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There are millions of RA890’s in service today. They are applicable to atmospheric burners up to 2500 MBH input. Some manufacturers applied them to power burners with the addition of purge timers, notably Gordon Piatt. With the introduction of the R4795 with purge cards, the practice of using an RA890 with a purge timer was for the most part dropped by most manufacturers. (See the attached modernization sheets for a Gordon Piatt T-3 or T-4 timer system.)Generally speaking, you don’t find RA890’s on burners less than 400 MBH input. Burners less than 400 MBH can use more cost-effective safety devices. Because of the larger amount of fuel being used on burners over 400 MBH input devices that react to flame failures faster than thermocouples or pyrostats must be used.The RA890F is used with rectification detectors: flame rods and photocells. The RA890G is used with UV minipeepers. RA890H and J are RA890’s with self-check for component failure. All RA890’s have the same operating sequence.
The safety switch of the RA890 is calibrated so that its timing is the same as the pilot flame establishment period: 15 or 30 seconds. If a pilot flame is not detected within that period, the control locks out. The control must be manually reset to try a re-start. Should the flame fail during the run period, the control will return to the ignition sequence and try to re-establish the flame once again for the safety switch timing of 15 or 30 seconds. Remember: The safety switch continues to heat while the pilot and ignition relays are energized. The safety switch will continue to heat until a flame is established or safety shutdown occurs. Figure 1 shows the typical wiring and internals of a RA890F.
Most RA890’s will be controlled with a line voltage controller and T-T will be jumped. The standby period is the only time operation that differs if a line voltage or low voltage controller is used. Once either type of controller closes, operation is identical.
On a call for heat, the relay coil 1K is powered through the thermistor. The thermistor resists current flow delaying the energizing of 1K for a few seconds. This delay provides time for the control to determine if the 2K coil is energized because of a flame signal indicating the presence of a flame or condition simulating a flame. If such a condition exists, contact 2K3 will open and the 1K relay coil cannot energize. This results in the famous RA890 "growl" as the thermistor changes resistance resulting in the 1K relay receiving an increase in current, causing it to "groan" as it energizes. This is normal and does not indicate a defective control.
When the 1K coil energizes, 1K4 closes and takes the thermistor out of the circuit. Contact 1K3 also closes, heating the safety switch and closing a parallel circuit to hold the 1K coil energized after 2K3 opens due to flame detection. 1K1 also closes and energizes terminal 3 and 4. 1K2 closes, but has no effect as long as the pilot link is intact. (This will be discussed later.) The ignition transformer ignites the pilot, and when the flame detector senses the presence of the flame, the 2K coil is energized.
When a flame is detected, the following events occur:
1. Coil 2K energizes.
2. Relay 2K1 cuts off terminal 4 (ignition).
3. 2K2 closes to power-up terminal 5 (main valve).
4. 2K3 opens to stop the heating of the safety switch.
The control is now in run and will stay there until the operating control opens or there is a flame failure. If the flame fails, the following occurs:
1. The relay coil 2K de-energizes.
2. 2K2 opens and cuts off the main valve.
3. 2K1 closes to re-ignite the burner.
This is the "re-light" control operation. (This is the .8 or 3 second flame response timing.) Contact 2K3 is closed and the safety heater begins to heat. If flame is not re-established in 15 or 30 seconds, lockout occurs. The lockout time specifications can be found on the RA890 tags. All the various combinations of "FR" .8 or 3 and "SS" 15 or 30 are available, due to certain code requirements and OEM choices.
Ignition connected to terminal 4 is "interrupted" ignition. Interrupted means the ignition is cut off during the "run" period. "Intermittent" ignition means ignition is on during the run period. If the ignition transformer is connected to terminal 3, we’ll have intermittent ignition; usually only used on direct spark ignited oil burners.
The RA890 is an intermittent pilot relay. The pilot is on during the run period. The pilot cannot be connected for interrupted pilot operation. It may, however, be used with a standing pilot. The hook-up for a standing pilot requires the pilot link to be cut. No connections are on terminals 3 or 4 since there is no ignition transformer or automatic pilot valve. Note that the safe start check for pre-existing flames is lost.
On some old RA890E and RA890F systems, a relay was added to continuous pilot systems to restore safe start check. The pilot was supervised during the off cycle.
The R482F relay used to restore safe start check on older RA890F systems. The R482F relay is no longer available, so there is no reason to detail this system except to say if it is ever encountered, an ignition transformer and pilot valve will have to be added. The RA890F will need to be re-wired as in Fig. 1, or the pilot link will need to be cut and re-wired. If a non-recycling system is required or desired, a R4795D should be used to replace these systems. (Upgrades to the latest RM7800 Series controls will be covered later.)
There are many things that can be checked when troubleshooting RA890F systems. After eliminating obvious failures, such as welded contacts, burned-out relay coils, stripped screws, etc. After finding the obvious failure, it is a good idea to look for the cause of the failure. While relays may just get old and wear out, many more are ruined by some external cause.
For instance, if the load relay contacts are welded or severely burned, it would be a good idea to check:
1. The loads
2. The pilot and main valves
3. The ignition transformer for excessive current draw
4. Low voltage could be a problem. Low voltage causes high amp draws. High voltage can be as much a problem as low voltage.
NOTE: Supply voltage should not exceed 10% or be less than 15% of the rated voltage.
5. Ambient temperature must not exceed 125 degrees F.
Most failures that occur when the relay is OK are due to scanner problems. Most RA890F’s are used with flame rods. Since this is a very popular scanner system, a discussion of flame rectification and flame rod systems is in order here.
