49 CFR § 230.58 - Flue plugs.

(a) When plugging is permitted. Flues greater than 2 1/4 inches in outside diameter (OD) shall not be plugged. Flues 2 1/4 inches in outside diameter (OD) or smaller may be plugged following failure, provided only one flue is plugged at any one time. Plugs must be removed and proper repairs made no later than 30 days from the time the plug is applied.

(b) Method of plugging. When used, flue plugs must be made of steel. The flue must be plugged at both ends. Plugs must be tied together by means of a steel rod not less than 5/8 inch in diameter.