About this calculator
How many watts does a radiator or an underfloor loop need to keep this room warm on the coldest days? That figure is the room's design heat load, and this calculator works it out by the simplified method of EN 12831 (in Lithuania LST EN 12831):
- transmission — heat lost through every wall, window, door, roof and floor of the room, each with its own area, U-value and correction factor;
- ventilation — heat needed to warm the fresh air coming in;
- an optional heating-up allowance for rooms whose heating is turned down at night.
The result is given in watts, and per m² and m³ so you can compare it with the usual rules of thumb. Each element's U-value comes from the U-value calculator: work out the wall, roof or floor build-up there and enter the result here.
Formula
ΔT = θ_int − θ_e
Φ_T = Σ f_k × A_k × (U_k + ΔU_TB) × ΔT each element k
Φ_V = 0.34 × n × V × ΔT ventilation
Φ_hu = A_floor × φ_hu heating-up
Φ_HL = Φ_T + Φ_V + Φ_hu
f_k— correction factor: 1.0 for elements to the outside air; for a floor or wall to an unheated space, a reduction factor (STR 2.09.04:2008 Table 4.1: 0.5 for a basement without windows or outside doors, 0.8 with them, 0.7 for an insulated attic, 0.8 for a ventilated crawl space).- Floor on the ground: the ground is warmer than the outside air in
winter, so EN 12831 uses
f = f_g1 × f_g2 × G_w, withf_g1 = 1.45,f_g2 = (θ_int − θ_m,e) / (θ_int − θ_e)andG_w = 1.15when groundwater is less than 1 m below the slab (otherwise 1.0). Enter the floor's equivalent U-value to the ground. ΔU_TB— optional thermal-bridge allowance added to every element except the ground floor.0.34 Wh/(m³·K)— heat capacity of air,c × ρ(EN 12831, STR 2.09.04).n— minimum air change rate per hour;V— room volume.
Default temperatures. 20 °C indoors is the heating-season temperature of STR 2.09.04:2008 for dwellings. The outdoor design temperature in Lithuanian heating design is the mean of the coldest five-day period from the building climatology norm (now STR 2.01.12:2024); depending on the location it lies around −21…−25 °C. The default −23 °C is only a typical value — enter the one for your location. The annual mean of 7.4 °C is the country average of STR 2.01.12:2024 (1991–2020).
Worked example
A 5 × 4 m room, 2.7 m high, on a ground-floor slab, with one external wall (10.5 m², U = 0.20) and one window (3 m², U = 1.0); 20 °C inside, −23 °C outside, 0.5 air changes per hour:
ΔT = 20 − (−23) = 43 K
wall = 10.5 × 0.20 × 43 = 90.3 W
window = 3.0 × 1.00 × 43 = 129.0 W
floor f = 1.45 × (20 − 7.4)/43 × 1.0 = 0.425
= 0.425 × 20 × 0.15 × 43 = 54.8 W
Φ_T = 274.1 W
Φ_V = 0.34 × 0.5 × 54 × 43 = 394.7 W
Φ_HL = 668.9 W
= 33.4 W/m² of floor, 12.4 W/m³
In a well-insulated house the ventilation share is often the larger one — as here.
FAQ
Which outdoor temperature should I use? The design temperature for your location, from STR 2.01.12:2024 or your heating designer: the mean of the coldest five-day period, roughly −21 to −25 °C across Lithuania. A warmer value undersizes the radiators; a colder one oversizes them.
Why is the floor on the ground counted with such a small factor? Because under a building the ground stays far warmer than the winter air. With a mean annual temperature of 7.4 °C, the factor for a 20 °C room is about 0.42 — the floor loses less than half of what the same U-value would lose to the outside air.
What air change rate should I enter? At least the room's minimum fresh-air rate: around 0.5 per hour for a living room or bedroom in typical EN 12831 national annexes, more for kitchens and bathrooms. With a heat-recovery ventilation unit the heat actually lost is smaller, which this simplified method does not reduce.
Do I need the heating-up allowance? Only if the heating is turned down at night or for longer periods and must bring the room back up in reasonable time. For continuously heated rooms (typical with a heat pump and underfloor heating) leave it at 0.
How does this relate to the U-value calculator? The U-value calculator gives one element's U from its layers. This calculator takes those U-values for every element of the room and turns them into watts.
Assumptions and limits
- One room at a time, simplified EN 12831 method. Heat flow to neighbouring heated rooms at a different temperature is not modelled (enter it as an element with the appropriate factor if needed).
- No internal or solar gains, no extra infiltration beyond the minimum air change, no heat-recovery credit.
- Default U-values, areas, factors and the 0.5 1/h air change are examples; the outdoor design temperature must be set for your location.
- The thermal-bridge and heating-up allowances default to 0 — EN 12831's own default values for them are not applied here; enter your designer's. EN 12831 requires thermal bridges to be accounted for (typically 0.05–0.15 W/(m²·K)), so with the default 0 the transmission loss is understated and a warning is shown.
- The result is the heat the emitters must supply to this room; boiler or heat pump sizing for the whole house also needs domestic hot water and system losses.