Heater Flame Sensing

Heater Flame Sensing

Do you have a gas heater where the heater briefly ignites and you get a puff of heat and then shuts down? You may also be getting error codes on your heater and notice 3 flashing lights after the heater attempts to light 2-3 times. If so you may have a flame sensing problem. This article will describe what may be happening and ways to fix it.

Heaters can fail in several ways due to sensor issues in their safety system. Most are easily diagnosed with error codes and troubleshooting described in the manual. Flame sensing errors can be much more difficult to diagnose.

Note that low gas pressure can show the same symptoms. If you use propane, check you have gas in your tank. Some members have discovered problems with their natural gas meter causing low gas pressure at the pool heater even when other gas appliances in the home were working.

The Fenwal box

Many gas heaters, including the Pentair MasterTemp, Jandy LXI, Jandy JXI, and others, use an electronic control box manufactured by Fenwal to safely control the gas valve and flame. The FENWAL 35-6x Series are 24VAC Microprocessor Based Gas Ignition Controls designed for use in all types of gas-fired equipment and appliances.

The Fenwal control box utilizes microprocessor technology to safely light and continually monitor and analyze the flame for proper operation of the gas burner. Features include on-board combustion blower relay, pressure switch monitoring, alarm output, non-volatile lockout with manual reset, system LED diagnostics for troubleshooting, automatic one hour reset and flame current test pins.

The flame sense circuit is extremely reliable and very sensitive. Testing for the flame sense signal is easy if you have the right equipment but if not and you’ve already replaced it with a new “known good” module it's easier just to assume you have a good signal with a new module. If this assumption is correct your problem is one of the following as long as you hear the flame ignite and briefly feel heat from the exhaust:

1. Corrosion in the grounding circuit
2. Broken or corroded current sensing wire
3. Interference in the ground signal

You can fix these as follows:

1. Take the blower flange bolts loose and strip a wire with a longish strip ( 1inch or so) then slide it into the gap between the flange and the flame tube, Connect the other end to the yellow ground wires that are ganged inside the control panel (usually next to the Fenwel). Then restart the unit and see if this works. This will ensure you have a good ground connection to the Fenwel which is required to measure the tiny signal. This technique was discovered by Fenwal and is described in their technical documentation on their website. Pentair also sells a kit with the proper parts to perform this modification.
2. If you have had potential issues like this from a rat problem in the past then double-check the flame sensing wire (usually one of the wires connected to the igniter) by replacing it.
3. If the power supply wire insulation to the unit is compromised you can have leakage to the ground circuit that is very small so the unit will actually work but the ground has sufficient interference that it prevents the flame sense circuit inside the Fenwel from working. This can happen from accidental shorting of the power supply, lightning strikes, and deterioration of the insulation and can cause the Fenwel test pin readings to indicate a bad module but it isn't bad it's your power supply quality. To test this simply rig up a temporary parallel supply wire. I used a piece of romex wire I had laying around in the garage. Run it to a different breaker if possible. This is a test wire only, don't leave it in place due to the obvious hazard. If your unit runs this way then just re-pull new supply wires.

Fenwal Sequence of Operation

Power-Up / Standby

Upon applying power 24 VAC to the R terminal, the control will reset, perform a self-check routine, flash the diagnostic LED and enter the thermostat scan state.

Start Up - Heat Mode

When a call for heat is received from the heater PCB acting as the thermostat and the Water Pressure Switch is closed supplying 24 VAC to the W (IND on some models) terminal, the control will check the air flow switch (AFS) for normally open contacts. The inducer blower is then energized and, once the AFS close, 24V will be put on TH and an optional prepurge period begins. After the pre-purge, the Igniter is energized for the heat-up period, and then the gas valve is energized for the Trial for Ignition (TFI) period.

When the flame is detected during the TFI, the igniter is deactivated and the gas valve and inducer blower remain on. The heater PCB, AFS and burner flame are constantly monitored to assure proper system operation. When the heater PCB is satisfied and the demand for heat ends, the gas valve is immediately de-energized, the control verifies the loss of flame signal and initiates an optional post-purge period before de-energizing the inducer blower.

