25,000BTU dumped into 2000cfm would yield a ballpark ~11degF heat rise, i.e., the "preheated" temperature of the air entering the radiator is increased by ~11degF. The temp rise varies inversely with CFM, e.g., at 1000 CFM the rise is ~22degF, at 10,000CFM it's ~2.2degF.
Other factors will cause this to vary but for a ballpark estimate, this should do.
Ambient conditions - Dry air at or around 68degF, thereby having density of 1.29kg/m^3 and specific heat 1000 J/kg*degK.
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