Once summer is behind us, and our commercial customers begin to use their heaters again, you’ll probably be getting calls like this: “I think my heater’s working because I can hear it turn on, but then it turns off again.”

If customers opted for a preseason tuneup, this complaint could be prevented. However, in many cases there is no preseason checkup, and you’ll be getting the calls. Let’s take a look at what happens.

The weather turns cool and your customer activates the inside thermostat. The thermostat sends a signal to the heater that it should turn on in order to increase the temperature so that the room temperature matches the thermostat. The heater then opens the gas valve. This signals 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. A sensor indicates to the controller that the flame has been lit.

However, if for some reason the flame doesn’t light, the heater automatically closes the gas valve so that the gas will not continue flowing without a flame to burn it off.

When you get the service call, the first thing you should test is whether or not the flame is present. There are a few ways a direct ignition system can confirm the presence of a flame, including a thermocouple, pressure-sensing bulb, fire eye, and flame rectification. This article focuses on flame rectification.

Flame Sensing

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

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 a flame is 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 re-ignite or lockout. The DC offset is small, measurable only in microamps.

It’s very important that the flame-sensing rod works properly. Dirt, corrosion, and bad connections in the flame-sensing circuit can trick the controller into thinking the flame didn’t ignite. The gas valve will be shut down prematurely. Controller manufacturers publish specifications for the flame diode DC offset current. You can measure the current in the flame-sensing rod by putting an instrument in series with the flame-sensing rod. The instrument must be capable of measuring 1 to 10 microamps DC and have a resolution of 0.1 microamps.

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. 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.

For the more common type of direct-ignition systems, where the flame rod and igniter are separate, a milliamp head (such as Fieldpiece Instruments’ AUA2, or any other Fieldpiece meter with microamps) can be connected in series between the controller and the flame-sensing rod. (The AUA2 comes standard with quick connects for easy hookup to already existing male plugs. The AUA2 milliamp head and the AQK3 flame diode test adapter kit includes a pair of adapters for connection of the AUA2 or a meter to quick connects, plus an adapter for controllers that require a mini-plug connection.)

When the flame is on, there should be a measurable mADC signal, typically less than 10mADC (mA = microamps). Compare this to the manufacturer’s spec.

What The Signals Mean

If the microamp DC signal is too low, the furnace will perceive this as a “no-flame situation” and the gas valve will close. Here’s what to check:

  • 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 90V), 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 them. Cleaning the flame rod and burner will remove impedance from the flame current circuit, thus should increase the flame current.

  • If the current is still too low, 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.

  • If the heater is still not operating correctly, the problem is probably the controller.

    Note: When these systems use the same hot surface igniter for sensing and ignition, you could put yourself and your equipment in danger. Many amps go through the hot surface igniter during the ignition part of the startup cycle. The hot surface igniter will only perform the function of a flame-sensing rod after the ignition cycle is finished. In practice, one instrument cannot conduct both very high and very low currents without mechanical switches.

    Robertshaw makes an adapter (Model 900-041) with 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.

    Always follow the manufacturer’s specifications, requests, and suggestions. While working on a furnace, never do anything that could potentially put you or anyone else in danger. Never leave the equipment unattended unless you have properly disabled the system or completed and tested the job to ensure proper safety requirements have been met.

    Wurts is a research specialist at Fieldpiece Instruments and is a field-certified HVACR technician. He may be reached at Fieldpiece Instruments, Inc., 580 W. Central, Ste. A, Brea, CA 92821; 714-257-9060; 714-257-9069 (fax); www.fieldpiece.com.

    Sources for this article include:

  • Howard Leonard, president, Total Tech HVACR Training, Phoenix.
  • Stephen L. Herman, and Bennie L. Sparkman,
  • Electricity and Controls for Heating, Ventilation & Air Conditioning, 3rd ed. Albany, N.Y.: Delmar Publishers, 1996.

    Publication date: 09/15/2003