• Main control asks for heat
• blower runs for 15 seconds prepurge
• ignitor heats up for 40 seconds
• gas valve is powered for 7 seconds while Fenwal looks to see if flame is proven

OEM Fenwal models used by Pentair, Jandy, Hayward and others may have different pre-inter & post purge timings as well as different ignitor warm up and trial for ignition timings.

The ladder diagram below for the Pentair Mastertemp illustrates how the heater PCB and Fenwal interact...

Failure to Light - Lockout

SINGLE TRIAL MODEL Should the burner fail to light, or a flame is not detected during the TFI period, the gas valve will de-energize and the control will go into lockout. The inducer blower will turn off following the optional post-purge period. The LED will indicate the fault code for ignition lockout.

MULTI TRIAL MODEL - (This is the model commonly used for pool heaters described in the introduction) - Should the burner fail to light or the flame is not detected during the TFI period, the gas valve will de-energize. The control will then go through an inter-purge delay before an additional ignition attempt. The control attempts two additional ignition trials before de-energizing the gas valve and entering lockout. The inducer blower will turn off following the optional post-purge period. The LED will indicate the fault code for ignition lockout.

FLAME FAILURE-RECYCLE MODE Upon loss of flame, the gas valve is de-energized and the control proceeds to inter-purge before attempting to relight the flame. Multi-try models permit three tries for ignition including interpurges. If the burner relights, normal operation resumes. If the burner does not relight, the control will enter lockout and the inducer blower will turn off following the optional post-purge period.

Lockout Recovery

Recovery from lockout requires a manual reset by either resetting the thermostat, or removing 24 VAC for a period of 5 seconds. On models with automatic reset, if the thermostat is still calling for heat after one hour, then the control will automatically reset and attempt to ignite the burner.

Combustion Airflow Fault

Combustion airflow is continually monitored during an ignition sequence by the pressure switch (PS terminal). If the pressure switch contacts remain closed for 30 seconds at the start of the ignition sequence without an output signal to the inducer blower, the LED will indicate an airflow fault and the control will remain in this mode with the inducer blower off. If the pressure switch contacts later open while there is still a call for heat, the control will begin the pre-purge period followed by a normal ignition sequence.

If the pressure switch contacts remain open for more than 30 seconds after the inducer blower output (F1 and F2 terminals) is energized the LED will indicate an airflow fault and the control will remain in this mode with the inducer blower on. If proper airflow is later detected from the pressure switch input (PS terminal), the control begins the pre-purge period followed by a normal ignition sequence.

If the airflow signal is lost while the burner is firing, the control will immediately de-energize the gas valve and the LED will indicate an airflow fault. The inducer blower will remain on for the post-purge period and the control continues to monitor the PS input waiting for airflow to return. If proper airflow is detected during the post-purge period, a normal ignition sequence will begin with the pre-purge period. Otherwise, the control will remain in an airflow fault as indicated by the LED with the inducer blower off.

Fenwal Diagnostics

Fenwal LED Light

The LED will flash on for 1/4 second, then off for 1/4 second during a fault condition. The pause between fault codes is 3 seconds. The LED light on the Fenwal box can indicate the following problems:

• Steady on - Internal Control Failure
• One flash - Air Flow fault
• Two flashes indicates flame no call for heat, then the Fenwal is detecting a flame current when it shouldn't be
• Three flashes indicates no flame current being sensed and ignition locked out

Testing the Flame Sense Circuit

The Fenwal has two test points for flame current FC+ and FC-. Flame current is the current that passes through the flame from sensor to ground.

The best way to measure the flame sense current is with a true RMS meter. Measure the current with a good true rms meter that can measure dc current in the microamp range.

To measure flame current, connect a True RMS or analog DC micro-ammeter to the FC+ and FC- terminals. Readings should be 1.0 µA DC or higher. If the meter reads negative or below "0" on scale, meter leads are reversed. Reconnect leads with proper polarity.

