Radiator Output Sizing
Calculator
Results
- Required heat output (W)
- 1,872
- Catalogue output at ΔT 50 K (W)
- 1,872
- Radiator correction factor
- 1
- Power (BTU/h)
- 6,387.529824
HVAC and plumbing results
| Required heat output (W) | 1,872 |
| Catalogue output at ΔT 50 K (W) | 1,872 |
| Radiator correction factor | 1 |
| Power (BTU/h) | 6,387.529824 |
formula-map diagram
- Required heat output (W)
- 1,872
- Catalogue output at ΔT 50 K (W)
- 1,872
- Radiator correction factor
- 1
- Power (BTU/h)
- 6,387.529824
HVAC and plumbing relationship
Formula
Q50 = V × f × ΔTroom ÷ (ΔTrad ÷ 50)^1.3= 1872
Note
This is a simplified model: it applies a standard engineering formula to the numbers you entered. Conversions between BTU/h and kW use the exact factor 1 kW = 3412.142 BTU/h, but every sizing figure is an estimate. Air conditioner sizing uses the common 20 BTU/h per square foot rule of thumb, not a room-by-room load calculation, and it ignores insulation, glazing, orientation, ceiling height, infiltration and local climate. Heating loads use a single volumetric heat-loss factor in W/m³·K rather than a fabric-by-fabric U-value calculation. Air properties are fixed at 1.2 kg/m³ and water at 1000 kg/m³ and 4186 J/kg·K, with no correction for temperature, altitude or glycol. Duct and pipe results use the ideal continuity equation and ignore fittings, bends, roughness and system effect unless you enter those losses yourself. The Hazen-Williams equation is valid only for water in full turbulent flow at ordinary temperatures. Water hammer uses the Joukowsky surge, an upper bound for instant closure. Tank drainage assumes a prismatic tank and steady free discharge. Hot water recovery and condensate figures ignore standing losses and coil bypass. Size real systems with a proper heat-loss survey and have the work checked by a qualified HVAC or plumbing professional.
More in HVAC and plumbing
See all →Frequently asked questions
How do I know what radiator output (in watts or BTU) a room needs?+
Radiator output should match the room's calculated heat loss — the rate at which the room loses heat through walls, windows, and ventilation at the design outdoor temperature. Matching output to heat loss keeps the room at the target indoor temperature on the coldest expected day.
Why do radiators list output at specific temperature differentials like Delta T 50?+
A radiator's heat output depends heavily on how much hotter it is than the surrounding room air. Manufacturers rate output at a standard temperature difference (commonly 50°C between average water temperature and room air) so outputs can be fairly compared between products.
What happens if my system runs at a lower flow temperature than the radiator's rated Delta T?+
Lower flow temperatures — common with heat pumps and modern condensing boilers — reduce the radiator's actual heat output below its nameplate rating, sometimes significantly, which is why radiators are often oversized when converting a system to a heat pump.
Should I size a radiator for the coldest day of the year or an average day?+
Radiators are sized for the design condition — a defined cold outdoor temperature specific to the climate — not the average. This ensures the room stays comfortable on the coldest days the heating system is realistically expected to handle, even though it will be oversized on milder days.
Is a bigger radiator always better?+
Not necessarily — an oversized radiator can make a room overheat or force the system to cycle inefficiently at low output, and it costs more upfront. Matching output closely to the calculated heat loss, with a modest safety margin, is generally preferred over significant oversizing.