Heater. Any device or assembly of devices or appliances used to heat the interior of any motor vehicle. This includes a catalytic heater which must meet the requirements of 177.834(2) of this title when Class 3 (flammable liquid) or Division 2.1 (flammable gas) is transported.
49 CFR § 393.5
As used in this part, the following words and terms are construed to mean: