14 CFR § 25.1125

Each exhaust heat exchanger must be constructed and installed to withstand each vibration, inertia, and other load to which it would be subjected in operation. In addition—

Each exchanger must be suitable for continued operation at high temperatures and resistant to corrosion from exhaust gases;

There must be means for the inspection of the critical parts of each exchanger;

Each exchanger must have cooling provisions wherever it is subject to contact with exhaust gases; and

No exhaust heat exchanger or muff may have any stagnant areas or liquid traps that would increase the probability of ignition of flammable fluids or vapors that might be present in case of the failure or malfunction of components carrying flammable fluids.

If an exhaust heat exchanger is used for heating ventilating air—

There must be a secondary heat exchanger between the primary exhaust gas heat exchanger and the ventilating air system; or

Other means must be used to preclude the harmful contamination of the ventilating air.