If you don't have a RMS meter you can just test for voltage between FC+ and FC- terminals to confirm if you have flame sense. Each micro-amp of flame current produces 1.0 VDC. For example, 2.6 VDC equates to 2.6 µA. Voltage is an indirect indication but easier to do with typical DIY equipment. But it's still very difficult to measure unless you make up some pin wires to clip your voltmeter to. Or you can order some insulated micro clips for the test.

The reading should be 3-8 volts and this corresponds to the millionth's of an amp signal your flame sense signal should reading. If you try to check this be very careful since you can easily short the 24 vac circuit which can blow the transformer before the 2 amp fuse blows.

When not operating, the flame current should read 0 volts dc and 0 microamps dc. DC amps and voltage should be zero when there is no flame.

When there is a flame, there are ions in the flame that can carry current from the flame sensor to the burner and then through the ground back to the Fenwal. This electro-chemical principle is used for flame detection in most newer gas heaters because it has proven reliable and safe since it detects an actual flame rather than the technique of measuring temperature to infer a flame used in older heaters.

The current mostly goes in one direction, so it's "rectified" from ac to dc. Rather than being a true dc current it is actually a signal in which half of the sine wave is dampened so that it reads close to a dc current. The reason for this is the difference in contact areas of the flame to the sensing surfaces involved. The small area of the flame sensor as opposed to the larger area of the burner to ground connection means that it is easier for the current to flow from the flame sensor to the burner than from the burner to the flame sensor.

This is called "flame rectification" and it's how the Fenwal verifies that the burners actually light when the gas valve opens.

If the Fenwal detects current when the gas valve is closed, the Fenwal determines that there is an error and refuses to operate.

Fenwall Configuration

The part number on your Fenwall box will tell you what its configuration is.

Fenwal Replacement

You may find that the easiest, although expensive, way to eliminate the Fenwal box from the problem is by replacing it. It is best to get a replacement box from an authorized Fenwal reseller. A member bought a replacement Fenwal box from a cheap eBay seller which did not work properly.

Igniter Corrosion

Some heaters use a flame sense rod. The Pentair MasterTemp and Sta-Rite heaters use the igniter as part of the flame sense circuit. Below is an igniter removed from a MasterTemp heater that would light and then shutdown with a flame sense error.

It is interesting to note that the igniter corrosion also occasionally caused the GFCI CB to trip when the heater lit.

Ground Problems

If you are confident you have an operating Fenwal box and a new igniter then it is likely your problem is a bad burner ground. A good burner ground that matches the control ground is critical for reliable flame sensing.

If you're getting flame no call for heat, it might be a bad ground or stray current or bad controller.

If there's corrosion in the connection between the burner and chassis or Fenwal and the heater chassis, the flame current won't make the path from the Fenwal to the flame sensor, through the flame, to the burner and back to the Fenwal.

A microcurrent has to travel from the Fenwal, though the flame sensor, through the flame, to the burner and then through the frame grounding back to the Fenwal. You need to make sure that the Fenwal is securely grounded to the frame and that there is a good ground path between the Fenwal and the burner.

The burner and the Fenwal need to share a common ground to reliably monitor the flame current. Since the current is so small, having a different ground reference can cause errors. You might also have some voltage leaking to ground causing random currents. Check for voltage between the main equipment ground and the heater chassis and between the bond wire and the heater chassis.

One member found the ground wires connected to the bottom of the box were absolutely covered in mouse urine. He disconnected them, cleaned them with some sandpaper and rubbing alcohol to make sure he had a solid connection to the box, and it fixed his heater problem.

Alternate Ground Circuit

Fenwal ground problems are common enough that Pentair has a ground wire kit for the MasterTemp and Max-E-Therm heaters (KIT P/N 475214Z), and Fenwal has a technical article on "Improve Grounding in Gas Ignition Controls".

Pool heaters often have a lot of ground wires that run from every different part of the metal firebox and sort of daisy-chain back to the control system ground. Problem is that severe service combined with a few small leaks can leave the heater a corrosion mess in short order.

The ground system is a very critical component for heater control since it works by producing a tiny signal to ground and then the control module detects it to be sure there's a flame. This technology is incredibly reliable so long as the ground system has no interference from corroded wires. If it doesn't detect this tiny signal (about 2 millionth's of an amp) due to corrosion the system will keep the gas valve closed.

