Service Tips

Examining Flame Rectification
By: Adolfo Wurts

Once summer is behind us, and customers begin to use their heaters again, you'll probably be getting calls for service on the heaters. "I think my heater's working because I can hear it turn on, but then it turns off again" is a common complaint. If customers asked for a pre-season tune-up on their heater, much like they do for their air conditioner before the hot weather starts, this complaint can be prevented. In most cases there is no preseason check-up 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, will turn into a flame. The flame will continually burn 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.

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

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, first of which is to close the gas valves to the main burners.

Here’s the way it 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 re-ignite or lock-out. The DC offset is small, only in micro amps.

It’s very important that the flame sensing rod works properly. Dirt, corrosion, or bad connections in the flame sensing circuit can cause the controller to think 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 micro amps DC and have a resolution of .1 micro amps.

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, the Fieldpiece Instruments’ AUA2 milliamp head or any other Fieldpiece meter with micro amps can be connected in series between the controller and the flame sensing rod. The AUA2 comes standard with quick connects for easy hook up to already existing male plugs. Some controllers require a mini-plug connection. The Fieldpiece AUA2 milliamp head and the AQK3 Flame Diode Test Adapter Kit includes both a pair of adapters to enable easy 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 under 10mADC (mA = micro amps). Compare this to the manufacturer’s spec.

If the micro amp 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:

1) 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 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.

2) 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.

3) 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.

4) If the current is still too low, replace the flame sensing rod. Now you should be able to measure the correct DC offset current.

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

However, when these systems use the same hot surface igniter for sensing and ignition you may put yourself and your equipment in danger. 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 the pressed. This should be done only after the ignition cycle has been completed.

The information written in this article is of a generic nature and may or may not apply to any particular situation. Always follow manufacturer’s specifications, requests and suggestions. Never do anything while working on a furnace 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.

Adolfo Wurts is a Senior Research Specialist at Fieldpiece Instruments and is a certified HVAC/R technician.
 

Fieldpiece Instruments, Inc.
580 W. Central, Suite A
Brea, CA 92821
(714) 257-9060 FAX (714) 257-9069
fporders@fieldpiece.com

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