Today’s units react to unsafe conditions in less than a second. This is accomplished through the use of “computerized control modules” and a flame sensing control system known as flame rectification.
With the growing use of furnace control modules, it is imperative that the modern service technicians understand the sequence of operation of the module as well as the phenomenon known as flame rectification. The modern HVAC industry relies on the process.
Millivolts versus Microamps
The safety in older furnaces with pilot lights was the thermocouple. Placing this device in the flame created a small millivolt signal due to dissimilar metals within the thermocouple housing touching when heat was present on the thermocouple.
Gas valves dependent upon thermocouples required this millivolt signal, which verified the presence of flame, to hold the internal safety valve open and keep the pilot lit or allow the main valve to open when it was energized from the thermostat. This voltage could be measured and was usually a nominal 18 to 20 millivolts.
The safety used in today’s furnaces is the flame sensor. It is nothing more than a stainless steel rod encased in porcelain. By itself, it does nothing. When attached to a control module, the module sends out an A/C voltage to the sensor. This voltage is always present at the sensor. When the burners ignite, flame hits this sensor creating a “path to ground through the flame” for the A/C voltage. When this happens, a pulsating D/C signal, in the form of microamps goes back to the control and the module decides if the signal is strong enough to allow the unit to continue operating. Nominal flame current on today’s furnaces is about 1 ½ to 3 ½ microamps. Because this system is “electronic”, it reacts to unsafe ignition problems in fractions of seconds instead of minutes.
Internally, in the control module, once flame is established, current begins flowing in the flame sensing circuit. The current energizes a relay in the control. A minimum amount of current is needed to energize this relay (microamps). When this relay pulls in, one set of contacts open to de-energize the ignition system and another set of contacts closes to keep the main gas valve energized. It is important to know the minimum microamps that are required to “lock in” the control module and keep the unit running.
If the proper amount of microamps is not present, the control will not allow the main valve to remain open and shuts the unit down. Depending on the control, it may or may not go into “retries” to prove that flame was established.
One of the most common causes of flame failure is the sensor. Keep in mind that the control is sending out an A/C voltage signal. If the sensor becomes dirty or corroded, it is like covering that stainless steel probe with insulation and the current can not pass to the flame (ground) and create the D/C signal back to the board. Or, if the wire is cracked or the porcelain is cracked, the voltage “bleeds” off to ground at the point of the “leak” changing the microamp signal to the board and not allowing it to lock in. This is why it is imperative for the MODERN SERVICE TECHNICIAN to have a microamp meter to see if the control module is getting the proper signal. Without this tool, diagnostics becomes very difficult.
Unlike the thermocouple which requires heat to “move” the bi-metals” inside the head to create millivolts, heat has nothing to do with the flame sensor. It could be glowing cherry red and still not allow the control to lock in. It is the ability of the sensor to pass voltage, through the flame to ground, and create a D/C signal back to the board that allows the system to operate. When this signal is not present, the control responds in a fraction of a second to shut the unit down safely. If the pilot blew out on a thermocouple, it could take 30 to 90 seconds for the safety to shut off the gas. That’s a lot of gas being allowed to enter the furnace unburned.
Since the flame sensor is the primary safety for ignition, the signal should be check every time a unit is cleaned and serviced. If the signal is getting weak, the sensor should be cleaned. Cleaning it is a fix if the signal does not reach the nominal microamps required by the control. Keep in mind that this is the primary safety in the furnace. It is important to make sure that it functions properly.
Will technology stop here? Not likely! Service technicians need to keep up with the latest diagnostics and have the proper tools to work on the furnaces of today as well as those of tomorrow.