Heating Capacity of a Radiator Formula and Calculator

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Heating Capacity of a Radiator Formula and Calculator

Heat Transfer Engineering and Design
Heat Exchanger Design and Engineering

Heating capacity of a radiator, Formula and Calculator

The heating capacity of a radiator, convector, baseboard, finned-tube heat-distributing unit, or radiant panel is a power function of the temperature difference between the air in the room and the heating medium in the unit.

Heating capacity is given as:

q = c ( t_s - t_a )ⁿ

Where:

q = heating capacity, W,
c = constant determined by test may also be obtained from manufacurer ,
t_s = average temperature of heating medium, °C. For hot water, the arithmetic average of the entering and leaving water temperatures is used,
t_a = room air temperature, °C. Air temperature 1.5 m above the floor is generally used for radiators, whereas entering air temperature is used for convectors, baseboard units, and finned-tube units,
n = exponent that equals 1.2 for cast-iron radiators,
1.31 for baseboard radiation,
1.42 for convectors,
1.0 for ceiling heating and floor cooling panels,
1.1 for floor heating and ceiling cooling panels.

For finned-tube units, n varies with air and heating medium temperatures. Correction factors n to convert heating capacities at standard rating conditions to heating capacities at other conditions are given in Table 1.0 and 2.0.

Table 1.0 Correction Factors c for Various Types of Heating Units

Table 19.2 Correction Factors c for Various Types of Heating Units [2016S, Ch 36, Tbl 2]

Steam Pressure (Approx.), kPa (absolute)	Steam or Water Temp., °C	Radiator Room Temp., °C			Convector Air Temp., °C
Steam Pressure (Approx.), kPa (absolute)	Steam or Water Temp., °C	25	20	15	25	20	15
9.5	45	—	—	—	—	—	—
15.8	55	—	—	0.40	—	—	0.33
25.0	65	0.40	0.47	0.54	0.33	0.40	0.47
38.6	75	0.54	0.61	0.68	0.47	0.54	0.61
57.9	85	0.68	0.76	0.83	0.61	0.69	0.77
84.6	95	0.83	0.91	0.99	0.77	0.85	0.93
120.9	105	0.99	1.07	1.15	0.93	1.02	1.11
169.2	115	1.15	1.24	1.32	1.11	1.20	1.30
232.3	125	1.32	1.41	1.50	1.30	1.40	1.50
313.4	135	1.50	1.59	1.68	1.50	1.60	1.70
415.8	145	1.68	1.77	1.86	1.70	1.81	1.92

Table 2.0 Correction Factors c for Various Types of Heating Units

Table 19.2 Correction Factors c for Various Types of Heating Units [2016S, Ch 36, Tbl 2]

Steam Pressure (Approx.), kPa (absolute)	Steam or Water Temp., °C	Finned-Tube Air Temp., °C			Baseboard Air Temp., °C
Steam Pressure (Approx.), kPa (absolute)	Steam or Water Temp., °C	25	20	15	25	20	15
9.5	45	0.15	0.21	0.26	0.14	0.19	0.24
15.8	55	0.26	0.32	0.37	0.24	0.30	0.36
25.0	65	0.37	0.44	0.50	0.36	0.43	0.49
38.6	75	0.50	0.57	0.64	0.49	0.56	0.63
57.9	85	0.64	0.71	0.78	0.63	0.70	0.78
84.6	95	0.78	0.86	0.94	0.78	0.86	0.94
120.9	105	0.94	1.01	1.09	0.94	1.02	1.11
169.2	115	1.09	1.18	1.26	1.11	1.20	1.29
232.3	125	1.26	1.34	1.42	1.29	1.38	1.47
313.4	135	1.42	1.51	1.60	1.47	1.57	1.66
415.8	145	1.60	1.69	1.78	1.66	1.76	1.86

Note: Use these correction factors to determine output ratings for radiators, convectors, and finned-tube and baseboard units at operating conditions other than standard.

Standard conditions in the United States for a radiator are 102°C heating medium temperature and 21°C room temperature (at center of space and at 1.5 m level).

Standard conditions for convectors and finned-tube and baseboard units are 102°C heating medium temperature and 18°C inlet air temperature at 101.3 kPa atmospheric pressure. Water flow is 0.9 m/s for finned-tube units. Inlet air at 18°C for convectors and finned-tube or baseboard units represents the same room comfort conditions as 21°C room air temperature for a radiator.

Standard conditions for radiant panels are 50°C heating medium temperature and 20°C for room air temperature; c depends on panel construction.

To determine output of a heating unit under nonstandard conditions, multiply standard heating capacity by appropriate factor for actual operating heating medium and room or inlet air temperatures. Related:

Derived from resources provided by:

ASHRAE Pocket Guide for HVAC SI, 2013