Installation of resistance heaters in air ducts shall be permitted, subject to the following requirements:
A. A heater which is to be installed in an air duct or plenum shall be approved for the purpose and shall be installed in the manner approved for the equipment;
B. Each such heater installation shall be provided with approved controls, consisting of an air‑flow switch or pressure differential switch, which will disconnect the power supply to the heaters in the case of failure of a normal air flow across the heaters for any reason, such as blocked filters, motor failure, broken belts, shafts, or other driving equipment, etc., and shall be provided with an approved temperature‑limiting control. In addition, a supplementary independent control shall also be provided in each duct heater to prevent overheating. This device shall be manually resettable or replaceable;
C. They shall not be located outdoors, unless specifically approved;
D. They shall not be installed in downflow systems;
E. Except for approved heater assemblies used in conjunction with a heat pump or central air conditioner, the heater shall not be located closer than 4 feet to the flange of the heat pump or air conditioner unit;
F. Two or more duct heaters may be installed in a group in duct work when specifically approved for this purpose and under such restrictions as may be necessary to ensure safe operation;
G. Duct heaters shall be approved for zero clearance from combustible materials, except that the terminal and/or control box of a duct heater shall have a minimum access clearance of 24 inches to permit servicing and adjustment to the controls;
H. An inspection panel with an area of not less than 140 square inches and a minimum dimension of 5 inches shall be provided in the duct immediately adjacent to, and on the upstream side of, each electric duct heater. The panel shall provide access for cleaning dust and debris from the heater and means for checking concealed heat limiters. The panel shall be marked, “Disconnect heater before removing this panel.”
EXCEPTION: An access panel will not be required for electric duct heaters of the slide‑out type if they are supplied by flexible conduit and ‑
a. Are readily removable.
b. Weigh less than 50 pounds.
c. Have a maximum dimension of 3 feet for that portion which extends into the duct,
2. A separate access panel will not be required where the electric duct heater is located within one foot of a removable room grill and the heater is removable through the grill opening;
Duct and plenum heater controller equipment shall be accessible with the disconnecting means installed at or within sight of the controller.