Flame rectification has become the primary means of flame sensing in virtually all new residential and commercial gasfired HVAC equipment. The days of the mercury filled autopilot and bimetal warp switch are long gone. Be it an intermittent pilot system, direct spark ignition, or a hot surface system, manufacturers have chosen flame rectification for their means of flame verification.
Why flame rectification? It may be the safest form of flame sensing available. It's virtually impossible to fool the ignition module with anything short of an actual flame simulator. It's very fast, with no cool down time such as with a thermocouple or autopilot. The gas flow can be stopped almost instantaneously. Any sensing circuit failure will result in a system shut down rather than an unsafe ignition.
Technicians have progressed into this modern age of flame sensing, and the meter manufacturers have been right there with us. There are a wide variety of ignition system analyzers on the market. However, while these tools can be very beneficial, there's nothing that can replace a solid understanding of the fundamentals of flame rectification.
The minimum requirements for a properly operating sensing circuit start with a good ground. I believe many condemned ignition modules are mistakenly replaced when the only problem is a poor ground to the pilot or the module.
Many mechanics will replace the ignition module, then think they've solved the problem because the ignition system begins to operate normally. In reality, what has often happened 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 was faulty.
A stable pilot flame engulfing the flame sensor and ground target is also important. 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.
Improperly routed ignition and sensing wires can also cause problems. 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-millionth of an amp, even insignificant defects can show up.
Different manufacturers may call for different minimum signals, 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.
Making Your Diagnosis
Where to start? Assuming you've determined there is gas to the unit, if there's no pilot lighting and you hear no spark, start at the point where it all begins: the 24 VAC or thermostat signal. Once you're sure that we have a call for ignition, you can proceed on to the module and pilot flame.
Using the intermittent pilot system as an example, a spark and the pilot gas should arrive simultaneously.
If there's no spark, check the ignition wire for grounds, cracks, or breaks. Check the spark gap. It may be too wide (no spark), or touching ground (dead short). If you're still not sure of the high voltage circuit, with the gas turned off, connect an alligator clip to the high voltage terminal of the module and lay the other end within 1/8-in. of the metal case. If the module is producing a spark, you'll see it jump the gap. This will help you determine if the ignition wire or rod is defective or grounded.
If there's no pilot gas, check for 24 VAC to the pilot valve coil. If you're not getting 24 VAC at the coil or the module terminals, replace the module.
If 24 VAC is present and no pilot gas is flowing, replace the valve. However, it's still a good idea to check the coil for continuity. If the coil has continuity, the valve is stuck. When you change the valve, check for debris in the gas line and valve. Ensure that a drip leg has been installed just before the unit.
If the pilot is lighting but the unit never gets main gas, it's time to check for the proper microamp signal. The microamp meter or adapter is wired in series with the flame rod. The four types of readings you can expect to see would be no signal or 0 microamps, a steady reading of 0.8 or more, a low reading of less than 0.8, or a fluctuating reading that won't stabilize. Here are the possible causes for each type of reading:
- 0 microamps. Look for an open or grounded sensor wire or flame rod, or a defective module. The wire and rod can be diagnosed with an ohmmeter. Make your diagnosis of a defective module after all other possibilities have been checked.
- 0.8 microamps or more, with a steady reading. With this reading and no 24 VAC to the main gas valve, the module is defective.
- Fluctuating reading. Check for a steady pilot flame. Drafts from air leaks can blow the flame away from the flame rod. A dirty orifice can also cause an unstable flame.
- Less than 0.8 microamps. Look for a too-small pilot flame that's not properly engulfing the rod or pilot hood. Also check the ground connection back to the module.
Condemn the Module Last
Beware of the impulse to change out the module. Make sure that you have checked all of the possible reasons for the pilot or burners not lighting. Many ignition modules that are replaced as defective are actually good. Electronics have come a long way in 25 years. New modules aren't perfect, but they're one of the most reliable components in a modern HVAC unit.
Doug Barnard, CMS, Fresno County Building Maintenance, Fresno, CA, won the " Top Tech 2005" title at the North American Technician Excellence (NATE) Certified Technician Competition held during HVAC Comfortech 2005 in Nashville, TN. He can be reached at 559/225-8824 or email@example.com
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