Definition
Metal shrouds fitted around portions of the engine exhaust system that capture heat from the hot exhaust pipes. Outside air is routed through these shrouds, warmed by contact with the exhaust surface, and then ducted into the cabin for heating and into the carburetor intake to prevent or remove carburetor ice.
Plain English
A simple metal sleeve wrapped around the exhaust pipe. Air passes through the sleeve, picks up heat from the hot pipe, and that warm air is then sent to the cabin heater or to the carburetor to keep ice from forming.
Context Anchor
You may encounter heat muffs during preflight inspection, cabin heat checks, carburetor heat checks, and discussions of the exhaust system.
Derivation
A 'muff' is an old word for a tube-shaped covering you slide your hands into to keep them warm. The exhaust 'heat muff' works the same way -- a sleeve that wraps around the pipe and traps warmth for use elsewhere.
Why Pilots Care
They supply cabin heat and carburetor heat while keeping exhaust fumes safely separated from the ventilation air.
Intuition Check
Do not think of heat muffs as small heaters that make heat by themselves. They are covers that collect heat already being produced by the exhaust system.
Example Sentence 1
On most light single-engine airplanes, cabin heat comes from air warmed by a heat muff around the exhaust.
Example Sentence 2
During the preflight inspection the pilot checked the heat muffs for cracks or holes that could allow exhaust gas into the cabin.