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Using Flame Meters to Check Flame Signals

All primaries, programmers, and new residential heating equipment have some device to check for a flame’s existence at the proper time when burning various fuels.  These detectors ensure that fuel valves are open or closed when they should be.  They are the heart of the combustion control system.

Because residential heating equipment uses relatively small amounts of fuel, “slow” reacting detectors or scanners were used, such as bi-metals (White Rodgers 3000 Series, stack relays, etc.), and thermocouples to detect the presence of flame.  However, when dealing with commercial and industrial equipment, faster detection systems are needed due to the larger amount of fuel being handled.  Fuel valves must be regulated much faster in order to prevent unsafe conditions.

Proper flame signal reading will vary according to manufacturer and to the various scanner amplifier combinations.   Figure 1 below shows the various combinations of amplifiers and scanners for Honeywell programmers.


A good micro ammeter is needed to read these currents.  The RM7800 Series of controls is read with a DC voltmeter.  The minimum acceptable signal is 1.25 VDC for all 7800 service controls.  The maximum expected signal is 5 VDC.  Flame signals should not fluctuate.  Self-check scanners will show a pulsing flame signal as the shutter opens and closes.

Fireye flame signals for the M Series controls were discussed in Info-Tec 4.  D10 and D20 Series have flame signals with a minimum of 6 VDC and normal readings between 20 to 25 VDC.  The D30 flame signal is 18 to 25 VDC.  The D40-41 Series have flame signals ranging from 20 to 25 VDC.  The E Series uses an arbitrary numbering system.  The flame signal is displayed on the message center.  Ten is the minimum signal, with 20 to 80 the normal signal.

Voltages measured at scanner terminals vary widely and can be AC or DC.  There is no need to measure voltages at scanner terminals.  We are interested in proper flame signal readings, not voltages at the scanner terminals.

Part of flame signal checking is a pilot turn down test.  This should be made on every installation using a pilot.  This ensures that the main burner will light with the smallest pilot that will hold in the flame relay; that is, produce enough of a flame signal.  The test for the R4140L would be typical of this test.  Low fuel pressure limits, if used, could be open.  If so, bypass them with jumpers during this test.


1.     Open the master switch.

2.     Close the manual, main fuel shutoff valve(s).

3.     Connect a manometer or a gauge to measure pilot gas pressure during the turndown test.

4.     Open the manual pilot shutoff valve.

5.     Close the master switch and start the system with a call for heat (raise the set point of the burner controller).  The burner motor (blower) should run, the programmer timer should start, and pre-purge should begin.

6.     When the IGN area of the timer dial is opposite the index notch, set the timer switch to TEST position to stop the timer.  Relay 2K should energize when the pilot ignites.

Note:  If the timer does not stop, recycle the programmer and make sure you set the timer switch as soon as the beginning of the IGN area of the timer dial reaches the index notch.  Remember that you have only about six seconds to stop the timer after the start of ignition.

7.     Turn the pilot pressure down very slowly, reading the manometer, or gauge, as it drops.  Stop instantly when relay 2K drops out.  Note the pressure at the dropout point.  The pilot is at the turndown position.  Immediately, turn up the pilot pressure until relay 2K energizes again.  With the timer stopped in this position, the lockout switch will heat when 2K is not pulled in.  If 2K is out for a total of about half a minute, safety shutdown will occur.

8.     Repeat step 7 to verify the pilot gas pressure reading at the exact point of relay 2K dropout.

9.     Increase the pilot pressure immediately to energize 2K, and then turn it down slowly to obtain a pressure reading just above the dropout point.

10.   This step may require two people, one to open the manual valve(s) and one to watch for ignition.  Set the timer switch in the NORM position and let the timer proceed.  When the MAIN area of the timer dial reaches the index notch, make sure the automatic main fuel valve(s) opens; then smoothly open the manual main fuel shutoff valve (and manually opened safety shutoff valve, if used) and watch for main burner ignition.   If the main burner flame is established, proceed to step 17.

11.   If the main burner flame is not established within 5 seconds, or within the normal light-off time specified by the burner manufacturer, close the manual main fuel shutoff valve(s) and open the master switch.

