BTU/°R
BTU per Rankine degree
Heat capacityexact by definition
The BTU per Rankine degree is exactly 1899.100535 J/K, identical in size to BTU/°F. One US gallon of water is about 8.34 BTU/°R, which is the origin of the 500 factor in the US hydronic heat equation.
| 1 BTU/°R | 1,899.1005 J/K |
Where the unit came from
The familiar Q = 500 × gpm × ΔT for water comes straight from this unit: 8.34 lb per gallon times 1 BTU/(lb·°F) times 60 minutes per hour is 500.4, rounded to 500 and printed in every North American hydronic manual.