Furnace Controller Phobia

This month, let’s talk about the furnace controllers that have shown up on 80%- and 90%-AFUE furnaces in the last 10 years or so. These are the mysterious boxes with all the wires emanating from them that control the entire furnace. I don’t know about you, but when I see one of these magic boxes for the first time, my stomach turns.

I have an innate fear of electronics. I don’t feel too bad, though, because I’m in good company.

The furnace manufacturers are well aware of the fear that these magic boxes instill in the technician at large. Most of the controllers they get back under warranty are still good.

Many techs are so intimidated by these controllers, they fixate on them. This is how it goes:

“Furnace broke? Must be box!” (Replace box.)

“Furnace still broke? New box bad!” (Replace box again.)

“Furnace still broke? Man, do those manufacturers make junky boxes!”

In this article, I am going to attempt to take some of the magic and mystery out of these furnace controllers.

HEY, MOE…

The first thing to remember about these controllers is not to let them intimidate you. If, while working on a furnace with a controller, you find yourself going brain dead, slap yourself a couple of times and say to yourself, “Knock it off.”

The second thing to remember is that all these controllers work pretty much the same. Every brand has different fault detection; their reactions to faults can be different, but once you know how one of them works, you pretty much know 90% of everything you need to know.

Here is a generic list of what these controllers do:

Test the safety controls.
Operate the combustion blower.
Confirm combustion blower operation.
Ignite the flame.
Confirm flame ignition.
Start and stop indoor blower motor on time.
Monitor safety controls.
Shut system off if the safeties trip.
Detect and display faults.

This list should give you a good overview.

SEQUENCE OF OPERATION

Now let’s look at the sequence of operation in more detail.

When the thermostat calls for heat, the controller checks the pressure cutouts to ensure that they are not inappropriately indicating combustion blower operation. It also checks the high-temperature limit (the one between the heat exchangers) and the rollout limit(s) (the ones on the burner bracket), as well as the reverse-flow limit switch (on the blower housing), if there is one.

If they are indicating any problem, the controller will lock out and flash a fault code on the idiot light — I mean service indicator light — mounted on most controllers.

If the controller is satisfied that the safety controls are OK, it will start the combustion blower motor, purging the heat exchanger of any gas buildup and rechecking the pressure cutouts to confirm combustion blower operation.

If the pressure cutouts confirm that the combustion blower is operating, the controller will energize the hot surface igniter. It will apply high voltage to the igniter for approximately 15 to 20 sec, then apply 24 V to the gas valve. The burners have only a few seconds to light. If the flame sensor does not detect a flame, the controller will de-energize the gas valve, leaving the combustion blower running to purge the heat exchanger of any raw gas that may have accumulated during the aborted ignition attempt.

After this purge, the controller will attempt to ignite the burners again. Most controllers will retry two or three times. If the burners won’t light, the controller will lock the system out and show a fault on the service light.

If the burners do light when the controller tells them to, the controller will wait a short time for the heat exchanger to warm up and then start the blower motor on heat speed. If any of the safeties are tripped during the heating cycle, the controller will shut the furnace down and try to light the burners again.

If the furnace does not lock out during the heating cycle, the gas valve will be de-energized when the thermostat stops calling for heat. After the burners turn off, the controller will operate the fan for a while to remove the heat from the heat exchanger. The amount of time the blower runs after the heating cycle can usually be adjusted by the technician.

You may have noticed that I’ve been rather vague about the timing of these sequences; usually the only thing you have to worry about is whether or not the controller is performing the sequence. In my experience, controllers rarely fail as a result of timing errors.

Now that you have an overview of the function of the controller, let’s talk about the more mystical aspect of operation: the flame-detection system.

FLAME DETECTION

The flame-detection system is probably the thing that gives the most amount of trouble for techs in the field, so I’ll go into more detail. The igniters on these controllers are very simple. The furnace has no pilot to light the burners, so it is called a direct-ignition system. The name of the flame-detection system used in these controllers is flame rectification; once understood, it is very easy to troubleshoot.