12.   Purge the combustion chamber to remove any unburned fuel.  Check all burner adjustments.

13.   Wait about 3 minutes.  Reset the lockout switch (if tripped), close the master switch, and let the programmer recycle to MAIN.  Repeat steps 10 and 11 (try lighting once more).

14.   If the second attempt is unsuccessful, adjust the flame detector position so that a larger pilot is required to hold in flame relay 2K.  This may require relocating the flame detector to sense further out on the pilot flame, or adding an orifice plate.

15.   Measure the pilot flame signal after adjusting the flame detector to make sure it is stable and above the minimum.

16.   Repeat steps 5 through 15 until the main burner positively lights with the pilot flame just holding in flame relay 2K.

17.   Repeat the light-off of the main burner several times (steps 5 through 10) with the pilot at turndown.

18.   When the main burner lights reliably with the pilot at turndown, disconnect the manometer (or gauge) and turn the pilot up to normal.

19.   If used, remove the bypass jumpers from the low fuel pressure limits.

20.   Run the system through another cycle to check for normal operation.


Test to make certain that a false signal from a spark ignition system is not superimposed on the flame signal.

Ignition interference can subtract from (decrease) or add to (increase) the flame signal.  If it decreases the flame signal enough, it will cause safety shutdown.  If it increases the flame signal, it could cause a relay to energize when the true flame signal is below the minimum acceptable value.


Start the burner and measure the flame signal with both ignition and pilot (or main burner) on, and then with the pilot (or main burner) only.  Any significant difference (greater than 1/2 microampere) indicates ignition interference.


1.         Make sure there is enough ground area.

2.     Be sure the ignition electrode and the flame rod are on opposite sides of the ground area.

3.     Check for correct spacing on the ignition electrode: 6,000 volt systems 1/16 to 3/32 inch (1.6 to 2.4 mm) and 10,000 volt systems 1/8 inch (3.2mm).

4.     Make sure the lead wires from the flame rod and ignition electrode are not too close together anywhere.

5.         Replace any deteriorated lead wires.

6.     If the problem cannot be eliminated, you may have to change to an ultraviolet flame detection system.


Test to make certain that radiation from hot refractory does not mask the flickering radiation of the flame itself.

Start the burner and monitor the flame signal during the warm-up period.  A decrease in signal strength as the refractory heats up indicates hot refractory saturation.  If saturation is extreme, the flame relay will drop out and the system will shut down as though a flame failure has occurred.

If hot refractory saturation occurs, the condition must be corrected.  Add an orifice plate ahead of the cell to restrict the viewing area.  If this doesn’t work, re-sight the detector at a cooler, more distant background.  You can also try lengthening the sighting pipe or decreasing the pipe size (diameter).  Continue adjustments until hot refractory saturation is eliminated.


Test to make certain that hot refractory will not cause the flame relay to stay pulled-in after the burner flame goes out. 

This condition would delay response to flame failure and also would prevent a system restart as long as a hot refractory is detected.

First, check the plug-in flame signal amplifier by starting a burner cycle.  As soon as the programmer stops for the run period, lower the set point of the burner controller to shut down the burner while the refractory is still at a low temperature.  Measure the time it takes for the flame relay to drop out after the flame goes out.  (Watch or listen to the flame relay to determine when it drops out.)  If the flame relay fails to drop out within 4 seconds, open the master switch and replace the amplifier.

To check rectifying photocells for hot refractory hold-in, operate the burner until the refractory reaches its maximum temperature.  Then terminate the firing cycle.  (Lower the set point of the burner controller, or set the fuel selector switch to the OFF position.  Do not open the master switch.)  Visually observe when the burner flame goes out.  After the flame goes out, measure the time it takes for the flame relay to drop out.  (Watch your meter and listen to the flame relay to determine when it drops out.)  If the flame relay fails to drop out within 4 seconds, the photocell is sensing a hot refractory.  Your meter will still show a flame signal.  This condition must be corrected as described in the last paragraph of this test.

