During the heating season one of the more common complaints is when gas furnaces turn on, but almost immediately turn off. Most people schedule a preseason tune-up for their air conditioning system before the hot weather starts. If they did the same for their furnaces before the heating season, this complaint could be prevented. Most don’t and contractors get the complaints.

One of the most common causes of this problem is flame rectification failure.

First, let’s take a look at how a normal system works. The weather turns cool and your customer activates their thermostat. The thermostat sends a signal to the furnace that it should turn on in order to bring the temperature of the room to the temperature set by the homeowner. The heater then opens the gas valve signaling the igniter, which, when it hits the gas, turns into a flame. The flame continually burns the gas until the thermostat tells it to turn off. There is a sensor that indicates to the controller that the flame has lit. However, if for some reason the flame doesn’t light, the heater will automatically close the gas valve so that the gas will not continue to flow without a flame to burn it off. But sometimes that automatic action occurs when the gas does ignite.

There are a few ways a direct-ignition system can confirm the presence of a flame, including a thermocouple, a pressure sensing bulb, or a fire eye, but the most common way with modern gas systems is flame rectification.

Flame-Sensing Rod

The job of the flame rectification sensing rod is to tell the controller that the main gas burners have ignited. If no flame is present after a specified amount of time, the controller needs to take the appropriate action, first of which is to close the gas valves to the main burners. 

Here’s the way the technology works: The controller applies alternating voltage between the flame sensing rod and the base of the flame (ground). The ions in the flame provide a high-resistance current path between the two. Because the surface of the base flame is larger than the sensing flame rod, more electrons flow in one direction than the other. This results in a very small dc offset current. If there is a flame present, the dc offset is detected by the controller, which tells the gas valve to remain open. If there is no current flow, the controller will close the gas valve and the system will purge itself of any remnant gas before trying to reignite or lock out. The dc offset is small and measured in microamps.

The Basics

According to engineers at Fieldpiece Instruments, there are important basic steps to keep in mind.

First, start with a good ground. Who knows how many ignition modules have been condemned and replaced when the only problem was a poor ground to the pilot or the module?

Sure that usually solves the problem, and the ignition system returns to normal operation. Actually, what often happens is that by reconnecting the wires to the module, they’ve changed the ground connection and improved the sensing signal. In many cases, they may have achieved the same results by simply disconnecting and reconnecting the wires on the ignition module they thought were faulty. In the end, the customer ends up paying more for what could have been a simple fix.

Another basic tip is to check to see if a stable pilot flame is engulfing the flame sensor and ground target. A fluctuating pilot flame or a sensing rod not making good flame contact can result in an intermittent flame signal. To avoid this problem, clean the pilot orifice and make sure the pilot is properly adjusted.

It’s also important to ensure the ignition and sensing wires are properly routed. It’s a good idea to keep the wires off the bottom pan of the heating section, if possible. On rooftop units, rain can accumulate in the pan and cause high-resistance shorts to ground. When we’re talking about a signal that can be as low as one one-millionth of an amp, even insignificant defects can show up.

There are different minimum signals, depending on the manufacturer, but if the sensing circuit is sound, you should get a signal well above their minimums. If the sensing circuit is working properly, it’s not uncommon to read a signal between 1 and 10 microamps. Newer meters can read the signal directly, or small (and inexpensive) adapters can be used to enhance the reading on standard digital meters. Both will give satisfactory results.

Diagnosing the Problem

After ensuring that the gas is flowing properly, the first thing to check is the sensing rod. Often they do not operate as they should, usually due to dirt, corrosion, or bad connections in the flame sensing circuit. When that happens, the flame sensing circuit tells the controller that the flame failed to light, even though it did, and the gas valve will be shut down prematurely.

“Before you connect any meter to the flame-sensing circuit you must determine if the system uses a separate flame-sensing rod, or uses the hot-surface igniter as both the igniter and sensing rod,” said Adolfo Wurts, engineering manager at Fieldpiece Instruments. “Typically, in hot surface ignition systems, the flame-sensing rod is separate from the hot-surface igniter, in which case it’s easy to connect to the circuit.

“To determine if the sensing rod is the problem, first measure the current in the flame sensing rod by putting an instrument in series with the flame-sensing rod. The instrument you use must be capable of measuring 1 to 10 microamps dc, and have a resolution of 0.1 microamps.

“For the more common type of direct-ignition systems, where the flame rod and igniter are separate, connect the meter in series between the controller and the flame-sensing rod. At Fieldpiece, our DMMs come standard with quick connects for easy hook up to already existing male plugs. Some controllers require a mini-plug connection. Any instrument used for this job should include a pair of adapters to enable easy connection of the leads to quick connects, plus an adapter for controllers that require a mini-plug connection. When the flame is on, there should be a measurable µADC signal, typically under 10 µADC (µA = microamps). Compare this to the manufacturer’s specifications,” said Wurts.

“If the microamp DC signal is too low, the furnace will perceive this as a no-flame situation and the gas valve will close. First, make sure there is ac voltage between the flame diode and the base of the flame. Measure ac voltage from the flame-sensing rod to the base of the flame. The value varies by model (in the neighborhood of 90 V), but the important thing is to ensure that there is voltage present. If there is no voltage, check to see that the wires from the module to the flame-sensing rod are correctly connected, or if the flame-sensing rod or the wire connected to it is grounded.

“If voltage is present, make sure the flame rod and burner are clean and free of dust, dirt, or debris. If they are not, take them out, clean them, and reinstall. Cleaning the flame rod and burner will remove impedance from the flame current circuit and thus should increase flame current.

“If the current is still too low, check to make sure the connections from the controller to the flame diode, and the connection from the controller ground to the flame rod ground, is low (it should be well under 100 ohms). Another way to check this is to install two temporary connections directly, one from the controller ground to the flame base and another from the flame rod to the flame-rod connection on the controller.

“If the current is still too low, replace the flame-sensing rod. Now you should be able to measure the correct dc offset current,” said Wurts. “If the heater is still not operating correctly, the problem is probably the controller.

“When these systems use the same hot-surface igniter for sensing and ignition you have to be particularly careful. During the ignition part of the start-up cycle there are many amps going through the hot-surface igniter. Only after the ignition cycle is finished will the hot-surface igniter perform the function of a flame-sensing rod. In practice, one instrument cannot conduct both very high and very low currents without mechanical switches.

“Robertshaw makes an adapter (Model 900-041) which has a switch that is normally open and routes the flame-sensing circuit through your multimeter when pressed. This should be done only after the ignition cycle has been completed,” said Wurts.

Remember, this information is of a generic nature and may not apply to all particular situations. Always follow manufacturer’s specifications, requests, and suggestions, when performing maintenance. When the job is done, you can mention to your customer the value of a preseason tune-up.

Publication date: 4/29/2013

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