There are two basic types of flame-rectification systems in use in residential furnaces. One has a dedicated flame-sensing rod with a separate ignition source. The other, less-common type uses the hot-surface igniter for ignition and flame detection. We’ll cover the more common system with the dedicated flame sensor.

Note that in the following description of “flame sensor operation” that it doesn’t actually sense anything. The ignition control applies an alternating current to the flame sensor. The voltage varies depending on the design.

When a flame is present, the current can pass from the flame sensor through the flame to the burner to ground. Because the flame sensor has a much smaller surface area than the burner, current passes through the flame more easily in one direction than the other, resulting in pulsating dc on the burner and therefore on ground — hence the name “flame rectification.” The ignition control monitors ground for the pulsating dc signal. If it sees the pulsating dc signal, it will allow the gas valve to stay energized.

Due to the high resistance of the flame, the flame-sensing current is extremely low — usually between 1 and 8 dc microamps (millionths of an amp), although some brands use as little as 0.25 dc microamps.

Timely tool tip: To troubleshoot these systems confidently, you must have a meter that will measure dc current in this very low range. Because many meters will not measure this low, a device can be purchased that will amplify the dc current, making it possible for you to diagnose these systems with almost any meter. This device is installed on your voltmeter and converts microamps to dc voltage so that your meter can read the signal. The device is called a microamp transducer. Some of the furnace manufacturers make these units available. Microamp transducers are also available through parts wholesalers. If you plan to buy a new voltmeter with microamp capabilities, make sure it will read down to tenths of a microamp.

Since the flame current is normally low, it takes very little to inhibit it. If the flame current is being inhibited by something other than no flame, a malfunction of the flame-detection rectification system will occur. The symptom for this kind of malfunction is that the furnace

will light correctly, but the gas valve will de-energize after several seconds.

Measuring the amount of flame current, as described later in this article, is one way of telling if a flame-rectification system is working. If there is no or very low flame current, the ignition control believes there is no flame and will lock the furnace out.

Flame lockout can be caused by many things: for example, reverse polarity (hot and neutral wires are reversed), oxidized flame sensor, rust or corrosion in the area where the current flows from the sensor to the burner, poor ground connection between burner and ignition module, ignition module not applying ac

to the sensor circuit, ignition module not sensing the pulsating dc on ground, cracked sensor insulator, degraded sensor wire insulation, or poor location of flame rod.

If you believe that a flame-rectification system is failing to sense flame, try the following:

1. Check for reverse polarity.

2. If polarity is OK, ensure that the ignition control is applying ac voltage to the sensor by measuring ac voltage from the sensor to ground. (The voltage will vary from one model control to another, but what is important is that there is ac voltage at the probe.)

3a. If there is no ac voltage, work your way back toward the ignition control to solve this problem.

3b. If there is ac voltage at the probe, place your meter in series with the flame sensor circuit and measure the dc amperage while the flame is lit.

4a. If there is adequate amperage, the control is not reading the dc voltage on ground; you have a defective controller.

4b. If there is not adequate amperage, something is inhibiting current flow. This is by far the most common cause of flame-rectification problems. Look on the label of the ignition control or somewhere else in the machine to (hopefully) determine the min/max flame current for that model of control.

To determine the cause of the low flame current:

1. Remove the flame sensor from the furnace and polish the flame rod with a cloth. Do not use an abrasive.

2. If, after replacing the polished flame sensor, the flame current does not increase enough to bring the flame current up to normal, remove the associated burner and clean it.

3. If there is still low flame current with the cleaned burner replaced, install a temporary ground from the burner to the chassis ground of the ignition control.

4. If this does not solve the problem, run a temporary flame-sense wire.

5. If that doesn’t solve the problem, replace the flame rod. If the flame rod is not indexed (located) by factory design but has been installed as a retrofit, adjusting the location might be helpful.