14 CFR 23.1125 - Exhaust heat exchangers.
For reciprocating engine powered airplanes the following apply:
(a) Each exhaust heat exchanger must be constructed and installed to withstand the vibration, inertia, and other loads that it may be subjected to in normal operation. In addition -
(1) Each exchanger must be suitable for continued operation at high temperatures and resistant to corrosion from exhaust gases;
(2) There must be means for inspection of critical parts of each exchanger; and
(3) Each exchanger must have cooling provisions wherever it is subject to contact with exhaust gases.
(b) Each heat exchanger used for heating ventilating air must be constructed so that exhaust gases may not enter the ventilating air.