HVAC For Beginners

Electric Furnace Sequencer Replacement

A Step By Step Repair Guide

With this electric furnace sequencer replacement guide, you will be able to diagnose, select a suitable replacement, and replace a faulty electric heat relay. The typical cost for an HVAC service company to perform this would be over $100. We will help you to keep most of that money.


An electric furnace sequencer is a device that turns one or more heating elements on and off. This is necessary to limit the electric current spike that occurs when an electrical device is energized. If all the elements in a unit were energized at the same time, the current surge would probably cause the breaker to trip or the fuse to blow.

Sequencer Replacement

Normal Operation

In order to tell whether a part is broken or not, we must know how it normally works. The heat relay consists of a heater coil, a bimetal switch, and connections for wires. The heater coil on modern units is powered by 24 volts from the furnace control board. On older units, some of the coils were powered by 120 or 240 volt power. When power is supplied to the heater coil, electric current flows and produces heat in the relay. This heat causes a small piece of metal in the bimetal switch to deform and close the contacts of the switch. When the contacts close, power is supplied to the electric elements. When the power to the heater coil is interrupted, the small piece of metal cools off and goes back to it's original shape. This causes the contacts to open and interrupts power to the elements.

Testing A Sequencer

If you suspect that a sequencer is bad, you should test it before replacing it. To do this, you should begin by shutting off the power to the furnace and verifying that it is off with an electric test meter. Then, you should disconnect the wires from the coil terminals and measure the resistance of the coil. This will normally be around 20 ohms. If the reading is zero or considerably higher than 20, you should continue with electric furnace sequencer replacement.

Electric Furnace Sequencer Replacement

To select a replacement, you should obtain an exact replacement if possible. The relay will have a manufacturer's part number on it. If there is not a part number on it, you should match the heater voltage, the length of on/off timings, and the number of switches. The on timing will normally look something like H1-20 which means that the switch will close between 1 and 20 seconds after power is applied to the heater. The off timing will usually look similar to C40-110 which means that the switch will open between 40 and 110 seconds after power is removed from the heater coil. To remove the old unit, start by labeling all of the wires that are connected to it and where they are connected. Then, removal is usually accomplished by removing a mounting screw. To install the new relay, you simply mount the switch and reconnect the wires. Then, you can turn the power back on to the unit. To complete your electric furnace sequencer replacement, you should observe a complete heating cycle to verify proper unit operation.