Infrared (lead sulfide) detectors can respond to infrared rays emitted by a hot refractory, even when the refractory has visibly ceased to glow.  Infrared radiation from a hot refractory is steady, whereas radiation from a flame has a flickering characteristic.  The infrared detection system responds only to a flickering infrared radiation; it can reject a steady signal from hot refractory.  The refractory’s steady signal can be made to fluctuate if it is reflected, bent, or blocked by smoke or fuel mist within the combustion chamber.  Care must be taken when applying an infrared system to ensure its response to flame only.

To check infrared (lead sulfide) detectors for hot refractory hold-in, operate the burner until the refractory reaches its maximum temperature.  If the installation has a multi-fuel burner, burn the heavier fuel, which is most likely to reflect, bend, or obscure the hot refractory’s steady infrared radiation.  (Burn a solid instead of a liquid, or a liquid instead of a gas.)  When the maximum refractory temperature is reached, close all manual fuel shutoff valves, or open the electrical circuits of all automatic fuel valves.  Visually observe when the burner flame goes out.  After the flame goes out, measure the time it takes for the flame relay to drop out.  (Watch or listen to the flame relay to determine when it drops out.)  If the flame relay fails to drop out within 4 seconds, the infrared detector is sensing hot refractory.  Your meter will still show a flame signal.  Immediately terminate the firing cycle.  (Lower the set point of the burner controller, or set the fuel selector switch to OFF.  Do not open the master switch.)

Some burners continue to purge their oil lines between the valve(s) and nozzle(s) even through the fuel valve(s) is closed.  Termination of the firing cycle (instead of opening the master switch) will allow purging of the combustion chamber, if available.  This will reduce a buildup of fuel vapors in the combustion chamber caused by oil line purging.

If the detector is sensing hot refractory, the condition must be corrected.  Add an orifice plate ahead of the cell to restrict the viewing area of the detector.  If this doesn’t work, re-sight the detector at a cooler, more distant part of the combustion chamber.  While re-sighting the detector, keep in mind that it must also sight the flame properly.  For an infrared detector, you can also try lengthening the sighting pipe or decreasing the pipe size.  Continue adjustments until hot refractory hold-in is eliminated.



Test to make certain that ignition spark is not actuating the flame relay.


1.         Close the pilot and main burner manual fuel shutoff valves.


 2.    Start the burner and run through the ignition period.  Ignition spark should occur, but the relay must not energize.  The flame signal should not be more than 1/4 microampere.


3.     If the relay does energize, re-sight the detector farther out from the spark, or away from possible reflection.  It may be necessary to construct a barrier to block the ignition spark from the detector’s view.  Continue adjustments until the flame signal due to ignition spark is less than 1/4 microampere.




Some sources of artificial light produce small amounts of ultraviolet radiation.  Under certain conditions, an ultraviolet detector will respond to them as if it is sensing a flame.  DO NOT USE AN ARTIFICIAL LIGHT SOURCE TO CHECK THE RESPONSE OF AN ULTRAVIOLET DETECTOR.  To check for proper operation, flame failure response tests (Safety Shutdown Tests 1, 2, and 3) should be conducted under all operating conditions.




With all initial start-up tests and burner adjustments completed, operate the burner until the combustion chamber is at maximum expected temperature.  (Observe the burner manufacturer’s warm-up instructions.)  Recycle the burner under these hot conditions and measure the flame signal.  Check the pilot alone, the main burner flame alone, and both together (unless monitoring only the pilot flame when using an intermittent pilot, or only the main burner flame when using direct spark ignition). Check the signal at both high and low firing rate positions and while modulating, if applicable.

Check the flame failure response time.  Lower the set point of the burner controller and observe the time it takes the relay to drop out after the burner flame goes out.  Four seconds is the maximum amount of time allowed.


If the flame signal is too low or erratic, check the flame detector temperature.  Relocate the detector if the temperature is too high.  The porcelain of a flame rod can lose its insulating ability and become a conductor at about 450 degrees Fahrenheit.  If necessary, realign the sighting to obtain the proper signal and response time.  If the response time is still to slow, replace the plug-in frame signal amplifier.  If the detector is relocated or re-sighted, or the amplifier is replaced, repeat all required checkout tests.

