B

Backup heating
See Auxilliary heating.

Balance point
The outdoor temperature at which useful solar and internal gains equal the heat loss; the building needs additional inputs of heat at outdoor temperatures below the balance point and is free-running at temperatures above it.

Base temperature
A temperature datum taken as a base for the calculation of heating degree days; for example, the setting on the main thermostat controlling the heating appliances, or a volume-weighted mean whole-house temperature or the balance point temperature.

Black body
A (theoretical) perfect absorber and emitter of radiation with absorptance and emissivity of 1.0 for all wavelengths.