Heater Cycling on Automatic Limit.

TM 55-1905-223-24-17

(d) If the shortage of heat is only in some areas you might have a condition that requires seasonal

damper adjustments to balance differing ratios of heat loss and heat gain. A galley may

require proportionately more summer cooling than winter heating. Crushed or broken duct

would affect a particular area. Also restricted air flow usually affects rooms at the end of the

duct system.

(e) Turning on the heat after the area has gotten cold and expecting it to warm up quickly is a

common complaint. Discharge air that has a low heat rise might feel, as it blows on a 980F

hand or face, as though the system is not working properly or as though there is no heat at all.

(3) Heater Cycling on Automatic Limit.

(a) The possibility of a defective or wrong temperature limit switch is always there, but more often

than not other conditions contribute to a heater cycling problem.

(b) Improper air flow caused by obstructions to return air, a loose or broken belt, or clogged filters

and/or evaporator coil, may cause the limit switch to cycle the heater OFF before the

thermostat is satisfied. This could cause a lack of heat in general, or just in the end-of-the-line

rooms, because of the reduced pressure in the duct system.

that a sufficient velocity of inlet air flows between the primary limit and heating element. Some

other conditions that contribute to cycling of a heater are as follows:

Heater too close to the blower outlet.

Baffles bouncing air off heating element onto limit switch.

Base of heater control box not flush with the air stream (reduces air curtain).

(4) Open Secondary Protective Device.

(a) An open manual reset, fusible link, or other secondary thermal device is usually the result of a

stuck contactor where the thermostat is satisfied and there is no more fan. The heater runs on

in this abnormal condition until the secondary device opens.

(b) Lack of proper air over the heater could cause enough cycling of the limit switch so that a

secondary would open after a buildup of enough residual heat.

Overcurrent fuses or circuit breakers may or may not open, depending on the amount of

element resistance to ground and the amount of related current it draws.

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