NOTE:  Repeat all required check out tests after all adjustments have been completed.  All tests must be satisfied with the flame detector(s) in its FINAL position.

False flame signals can be induced into scanner wires, either adding or subtracting from a flame signal causing fluctuations.  Never run scanner wires with other line voltage wires.

Scanners used with combustion controls are very susceptible to electric noise, even short runs of scanner wires.  Scanner wires run into panels will usually be very close to, even touching other wires.  Scanner wires not run to panels, but to burner mounted or wall mounted controls should still be shielded.  The best practice for shielding scanner wires is to run them alone in an iron conduit.  The conduit must be iron, not aluminum, plastic, etc.  Insulate the conduit from the scanner head and ground at the scanner.  Ground only at the control end.  See Figure 2 below.




Your flame signal meter will show you the information you need about flame signals.  The new electronic controls that have displays, such as the Fireye E series or Honeywell 7800, annunciate the flame signal, but still have the plug-in jacks for a flame meter.  You cannot properly service primaries or programmers without flame signal meters.  Other good and inexpensive tools are flame simulators.  They can provide a quick check as to whether a problem exists in the control or scanner.  If a primary or programmer functions when using the flame simulator, the “problem” is not in the controls, but with the scanner, its wiring, sighting, interference, etc.  (An infrared simulator is simply a jumper wire pulsed across the flame detector terminals.)  Don’t just eyeball flames.  Use flame meters.

