Hot surface vs intermittent pilot?

Don_197

The signal sent out of the module is an AC voltage (you'd be surprised if you knew how much......on Lochinvar Knights its in excess of 200VAC!) and the voltage is rectified to DC (half Wave.....the only way to get full wave is with a bridge rctifier as stated previously.......which is 4 diodes arranged in a diamond shape......built many of them in H.S. Electronics class 8-) ) and the module is looking for a uA (microamps) signal back from the flame sensor......and was sometimes milliamps on the old burner controls (think Honeywell circa 1963-74 or so.....they even sold a milliamp meter as a service tool....still have one laying around somewhere)

I built a simulator a few years ago that sent a false flame signal to the control board (this was specifically for Lochinvar Knights......used potentiometers for the sensors.....switches and lights for the flow switch and other "switched" features, and I needed the boiler to "see" flame in order to fake it out) and I simply took the flame sensor wire and plugged it into my simulator, which was wired directly through a 250V diode of the magic value..to the grounding wire on the power connector (would only work if the boielr was powered BY the simulator....so I added a 115V female plug) ..and wallah......the boiler would see 4 or 5 uA continuous and not lockout. Was fun for training, and a great learning exercise.

Tim McElwain

Some more info which should be helpful in understanding Flame Rectification:

All flames have certain characteristics in common, including the following:

• Production of Heat
• Expansion of Gases
• Production of By-products
• Emission of Light (Infrared to Ultraviolet)
• Ionization of the Atmosphere in and Around the Flame

Flame detection systems have been developed using several of these characteristics. The flame detecting portion of the system emits a signal or originates some physical action in the presence of the detected characteristic.

Many flame detection systems designed for use on domestic heating systems in the past used the thermal effect of the flame as the method of detection. The flame for operation of the system to continue must heat the detecting element. This is true whether the heat is converted to a physical force, as in a bimetal or hydraulic (mercury) pilot sensor, or to an electrical signal as in a thermocouple. Considerable time is required for the sensor to heat, and a similar time period is required for it to cool on loss of flame.

Larger systems (commercial and industrial) require faster flame proving techniques. Fast responding systems have been developed which use light emitted by the flame (infrared, visible or ultraviolet) and the ionization characteristic of the flame. An example of a visible light detector is the "cad cell" used on oil burners. Visible light detectors are not feasible for use on residential systems. The use of flame rods for flame detection is a less expensive and yet dependable way to prove the flame.

Flame Rod Vs Thermal Sensors

Flame rod systems depend on the ability of the flame to conduct a current when a potential is applied across it (Flame Ionization). The flame rod must be used with a suitable electronic safeguard control to amplify the signal from the flame rod. The flame rod usually is used to detect a gas flame. Oil flames are not generally suitable for the application of a flame rod because of their higher operating temperatures. Flame rod detection systems have 4 important advantages over thermal type pilot sensors:


• 1. Quick Response to Flame Failure - the bimetals pilot and the thermocouple pilot has a response time of up to 3 minutes. Rarely does this type of pilot respond in less than 1 minute. The mercury pilot system is a little less. On domestic installations, where these flame detection devices are used response time was less critical due to their low fuel consumption.
• 2. Proves Flame at Ignition Point
• 3. Protects Itself Against Failure of its Component Parts
• 4. Long Life of Consistent Operation


The flame rectification system also uses 2 electrodes, but with 1 important difference-the ground electrode is always designed to be much larger than the flame electrode (flame rod). For effective operation, the area of the ground electrode must be at least 4 times that of the flame rod. Usually, the ground electrode will be the burner hood. Because of the difference in electrode size, more current flows in one direction than in the other.

With the current in one direction so much larger than the current in the other direction, the resultant current is, effectively, a pulsating direct current which operates the electronic network. The flame relay pulls in, indicating the presence of a flame and allowing the burner sequence to continue. The larger the ratio of ground area to flame electrode area, the greater the flow of current in the proper direction-in other words, a rectified current.

Only the ionized path through a flame and the different sized electrodes can provide the rectified current required for the operation of the electronic network in a rectification system. Should a high resistance leakage to ground occur in the flame circuit, it sends an ac signal into the network, and the system shuts down safely. The rectification system does recognize the difference between a high resistance leakage to ground and the presence of a flame. The flame current is measured in microamps. Normal microamps would be 2 to 10.

One of the most important aspects of this system is an adequate ground connection. The gas valve, burners etc can be determined as effective ground connections.

Tim McElwain

Requirements for Flame Rod Rectification Systems
1. A stable flame
2. Adequate ground area 4 times greater than rod area.
3. Flame rod properly located in the flame envelope.
4. A circuit to carry the flame signal to the amplifier in the primary control (module).

ChrisJ

Tim,

Has anyone said you were amazing or told you thanks lately?