How to Test an Electric Heat Strip in an Air Handler
Most air conditioning systems use an electric heat strip as a heating source. Heat pumps, a type of air conditioner that uses refrigerant as a primary heat source, use them as an emergency-heat source. A heat strip uses a cadium/nickle alloy element that turns hot as electricity flows through it. The air handler's blower motor pushes air across the wire coil and into the duct system. If the fan does not turn on quick enough the heat-strip's element will turn red hot. If the element stays red hot long enough, or enough times, the element will burn into two pieces. Sometimes the ceramic insulators that hold the element in place will crack and fall apart. An air conditioner's electric heat strip's size, measured in kilowatts (KW), depends on the air handlers BTU rating.
To test and replace an electric heat strip, or its element, in an HVAC air handler a technician must have a basic knowledge of electrical principles and safety. The number one rule when working with electricity is - if you are not sure, do not do it and call someone who does know. Working with electricity is unforgiving and the results of an accident happen immediately. Some of the testing procedures involve working with live electricity. Another rule of working with electronics is to keep one hand in your pocket.
The tools needed to test and replace an electric heat strip in an air handler include a nut driver set, a flat-head screwdriver, pliers and a multimeter. Use a multimeter that can test AC Voltage, continuity and has a clamp-on amp probe. The ohm meter part of the multimeter can test for continuity. Digital meters are recommended.
Turn the thermostat's operating switch to the "Off" position and open the air handler's cover panels. The air handler, also known as the furnace, will be located in the attic, garadge or an interior closet. The ductwork and air conditioning refrigeration lines will connect to it. The panels can be removed with either a 1/4- or 5/16-inch nut driver. Inside of the air handler's panels you will find the evaporator coil, blower motor and the electrical service area. Some models will need a cover removed to access the electrical service area. This cover can be removed with a nut driver. In the electrical service area you will find the electric heat strip mounted to the back wall. The element has black disks that the wires connect to.
Turn the multimeter to the VAC selection. Test the voltage entering the air handler at the terminal block. The terminal block is the place where the wires that enter the unit from the circuit breaker and the air handlers high-voltage wires meet. The voltage -- usually around 240 volts -- at the terminal block should roughly match the air handler's operational voltage, as stated on the air handler's identification sticker. If not, then test the circuit breaker and the wires between it and the unit. If so, then turn the thermostat to the heat position. Remember, when working with 240 volts, this is a dangerous voltage, so pay attention.
After the fan turns on, test the voltage across the elements wire terminals, the black disks. The voltage should equal the voltage at the terminal block. If not, then inspect the wires for damage and test the sequencer. If so, then turn off the circuit breaker. Verify the voltage has been turned off by checking it at the terminal block.
Label each heat strip wire with masking tape. Pull the heat strip's wires off of the round disks. Turn the multimeter to the ohms setting. Test the heat strip for continuity. If the heat strip does not have continuity, then replace the heat strip. If the heat strip does have continuity and the correct voltage, then the problem is not the heat strip.
Testing heat strips is the kind of job a professional should complete. It involves working with deadly voltages and taking precise measurements. A visual inspection of the heat strip does not give a complete diagnosis, and can only show a complete short in the element.