This SAE Recommended Practice establishes uniform test procedures and performance requirements for engine coolant type heating systems of enclosed truck cabs. The intent is to provide a test that will ensure acceptable comfort for cab occupants. It is limited to a test that can be conducted on uniform test equipment in commercially available laboratory facilities. There are two options for producing hot coolant in this document. Testing using these two approaches on the same vehicle will not necessarily provide identical results. Many vehicle models are offered with optional engines, and each engine has varying coolant temperatures and flow rates. If the test is being conducted to compare the performance of one heater design to another heater design, then the external coolant source approach (Test A) will yield the most comparable results. If the test is being conducted to validate the heater installation on a specific vehicle model with a specific engine, then using the engine to heat the coolant (Test B) will be more appropriate. This document will be reviewed and revised as required with advancements in technology and changes in environmental laws.