• Check that the controlling T'Stat is set to call for Heating. If the T'stat has a Fan Switch, place it in the 'ON' position. If the Fan runs, there is power to that section and the Transformer, Fan Relay and Motor are apparently OK.
  • In order to check the Heater, the Fan Switch must be returned to the Auto position.
  • Have a qualified service person ensure that there is Power and Control Voltage to the heater. 
  • Then, once ALL HIGH VOLTAGE POWER IS OFF, various components can be checked with a Continuity Meter. Replace parts that have failed but remember the following:
    • An open Fusible Link or Manual Reset is usually caused by a stuck contactor that allows the Heater to "run on" after T'stat is satisfied and the Fan goes off.
    • Our U.L. Label is voided and liability passes from us when an exact replacement part or, an Approved Alternate Part Listed in our U.L. Procedure, is not used. The same holds true if any safety device is by-passed. All parts are readily available from our Distributors or manufacturing facility


  • Check that the Ampere draw is reasonably close to that on the Heater data plate.

If that is not feasible, check Heat Rise against the Performance Formula. Some systems have slower Fan speeds on Heating, so calculate the Heat Rise accordingly. You shouldn't be more than 10% short on either check unless the Supply Voltage is lower than the Heater Data Plate rating, or part of the Heater is not operational. Replace any nonfunctioning parts with EXACT BTU PARTS or Approved Alternates listed in our U.L. Procedure                                                              

  • If the heater cycles off before the set temperature is reached, the Heat Anticipator current draw may be set too low causing the Heater to "short cycle". (See Section on "Heater Cycling")
  •  Be sure to figure the extra components that may be energized during the defrost cycle on Heat Pumps. DO NOT FORGET that if  exact replacement components are not used, you may require a change in the Heat Anticipator setting.
    • If the shortage of heat is in only some areas, you may have a condition that requires seasonal damper adjustments in order to balance differing ratios of Heat Loss and Heat Gain. For instance, a kitchen may require proportionately more summer cooling than winter heating.
    • A recently crushed or broken duct would affect a particular area.
    • Also, restricted airflow usually affects rooms at the end of the duct system first.
  • TIP: Your customer may have allowed the area to be heated to get too cold, prior to turning on the heat, and expect it to warm up quickly.  Discharge Air with a low Heat Rise, blowing on a  hand or face, might feel like the system is not working properly or that there is no heat at all. This is a typical complaint in southern climates and in commercial applications where the Heat Rise is relatively low. If the system is functioning properly, your customer will need to turn the Heat on before the area gets so cold. U.L. Listed "Zero Clearance" Duct Heaters are more acceptable than those with more Heat Rise but without Zero Clearance to combustible surfaces. Maintaining a satisfactory Heat Level on the coldest days, with safety, is the primary concern.


  1. 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.
  2. Improper airflow caused by: obstructions to Return Air, Loose or Broken Fan Belt, Clogged Filters and/or Evaporator Coils may cause the Limit Switch to cycle Heater off before T'stat is satisfied and could cause riot enough heat in general or just in the "end of the line" rooms because of the reduced pressure in the duct system.
  3. Improper Installation -with insufficient or uneven airflow over the entire Heater. Borderline installations may start cycling after Coil, Filter and Blower get dirty. Heaters are designed not to nuisance cycle provided that sufficient velocity and thickness of inlet air curtain flows between the Primary Limit and Heating Element. The LIL rule for Duct Heaters to be located 4 feet downstream from an A/C unit and 2 feet before or after an elbow has its place in elimination of nuisance cycling. Some other conditions that contribute to cycling of a Heater are listed below:
  • Heater in a large plenum but too close to a small blower prevents even airflow.
  • Baffles bouncing air off Heating Element onto Limit Switch can add to problem.
  • Base of Heater Controls Box not flush with the air stream reduces air curtain
  • Heater connected to higher than rated voltage results in Superheated Elements.
  • Installing Heaters in the top of a horizontal duct is not authorized. Primary Limit Switch cut-out point is lowered by its location on top.


  1. An open Manual Reset, Fusible Link or other Secondary Thermal Device is usually the result of a stuck Coritactor where the Tstat is satisfied and there is no more Fan. The Heater "runs on" in this abnormal condition until the Secondary Device opens.
  2. Lack of proper air over the Heater could cause enough cycling of the Limit Switch so that a Secondary would open after a build up of enough residual heat. This is particularly so when a Heater is installed from the top of the duct.
  3. Grounded Heating Element may generate enough heat without Fan to open a Secondary. 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.


  1. Improper Wiring
  2. Insufficient Transformer Capacity (This may occur during defrost on some heat pumps when many extra components are temporarily energized OR if chatter happens after a component substitute change the substitute may draw more VA overloading the transformer.)
  3. Vibration or Contamination of Mercury Thermostat.
  4. Excessive Static Pressure on an open face Limit Control (Rare)

ELEMENT FAILURE (Not prevalent if properly designed and not abused)

  1. Excessive overheating with wrong temperature or by passed Primary Limit Control.
  2. Improper installation with enough air directed over the Primary Limit to keep the Heater operating but severely overheating other places not getting enough air.
  3. Improper filtering so that Elements can not dissipate heat through dirty covering.
  4. Corroded hardware or loose connections causing bumed-off wires or Elements.
  5. Any physical damage, grounding, a nick or an impurity in the Element itself.
  6. Bad Element Design with too much KW in a given space and/or riot enough Element mass to dissipate heat. The hotter an Element operates, the quicker It deteriorates.
  7. Very light gauges of Element wire may be sufficient electrically but they simply flutter in the high velocity air until they are grounded out and burn through.