In Order to Understand Why the Burners Won’t Stay Lit You Need to know:
You need to know and understand how most electronic ignitions systems work in order to diagnose why the burners won’t stay lit. Nearly all electronic ignition system on forced air furnace used a method of proving the flame called flame rectification. Proving the flame is the act of verifying that a flame existing. That can be a pilot flame as in Honeywell electronic ignition kits or main burner flame as in direct spark ignition system. It is the same thing. Flame rectification is the act of converting alternating current (the convenience outlet in your wall) to pulsating direct current (direct current is battery power). It turns out that if you send alternating current through a natural gas or lp gas flame the flame converts it to pulsating direct current. I don’t know how and don’t care, but it does. This direct current, which is very small, is grounded out at the burners of the furnace. The circuit board then reads this direct current through its grounding point. About 99% of all electronic ignition system work this way. The only way the circuit board can read direct current is if a flame is present. This proves the flame. As long as the circuit knows there is a flame it will deliver electricity to the main gas valve allowing gas to all of the burners. With this in mind we can discuss the problems that occur with electronic ignition systems.
The Pilot Flame is Not Correct
Most electronic ignition systems are tied directly to the pilot light. They have no standing pilot. The electronic ignition system lights the pilot and then, once sensing the pilot flame, allow the main gas to flow. This is a throwback to the days of the pilot assembly. There is nothing wrong with this, but you would think the engineers would have been a little more creative in their approach to electronic ignition and would have skipped the pilot process avoiding a redundant possibility of failure.
There are two types that function in pretty much the same way. The first system uses a pilot rod or spark rod to ignite and prove the flame and the second method uses a spark rod and pilot assembly shell to prove the flame. In both cases the receiving end of the rectifying portion of the pilot assembly needs to be fully engulfed in flame. The rod at the pilot assembly is energized with alternating current the entire time the furnace has a call for heat. The electricity passes through the gas flame and connects to the grounded electrode or the grounded pilot assembly hood. This creates the path for the alternating current to turn into direct current and be grounded leading to the ground wire connection on the circuit board.
If the pilot is not big enough or is not forceful enough, it needs to be like a mini blow torch, the flame can wisp away from the grounded section. Any disconnect from the flame to grounded section, no matter how slight, will cause the circuit board to shut the burners down. Electricity travels at the speed of light and therefore any gap in the flame will stop the flow long enough for the circuit board to safely stop the main burners.
The Ignitor Sensing Rod is Defective
The ignitor sensing rod is little more than a piece of high carbon steel surrounded by a ceramic insulator. If it gets dirty or corroded it can fail to allow enough electricity to travel through it for the circuit board to pick up the presence of the flames and this will shut down the furnace. If the ceramic insulator holding the sensing rod is cracked this creates a short which is an easier path for the electricity to travel and this will consume all of the electricity that travels through the gas flames and the burners will shut down.
One other weird thing happens to this rod. The heat and corrosion over the years change the molecular structure of the rod. It sounds crazy, but I had to sit down with an engineer that specializes in these to understand the explanation for it. It is complicated, but given the right conditions the heat alters the structural makeup of the spark sensing rod, it is not standard high carbon steel, and this inhibits the flow of electricity. Sometimes a perfectly clean spark sensing rod will not be functional. The only way to tell if this is the case is to replace the rod. Conventional multi-meters will not be able to diagnose the rod as being good or bad.
The Furnace Is Not Grounded Properly
This is a classic problem with a new furnace installation where it replaces one that was installed before electronic ignition systems were popular in home furnaces. It didn’t use to matter whether or not the power and neutral legs of the 120v circuit serving the furnace were on the correct sides of the convenience outlet. Now it does. If these are backwards the furnace will usually turn on for a short time and then shut off. Occasionally we will see one that will work for a period of time and then shut off. This can usually be traced to a shared neutral wire with another fixture. The furnace runs just fine until the other circuit, usually a light, is switched on. This turning on means that electricity is flowing back to the main power meter on the neutral circuit completing the power leg. This also drops out the grounding ability of the furnace when it is wired backwards. The result is that the circuit board will fail to realize or prove that a flame is present. The burners turn off. A loose or corroded ground wire will give an intermittent problem with the furnace.
The Burners Are Not Grounded
Many of the furnace electronic ignition systems use the power from the ignition at the pilot assembly to the burners as the electrical path back to the circuit board for flame rectification. Since the burners actually float in most furnaces, they are not bolted to the furnace or sheet metal, the connection is very poor. On these a green grounding wire is attached between the gas valve and the burners and occasionally directly to the circuit board. For some strange reason when the gas valve is changed out many service technicians just cut or remove the burner ground and fail to attach it back to the gas valve. I think the reason may be that the gas valve is threaded onto the burner distribution pipe and the belief is maybe that this ground is good enough. It is not and this situation can lead to intermittent problems with the main gas burners staying lit.
There is Not Enough Gas to the Furnace Valve
Inadequate gas flow to the furnace can cause the main burners to fail to ignite to the level that they should and this failure means that the flame will be pulled away from the pilot assembly resulting in the circuit board stopping the flow of gas altogether to the main burners for safety. Time after time I have seen this and witnesses the homeowners fail to realize that when you start adding onto the gas system in the home the lines will need to be upsized to accommodate the increased gas load. The fever pitch of switching to tankless water heaters right now is causing horrendous problems in people’s homes. Tankless water heaters generally require 199,000 btus of gas or roughly 181 cubic feet of gas per hour with the standard tank type only requiring 35,000 btus or 32 cubic feet of gas per hour. The nearly 6 fold increase in gas usage has a serious effect on the other appliances in the home that use gas. It starves them for it.
The other thing that starves systems for adequate gas is water. Natural gas, not lp or propane, is full of water. If the temperature drops outside, the water in the gas condensates in the line and it can fill the line up blocking or impeding the flow of gas to your appliances. Yes, we can remove the water and suggest ways preventing it from building up again, but this can be a real hassle and a real dangerous thing to happen to your gas lines. Gas lines are supposed to be installed in a manner that allows the flow of water back toward the gas meter and not allow it to buildup in the lines inside the home. Yes, the gas meter can then fill up, but that is what is supposed to happen. |