You may be asked how long purge timing should be for the plug-in purge card.  Purges are supposed to be long enough to make four air changes before lighting the burner.  If you knew the CFM rating of the blower, and the volume of air in the boiler and chimney, you could calculate the purge timing needed to charge the air four times.  Since most of the R4795’s will be replacements, use the same timing as the existing card. 
Low voltage controllers cannot be used with R4795’s.  The T & T terminals on an RA890 are now 6 & 7, where an airflow switch is connected.
Purge timing does not start counting until the airflow switch closes these contacts.  Once the purge has timed out, the lighting sequence is the same as the RA890’s.  The amplifier circuit is energized during purge so we have safe start check.  If a flame simulating condition is present during purge, the flame relay coil, 2K, will energize preventing ignition, but the burner motor will continue to run.  This will give continuous purge, a “safe” failure condition.  The relay will not “lock-out”.  If the flame simulating condition, or real flame, goes out, the start-up will proceed.  If a purge card fails or is not installed correctly, the burner motor, on a call for heat, will run but pre-purge cannot be completed so ignition cannot occur resulting in a continuous purge.
If the airflow switch doesn’t close, or opens during pre-purge, the purge cannot be completed, and once again, the burner motor will run but no ignition can take place.
If the airflow switch opens during the run period, terminals 3, 4, and 5 will be de-energized, dropping out the main valve, pilot valve, and ignition.  Terminal 8 will remain energized so the burner motor will continue to run.  If the airflow switch closes, the purge timing will start and the start-up sequence will begin again.  Note that no lockouts have occurred which have to be manually reset.  Lockout requiring manual reset happens when no flame is detected after purge.  Flame relay, 2K, will not energize and the safety switch will heat and lockout the control in about 15 seconds. If there is a flame failure during run, terminals 3, 4, and 5 are de-energized; pilot, ignition, and main valve.  If airflow is still proven, an R4795A will begin purge timing and attempt to re-light.  It will make only one try.  An R4795D will not recycle.  An R4795D will lockout on flame failure during run.
An R4795D differs from the A series in safe start check.  If a flame is detected during pre-purge (2K relay energizes), the purge will stop and safety lock-out will occur in about 15 seconds—the time it takes the safety switch to heat up.  These two things are the only differences between R4795A and D.
The next upgrade of the R4795’s was the R7795 series.  The R7795 series used more solid-state technology.  The R7795’s still used plug-in purge timers, ST795A’s, but the amplifier is not plug-in or interchangeable.  Therefore, an R7795 has to be selected with the correct amplifier to match the scanner.  R7795A’s are used with UV detectors and B’s are flame rectification.  A’s and B’s are intermittent pilot models.  R7795C’s and D’s are interrupted pilot models, the C’s for UV detectors, the D’s with flame rectification detectors.  R7795’s require a Q795 sub-base.  Their operation is the same as the R4795’s.
In light of the RM7800 series, do not upgrade a customer from an R4795 to an R7795.  Always upgrade to the 7800 series.  Honeywell is only keeping the R7795 available due to O.E.M. demand.  To an O.E.M., the R7795 is less expensive than the 7800 series and OEMs are very, very price conscious.  With the demise of the R4795 series, the 7800 series will be the service industry’s control of choice.
To select an RM7895 system to replace an R4795 system, some decisions have to be made.  All R4795’s were intermittent pilot.  We can now choose intermittent pilot, the RM7895A or B, or interrupted pilot, the RM7895C or D.  Intermittent pilot means the pilot is on during the run period.  Interrupted pilot means the pilot is shut off during the run period.  All RM7895’s have an initiate sequence that lasts at least 10 seconds on initial powering of the relay.  During this ten seconds, the relay is checking that the line voltage is within 132 VAC and 102 VAC and line frequency is within plus or minus 10%, or 66 HZ and 54 HZ.  If any of these tolerances are not met, the 10 second initiate sequence will go into a hold condition until the tolerances are met, and if not met the RM7895 will lock-out in four minutes.  If, at any time during this hold period the tolerances are met, the 10-second initiate sequence will restart checking voltage and frequency again.
After passing the initiate sequence, the relay goes into stand-by.  Stand-by can be any length of time.  Stand-by simply means the control is waiting for a call for heat.  On a call for heat, terminal 4 is powered; the blower motor and pre-purge begins.  Pre-purge timing is whatever ST7800A plug-in card was selected, from 2 seconds to 30 minutes.  The airflow switch (AFS), installed between terminals 6 and 7, must close within the timing of the short timing purge cards, 2, 7, or 10 seconds, or within 10 seconds for longer timing purge cards.  The purge timing does not start to count until the AFS closes.  Should the AFS not close within the specified time or 10 seconds, whichever is shorter, the control will recycle or lock-out, depending on jumper 3 being intact; recycle or cut; lock-out.
All RM7895’s have three configuration jumpers.  Jumper number 3 is the jumper that governs what happens if there is AFS failure.  If the AFS opens at any time after it has been made, that is in pre-purge, ignition trials, or during run, the RM7895 will recycle if jumper number 3 is left intact or if the jumper is cut the control will lock-out.
All RM7895’s have three jumpers that can be cut or left alone.  They are labeled JR1, JR2, and JR3.  Cutting a jumper enhances the level of safety.  Cutting a jumper never makes the control inoperative!  Jumper number 1 configures the PFEP (Pilot Flame Establishing Period).  Left intact, terminal10 will be powered for 10 seconds, the terminal that the ignition transformer is connected to.  If this jumper is cut, terminal 10 is powered for only 4 seconds.  Jumper number 2 configures the control to be a recycle or lockout control.  If left intact, the control will recycle on flame failure.  If cut, the control will lockout on flame failure.  Just like the RM7890, this jumper must be cut if an amplifier with 3 second flame response timing is used.  Jumper number 3 has been discussed.
The RM7895B and D have a feature the A and C series do not; an air flow switch check.  What this means is that on a call for heat or in stand-by, the control checks for a closed circuit between terminals 6 and 7.  If this circuit is closed, the RM7895B or D will lockout in 2 minutes.  Remember this: In the “old days”, to check R4795 nuisance shutdowns, we often jumped out the AFS for a while to see if bouncing contacts in the AFS were causing the problem.  Obviously, you can’t do this when dealing with the RM7895B or D.
The block diagram, Fig. 6 on page 11 of Honeywell’s form 65-0086 on the RM7895 has an error.  The “Airflow Interlock” is shown as a closed circuit.  It should be shown as an open circuit.  Another error is on the top of page 4.  For the RM7895B under “Flame Establishing Period” “main” it says “yes”.  This should be “no”.  Under “AFSC”, it says “no”.  This should be “yes”.
Attached is the Gordon Piatt diagram that shows the results of converting from the T3 or T4 timer system to the R4795
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