How radiant floor heating sizing works
Radiant floor sizing involves two linked problems: how much heat does the room lose (heat
loss), and how much heat can the floor actually deliver (output capacity). A floor surface
above 85°F becomes uncomfortably warm underfoot, which caps the BTU/hr per sq ft the floor can emit.
If room heat loss exceeds floor output capacity, a larger zone or supplemental heat is needed.
Heat loss calculation
Room heat loss is estimated using a BTU/hr per sq ft approach, adjusted for insulation quality and the
design ΔT between your target indoor temperature and the coldest outdoor design temperature for your
climate zone. Under-slab heat loss (when insulation is absent) is accounted for separately as a downward
loss penalty of up to 25%.
Design temperatures by climate zone
| IECC Zone |
Example Cities |
Design Outdoor Temp |
ΔT to 68°F |
| Zone 2 | Houston, Phoenix, Miami | 25°F | 43°F |
| Zone 3 | Atlanta, Dallas, Los Angeles | 15°F | 53°F |
| Zone 4 | NYC, DC, Seattle | 5°F | 63°F |
| Zone 5 | Chicago, Boston, Denver | −5°F | 73°F |
| Zone 6 | Minneapolis, Burlington | −15°F | 83°F |
| Zone 7 | Duluth, Fairbanks | −25°F | 93°F |
Floor output capacity and recommended supply water temperature
| Floor Covering |
Added R-Value |
Recommended Supply Temp |
Max Output (BTU/hr/sq ft) |
| Tile / stone | R-0.05 | 85–100°F | 35–40 |
| Engineered hardwood / LVP | R-0.5–0.7 | 95–115°F | 25–32 |
| Solid hardwood (max ¾ in.) | R-0.68–1.0 | 100–120°F | 20–28 |
| Thin carpet + pad (R < 1.0) | R-1.0 | 115–130°F | 18–22 |
| Thick carpet + pad (R 1.0–2.5) | R-2.0 | 130–140°F | 12–18 |
Supply temperatures above 140°F are not recommended for embedded radiant systems. If room load cannot
be met at 140°F supply, supplemental heating is required.
Standard condensing boiler sizes for hydronic radiant
| Boiler Output (BTU/hr) |
Typical Application |
Approx. Area (Zone 5, good insulation) |
| 40,000–50,000 | Small condo or apartment | 800–1,200 sq ft |
| 60,000–80,000 | Medium home | 1,200–2,000 sq ft |
| 90,000–110,000 | Large home | 2,000–3,000 sq ft |
| 120,000–150,000 | Very large / multi-zone | 3,000–4,500 sq ft |
| 175,000+ | Estate / commercial | 4,500+ sq ft |
Frequently asked questions
How many BTU per square foot for radiant floor heating?
In a well-insulated home with tile floors in Zone 5, plan for 30–35 BTU/hr per sq ft.
Poorly insulated homes or Zone 6–7 climates can need 45–55 BTU/sq ft. Carpet-covered floors
are limited to 15–20 BTU/sq ft because the carpet layer insulates against heat transfer —
rooms with thick carpet may need supplemental heat in cold climates.
What size boiler do I need for radiant floor heating?
Total the BTU/hr heat loss for every room, then add 20–25% as a system buffer for cycling losses.
A 1,500 sq ft well-insulated home in Zone 5 typically needs a 60,000–80,000 BTU/hr
condensing boiler. The calculator above totals all rooms and recommends the nearest standard
boiler size. Most residential models are sold in 40k, 60k, 80k, 100k, 120k, and 150k BTU increments.
Can you put radiant floor heating under hardwood?
Yes, with the right choice. Engineered hardwood (up to ¾ in. thick) is the
safest option — it’s dimensionally stable and rated for temperature cycling. Supply water
should stay below 115°F. Solid hardwood can work if supply temperature is kept
below 85°F and indoor humidity is controlled year-round (35–55% RH). Avoid solid planks
wider than 3 inches, which are more prone to gapping and cupping.
What water temperature for radiant floor heating?
Tile: 85–100°F supply. Engineered hardwood: 95–115°F.
Thin carpet: 115–130°F. Thick carpet: 130–140°F.
These are all far lower than baseboard radiators (160–180°F) — which is why condensing
boilers and air-to-water heat pumps are the ideal boiler choice for radiant floor systems.
How much does radiant floor heating cost per month?
With a 96% condensing boiler at $1.40/therm, a 1,500 sq ft Zone 5 home typically spends
$80–$150/month on heating during peak winter. Electric radiant for the same
area at 16¢/kWh would cost $200–$400/month — which is why electric radiant is best
reserved for bathrooms, kitchens, and other zones under 200 sq ft.
What floor coverings work with radiant heat?
Tile and stone are the best — near-zero thermal resistance (R-0.05) allows high heat output at low
supply water temperatures.
Electric radiant mats
under tile are the easiest retrofit. Engineered hardwood and luxury vinyl plank (LVP) also work well.
Solid hardwood works if supply temperature stays below 85°F. Thick carpet (R-2+) significantly
restricts output and may require supplemental heat in cold climates.
How many watts per square foot for electric radiant floor heating?
For supplemental floor warmth in a bathroom or kitchen: 10–12 W/sq ft is standard.
For primary heat in a cold climate: 15 W/sq ft or more.
A 100 sq ft bathroom at 12 W/sq ft draws 1,200 watts — about 10 amps on 120V, or 5 amps on 240V.
Most pre-made mats are sold at 10–12 W/sq ft.
See the space heater calculator for wattage-to-room sizing context.
Is radiant floor heating compatible with a heat pump?
Yes — it’s one of the best pairings available. Air-to-water heat pumps typically deliver
water at 100–120°F, which aligns perfectly with tile- and hardwood-covered radiant floors.
The low water temperature allows the heat pump to run at a higher COP (coefficient of performance),
which lowers electricity consumption compared to running the same heat pump into high-temperature
baseboard radiators.