By:  Eric Shidell, HVAC Service Mentor

A lot of HVAC service technicians learn early on that cleaning a flame sensor is a standard maintenance practice and that neglecting this basic task can lead to nuisance burner shutdowns and no heat calls.  Let’s look a little deeper into this situation and examine how and why flame failures occur and how to prevent them.

During a normal gas burner sequence of operations, the ignition device will activate, (spark or hot surface igniter) and gas will be released to the burner.  When the fuel / air mixture reaches the ignition source, flame will become established.

One of the very important safety features of modern gas burner ignition systems is known as “Flame Proving”.  This is a method whereby the ignition controller is able to recognize that the burner flame has been safely established.  This “knowledge” informs the controller that it is time to stop the ignition source and that it is safe to continue with the burner “run” operation.  In the event of a problem or failure to ignite, the flame proving system will shut down the flow of gas to the burner.

Problems that occur with the flame proving system can result in a nuisance shutdown of the burner system.  This results in a no heat situation.

A very common method of proving flame is called “flame rectification”.  A special metal rod is mounted in the path of the flame.  This is known as a “flame sensor” or “flame rod”.  Flame rods are found on nearly all induced draft burner systems and on many forced draft burners.

In a nutshell, the flame rectification system is an electrical process that causes a low level DC current to be conducted from the flame rod through the flame, and back to ground.  Technicians can measure this flame current by placing a meter that measures DC microamps in series with the flame rod.

The ignition controller is programmed to look for this DC current and make a “go – no – go” decision based on the strength of this current.  Normal flame current values for induced draft burner systems will vary between manufacturers and system styles.  A normal signal strength between 1microamp DC and 7 microamps DC is common.  If the flame current is too low, or not present, the ignition controller will stop the ignition operation and stop the flow of gas.  This prevents the possibility of explosion.

Two of the great strengths of this system is that is has very fast response (within microseconds) and it is impossible to bypass or defeat.  One of the weaknesses is that the amount of flame current is very low and can be diminished fairly easily.

Since the flame rod, the flame, and the metal parts of the burner and manifold are all part of a very low power electric circuit, they are subject to the same problems that all electric circuits are subject to.  As these components become dirty or rusty or corroded, the electrical path becomes corrupted and the flame current can be diminished even though the flame has been successfully established and everything is operating normally.

This results in a nuisance shutdown or lockout and a no heat situation.  In this condition, you will observe the burner go through its normal ignition sequence, ignite the flame, and then shut down within seconds.  Some burners will go into a retry mode and repeat the process a number of times.  Others will lock out until a power reset.  For the brief period that the flame is lit, you will be able to measure the flame signal and see that it is weak.

This shutdown is a normal response to a low flame signal.  The ignition controller is doing its job and working to keep things safe.

Flame rods don’t normally need to be replaced unless they are physically damaged or broken in some way.  To correct this condition, the flame rod needs to be cleaned as well as the burner tip.  The quality of flame should also be verified as a poor flame that is lazy or lifting off the burner will also interrupt the flame rectification circuit.  The same goes for the flame rod electrical connections and the ground connections.

Flame rods should be cleaned with a stiff steel wire brush or steel wool.  Never use sandpaper, plumber’s roll, or any other abrasive.  This will scratch the surface of the flame rod.  Once cleaned in this way, the contaminants will quickly fill in the scratches, causing the flame rod to foul again very quickly.  A scratched up flame rod should be considered damaged and it should be replaced.

The underlying cause of a fouled flame rod is actually contaminated combustion air.  Gas burning appliances that obtain all of their combustion air supply from indoors are more susceptible to nuisance flame failures than those installed in ventilated attics, crawlspaces, outdoors, or those that use outdoor air for combustion.

There are many chemical contaminants to indoor air. When these chemicals are burned in the flame they will leave a nearly invisible insulating coating on the flame sensor.  This is what leads to low flame signal.  Common culprits are:  Cleaning supplies, Laundry detergents and fabric softeners, Cat litter boxes, Pet food, Permanent wave solutions, Pool and spa chemicals, Fertilizers, and others.

The best long term solution to nuisance flame failures and dirty flame sensors is to find and remove contaminants from the combustion air supply.  It may be necessary to pipe in clean outside air for combustion into the appliance enclosure, or use a direct vent appliance that uses outside air for combustion.

Realize that while cleaning a flame rod is a pretty easy fix, there is often more to the story.  That represents a potential to turn your knowledge of the underlying cause of flame rod failures into a long term solution for your customer and more work for your company.

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