Q: I need to replace a few of the connectors that allow the ignition cable to interface with the spark plugs. Can you explain the differences between the various types you carry? Specifically, when should you use “interference suppressed” models?
A: The electrode connectors you’re referring to are also typically called “electrode adapters,” “spark plug adapters,” “electrode boots,” and “spark plug boots.” These parts are used, as you noted, to create a connection between the ignition cable and the spark plug (in some pilot burners, the connection is made from the ignition cable to the electrode via an adapter, without a spark plug). The other end of the ignition cable is fed to either the ignition transformer (in the case of an ignition electrode) or to the flame relay (in the case of a flame-rod, or ionization electrode).
There are several differences between electrode adapters, and the type you should use is typically dictated by what you are replacing. Combustion 911 primarily carries electrode adapters manufactured for Elster Kromschröder burners. Many of these are made of plastic, though some models also incorporate rubber pieces designed to limit the amount of moisture that can enter, and a ceramic model is available for applications where temperatures are seen at the spark plug that would be prohibitive for plastics.
Electrode adapters also vary in their connection sizes. Depending on the spark plug to which you are connecting, you will require either a 4mm or 6mm connection. Recently, Elster Kromschröder began offering a small additional piece that allows a 6mm adapter to essentially become a 4mm adapter. At the other end of the electrode boot, where the ignition cable is inserted, you will find a 7mm opening. The ignition cable carried by Combustion 911 is ideal for this size; if you plan to use a different ignition cable, please make sure that its diameter will allow easy connection with the new electrode adapters, before you purchase them.
There is another distinction that we should mention briefly, and that is the means by which the ignition cable is fixed inside the adapter. In one type, the connection is a threaded pin, with the point facing the 7mm opening, designed to bore into the cable as you twist it, like a screw. In the other type, the pin is seated perpendicular to the 7mm opening; when you look inside this opening, the pin resembles a tooth, or a spike. The spark plug connection, inside the opposite opening, is slotted in such a way that a flat-bladed screwdriver may be used to turn it to the left or right, drawing the tooth down (opening the cavity) or pushing it up (obstructing the cavity).
An interference suppressed model includes a resistor (usually 2k Ohms) that helps to minimize the electrical “noise” generated when a spark arcs from an electrode to ground. This protects sensitive electronics that may be nearby or connected to the same power source as the ignition transformer. For this reason, Elster Kromschröder recommends interference suppressed electrode adapters for use with ignition electrodes.
The transformer applies between 5kV and 10kV to the sparking (ignition) electrode, causing a spark to jump from the electrode to ground. The energy in this spark is used to ignite the burner. Adding a suppressor to the spark plug boot allows for conducted electrical noise to be quenched, rather than injected back along the electrode and any wires connected to it. At the same time, the relatively low resistance of the resistor, compared to the gap a spark must jump, ensures that most of the energy is still released in the spark (and not the resistor). So, this is not only a recommended practice, but also a really good idea.
If your burner uses an ionization electrode (or “flame-rod,” or “sense electrode”) to monitor the presence of flame, then there should be an electrode adapter connected here, as well. This sensing electrode typically has between 230VAC and 700VAC at its tip, with the precise voltage dependent on the particular flame relay connected to the electrode. This voltage causes a current to flow toward ground through a flame envelope, thus providing a method to detect a flame. Since the resistance of the flame is typically greater than 1 MegOhm, using a suppressed boot on the ionization electrode has no negative effect on its ability to detect flame. Using an interference suppressed electrode boot on the sense electrode is not necessary, but it does allow you to have the same model on both electrodes, reducing your spares requirements and eliminating the possibility that a non-suppressed model will be installed on the ignition electrode accidentally.