In which situations is it acceptable to use a single stage regulator?

The use of a single stage regulator is specifically suited for small portable appliances and outdoor cooking appliances that operate at lower BTU levels, such as 100,000 BTU or less. This is because single stage regulators are designed to handle less complex pressure regulation needs and are typically sufficient for appliances that do not require high gas output or do not have extensive piping systems.

In large commercial appliances, a multi-stage or more complex regulator is often necessary to accommodate higher demands and varying pressure requirements. Similarly, indoor heating systems typically require more precise pressure regulation and the ability to adapt to fluctuations in gas demand, which again points to the need for more advanced regulators than a single stage. As for all types of LP gas installations, this includes a wide variety of applications that could exceed the capabilities of a single stage regulator, making it unsuitable for general use across the board.

Therefore, the specified limitation of using a single stage regulator with smaller appliances is the reason this answer is correct, highlighting its appropriate application in low-demand scenarios.