimate your own costs with the Warmup running cost calculator or read more about floor heating running costs.
Is floor heating better as primary or supplemental heat?
Both work well. The right choice depends on your room’s heat loss and whether another heating system is already in place.
| Primary heat | Supplemental (comfort) heat | |
| Purpose | Heats the room on its own | Warms the floor; another system heats the air |
| Sizing | Must match the room’s heat loss | Sized to the floor area for comfort |
| Floor coverage | As much open floor as possible | Main walking areas |
| Insulation under heater | Strongly recommended | Recommended |
| Typical voltage | Usually 240V | 120V or 240V |
| Heat loss calculation | Required | Not usually needed |
Is floor heating allowed as the primary heat source under building code?
Generally, yes, as long as it meets your local heating requirements. In areas that adopt the International Residential Code (IRC), habitable rooms in colder climates must have heating that can maintain at least 68°F at 3 feet above the floor (IRC Section R303.10). A floor heating system sized to the room’s heat loss is designed to meet that requirement. Requirements vary by jurisdiction, so confirm with your local building department. California projects should also review Title 24 compliance.
Other frequently asked questions
Can radiant floor heating heat a whole house?
Yes, when every room’s heat loss is covered by its heated floor area. Electric floor heating is well suited to well-insulated homes, additions, basements, and smaller homes. For large homes in cold climates, compare operating costs with other heating options during design.
Is electric floor heating efficient as a primary heat source?
Yes. Electric floor heating converts nearly 100% of the electricity it uses into heat in the room, with no duct losses. Radiant heat also warms people and objects directly, so many people feel comfortable at a slightly lower air temperature. Running costs depend on your electricity rate and how well the room is insulated.
Do I need a backup heat source?
Not if the system is sized to your room’s heat loss at your winter design temperature. Some homeowners in very cold climates keep a backup source for extreme cold or power outages.
How warm does a heated floor get?
Floors are typically kept between about 77°F and 84°F, which feels warm to bare feet without being hot. Your thermostat limits the floor temperature to protect the floor covering. Wood, laminate, and vinyl floors may have lower limits set by the flooring manufacturer.
Can I use floor heating as primary heat in a basement?
Yes. Basements are a common primary heating application. Insulate the concrete slab before installing the heater, or use the In-Slab Cable System in a new slab.
Can floor heating be the primary heat source under carpet?
It can work under low-pile carpet with a thin pad, but carpet reduces the heat reaching the room. In rooms with high heat loss, tile or luxury vinyl is a better choice.
Is 1 watt equal to 3.41 or 3.412 BTU?
Both are correct. 1 watt equals 3.412 BTU per hour, which is often rounded to 3.41.
How many watts do I need to heat a room?
Divide the room’s heat loss in BTU/hr by 3.412. For example, a room that loses 3,960 BTU/hr needs at least 1,161 watts of delivered heat.
Who can help me design a primary heating system?
Contact Warmup at (888) 927-6333 or ussales@warmup.com. Share your room dimensions, floor covering, and heat loss calculation, and the team will design a system for your space.