To positively fix this you just run a wire from the burner base or from the burner flame holder on many of the new high efficiency heaters to the ground connection on the control module.

Ground Interference

You have replaced the igniter and Fenwal box, you checked and cleaned all the ground connections in the heater, you ran an alternate ground circuit, and your heater still does not stay lit. You are no doubt pretty frustrated!

If you are at this point let's confirm you are troubleshooting the right problem - your heater lights briefly, gives a puff of heat, but does not stay lit. That confirms you have gas and your igniter works but the flame rectification circuit is not saying the flame is staying on and so it shuts off the gas.

One member got to this point. He ran a temporary power supply all the way back to a separate circuit on the main panel. The heater is actually a very low load circuit so he just tapped into a nearby wall receptacle. With this simple change the heater worked perfectly. So he pulled 3 brand new conductors to his heater from his electrical sub-panel.

As discussed above the Fenwal is looking for above 1.3 micro amps, typically 2 - 3 microamps. If there are problems with the ground circuit to the electrical subpanel that small signal can be lost.

You can read the complete thread at Help Jandy JXI 260N Frustration!

The problems of EMI and gas ignitions is discussed in more depth in Applicance Engineer - EMI and Gas Ignition: The Impact of Electrical Noise.

Measuring Flame Rectification True RMS

AC voltage is a sinusoidal wave form that is continuously variable. It can deformed by different variables in the circuit so that it is no longer a pure sinusoidal wave. Pure sinusoidal AC waveforms can be accurately read by both averaging and true RMS multimeters. NonPure AC waveforms can only be read accurately by a true RMS multimeter. See What is true-RMS?.

When flame rectification occurs to prove a flame it is not an actual DC current even though it can be read as a DC current. See Troubleshooting a flame rectification system (note: this reference has limited access and will disappear if not copied and pasted). What is actually happening is that you are getting essentially a half wave AC voltage with a small blip on the opposite side of the wave because although the opposite side of the wave is resistant to flow because of the difference in surface area of the contact points there will still be a a small flow.

It seems to me that most pool heater ignition modules are Fenwal ignition modules. Fenwal states that flame current can be read at their FC+ and FC- terminals with an analog DC ammeter or as voltage with a true RMS multimeter.

True RMS multimeters can be somewhat expensive. For a cheap substitute you might try this alternative to a true RMS multimeter; Mecion DC 0-50μA Ammeter 85C1 Pointer Type Analog Current Panel, but realize that this is meant to be mounted in a panel and the most accurate reading would be if it was read in the upright position as if mounted in a panel and the readings that you will be getting are at the lower end of the scale but should suffice (a 0-30 scale would be better).

For a true RMS meter without spending hundreds of dollars since you will not be using it on a daily basis you could try these which Amazon just started offering:

• AmazonCommercial 4000 Count Compact Digital Multimeter, IP67, True RMS, CATIV 600V: Industrial & Scientific
• AmazonCommercial 6000 Count Compact Digital Multimeter, IP67, True RMS, CATIV 600V: Industrial & Scientific.

The 4000 count meter will be less accurate than the 6000 count meter, but should suffice for what you are using it for.

Also both of these meters will measure capacitance so you will be able to check the capacitors on your pump motor if it won't start and just sits there and hums before shutting down from overheating.

For a little more information on flame rectification you might try these sources: Flame Sensors — Revisited and CBFF - Flame Rectification. But realize that though the UK article is talking about 240 volt supply ( and obviously 50HZ [ cycles per second]), it would also hold true for US 120 volt 60Hz supply.

Millivolt Heaters

Some heaters need no AC power connection and power themselves through the pilot flame generating voltage on a thermocouple.

You need a minimum of 600-650mv for the heater to operate normally.

Pilot valve takes 100mV, main valve takes 400-450mv. Then you need to have a reading of about 100mv left over with the heater running to keep it running.

If you have 500mV or less it is not enough to keep a millivolt heater firing. As soon as the main valve opens all the voltage is used up, main valve closes, and the heater goes out.