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Electric resistance heat shows up all over HVAC and refrigeration work, and much of it is hidden. The open coil elements in an electric furnace or air handler heat strip provide the building's heat. Inside a walk-in cooler or reach-in case, small heaters keep doors from freezing shut and glass from fogging, keep condensate drain lines from icing over, and clear frost from evaporators during defrost. When any of them fail, the symptoms range from a cold house to a door that sweats, an iced-up drain pan or a freezer full of frost.
This category carries Supco heaters and heating elements: duct heaters, heater restring coil kits for open coil elements, ceramic insulators, refrigeration door and frame heaters, drain line heaters and foil defrost heaters. HVAC techs buy restring kits and insulators to repair heat strips. Refrigeration techs and appliance repair shops buy door heaters, drain line heaters and defrost heaters for commercial and residential refrigeration.
Every heater in this category works on the same principle: current flowing through a resistive conductor produces heat. Wattage depends on voltage and resistance. The relationship is simple and useful in the field: watts equal volts squared divided by ohms, and amps equal watts divided by volts. If you know the rated voltage and wattage, you can calculate the resistance a good element should read on your meter.
Electric furnaces, air handler heat kits and duct heaters use coils of nickel-chromium resistance wire strung across a frame and supported by ceramic insulators. Air from the blower passes directly over the coils. Each element circuit has safety controls in series: a limit switch that opens on high temperature and usually a thermal fuse or one-shot cutoff as backup. Sequencers or relays bring elements on in stages. Duct heaters mount in the ductwork and provide primary, supplemental or zone heat.
When an open coil element breaks, the frame, controls and wiring are often still fine. A restring coil kit provides replacement resistance wire of the correct gauge and length so a technician can restring the element instead of replacing the entire heater assembly. Ceramic insulators hold the coil off the frame and electrically isolate it. Cracked or missing insulators let the coil sag, touch the frame and short.
Freezer and cooler doors use low-wattage heater wire in the door frame and sometimes the door itself. These anti-sweat heaters keep surfaces above the dew point so condensation does not form and gaskets do not freeze to the frame.
Freezer and walk-in evaporator drain lines pass through below-freezing space. A drain line heater runs inside or along the drain line so defrost water can flow out before it refreezes.
Many refrigerators and some commercial units use a heater element bonded to aluminum foil, attached to the evaporator or drain area. The foil spreads heat evenly during the defrost cycle.
Match the voltage and wattage of the original element. A heater installed on the wrong voltage will either underperform or overheat. Higher wattage can overload wiring, breakers and controls.
For restring kits, match the wire gauge and length to the original coil or the kit's application. For door and frame heaters and drain line heaters, measure the required length. For foil defrost heaters, match the size, shape and lead configuration.
Check terminals, lead length and mounting method. Note how the original was installed, including clips, adhesive foil or brackets.
Use the OEM part number from the equipment's parts list. Supco cross-reference data and product specifications help match equivalents by voltage, wattage and dimensions.
| Rated Wattage | Voltage | Expected Resistance (approx.) | Expected Amps (approx.) |
|---|---|---|---|
| 5,000 W | 240 V | 11.5 ohms | 20.8 A |
| 5,000 W | 208 V | 8.7 ohms (if rated at 208 V) | 24.0 A |
| 3,000 W | 240 V | 19.2 ohms | 12.5 A |
| 1,500 W | 120 V | 9.6 ohms | 12.5 A |
| 500 W | 120 V | 28.8 ohms | 4.2 A |
Values calculated from ohms equal volts squared divided by watts. Cold resistance can read slightly lower than these figures. A 240V-rated element operated on 208V produces about 75 percent of its rated wattage.
Electric heaters operate at line voltage and high current, and many are installed in equipment that also contains refrigerant circuits. This work should be done by a qualified technician.
Supco has made replacement heating components for HVAC, refrigeration and appliance repair since 1945. HVAC365 lists voltage, wattage, dimensions and specifications on product pages, with cross-reference and spec sheet details where available. Orders ship in 3-5 business days.
Often, yes, when the frame, insulators, limits and wiring are in good condition. A restring kit with the correct wire gauge and length restores the element. If the frame or controls are damaged, replace the heater assembly.
With power off and at least one lead disconnected, measure resistance across the element. Compare the reading with the value calculated from its rated voltage and wattage. An open reading means the element is broken.
Low airflow is the most common cause. Dirty filters, blocked returns, a weak blower and closed registers all cause elements to run hot. Cracked insulators and coil sag also lead to repeat failures.
The door frame heater has likely failed or lost power. Also check the door gasket and closer, since poor sealing lets moist air in.
It keeps the evaporator drain line above freezing so defrost water drains out of the freezer instead of refreezing in the line or pan.
It will operate, but at roughly 75 percent of its rated wattage. Match the element to the supply where possible, and follow the equipment manufacturer's ratings.
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