A flame rod is a small-diameter metal rod supported on an insulator that can be mounted so that the tip of the rod projects into the flame. The flame rod is used in conjunction with the grounding area of the burner to provide two electrical leads to a burner flame so that the ions within the flame can be detected with an amplifier in the control unit. The main requirement for a flame rod is that it should not melt or deteriorate when immersed in the flame. Since the flame rod does not require any change in temperature in order to detect a flame, this method is not limited in speed.
Flame rods are usually made of high temperature alloys such as Kanthal. These will stand temperatures upwards of 2400 degrees F. They form an oxide coating on the surface of the rod which protects it against further oxidizing action. The oxide coating does not interfere with its use as a flame rod. The electrical conductivity of the rod is not important because the supply voltage is usually above 250 volts and the current is very low in the order of a few microamperes.
Even an insulating coating on the flame rod will not interfere with its operation because at the operating temperature the conductivity of the coating is high enough to carry the few microamperes required.
Flame rods are usable only in small gas flames. They are limited to the smaller size gas burners because of the temperature limitations of the flame rod of 2460 degrees F. The best application for a flame rod detector is on a small gas burner or on a gas pilot. The gas pilot may ignite either a larger gas flame or an oil flame. When the gas pilot is used with an oil burner, the flame rod is used in combination with a photoelectric detector in the visible region. The flame rod is used in the gas pilot, and the photocell is used to detect the oil burner main frame.
NOTE: Be careful not to get the porcelain of a flame rod in the flame. The porcelain will lose its insulating capability and become a conductor at about 600 degrees F.
Flame rods have been used in two different electrical circuits. One is called the conductivity circuit and the other is called the flame rectification circuit. The conductivity circuit operates on the current flowing through a flame without regard to the electrical polarity of the flame rod. While the conductivity circuit is a little easier to apply to all types of flames, it is also easier to develop a resistance leakage condition that will give the same signal as an actual flame. This would be a false indication of flame. Conductivity circuits are no longer used. The rectification principle where the flame conducts more current in one direction of electrical polarity than it does in the other direction is the circuit now used and has been for many years. In this circuit, the flame rod is supplied from an AC source so that a flame will develop a DC signal due to the rectification of the flame. Electrical leakage in this circuit will not produce the DC signal and therefore will not give false indication of flame.
The signal that is used for indicating a flame is the current that flows when the flame rod is positive. A positive flame rod will attract the negative ions in the flame, and the positive ions will be attracted to the grounding area of the burner. While the concentration of the positive ions and negative ions throughout the flame varies from area to area, this is not a factor in developing a good flame rod current. The rectification properties of the flame are mainly due to the size of the ions. The limiting factor is the area of the electrode that attracts the large positive ions, which in all cases is the grounding area of the burner. The larger this grounding area is made, the more positive ions will be collected.
TECH TIP: In order to produce the most flame rod current for any flame, the ratio of the grounding area to the area of the flame rod should be as large as possible. It is good practice to have a grounding area at least four times as large as the flame rod area.
One method of controlling the ratio of the flame rod area to the grounding area is to limit the amount that the flame rod is immersed in the flame. If the end of the flame rod is inserted into the flame partway through the flame, the flame rod current will be greater than if the flame rod is passed all the way through the flame. Another method of improving the ratio of the two areas is to add metal grounding plates or wires to the burner. The grounding plates can be made from stainless steel and they may be welded directly to the pilot or main burner nozzle so that the initial part of the flame wipes the grounding plate. These grounding plates will also aid the pilot by stabilizing it and preventing deflection of the pilot by air drafts. Another grounding plate arrangement that can be used is one where the flame impinges on a flat plate so that the pilot spreads out at right angles to the axis of the burner nozzle.
The flame rod should be located so that it is at the junction of the pilot flame and the main flame when used in a gas pilot-gas burner installation. The flame rod should be located so that it is beyond the inner cone region in order to get a good flame rod current, and it should be located at the side or in front of the pilot to insure a satisfactory pilot flame that will ignite the main burner.
The flame rod should be mounted so that the main body of the rod will not exceed 1500 degrees F. If it gets too hot, it will bend under its own weight. The flame rod should also be located so that in case it droops during use, it will not short out to a metal ground or hot refractory. Also, it should be located so that it is not in close proximity to the spark ignition electrodes. If a spark jumps to the flame rod, it will interfere with detection of a proper flame signal.
Since the flame rod circuit operates at a very high impedance level (in the range of 100 megohms), it is necessary that all wiring to the scanner be protected against electrical leakage. Moisture or dirt on the insulator will interfere with the operation. Poor quality of wire insulation for the flame rod lead will also cause trouble. Do not use asbestos insulated wire that might seem attractive because it withstands high temperature. This insulation will cause an electrical leakage problem because the asbestos is porous and absorbs enough water to short out the flame rod current. Paper and cloth insulation is equally bad. The proper wire to use is plastic insulated.
PVC wire rated for the maximum temperature encountered is recommended. This is available up to 105 degrees C. The flame rod wiring should be arranged with a service loop so that it can be removed for servicing. Yellow flames from burners without air premix should be avoided; as it is usually necessary to add grounding area in order to get a sufficient flame rod current. When a flame rod is used in a rectification circuit, it is necessary to separate the positive ions from the negative ions. Because of their size and slower speed, the positive ions are easier to collect at the larger electrode. In order to obtain the highest flame rod currents, it has been found that the ratio of the area of flame touching the grounding terminal to the area of flame touching the flame rod should be at least 4 to 1. A higher flame signal is better.
Every chance you get try to upgrade RA890’s with the RM7890.
Some important points to remember that are not clearly mentioned in the presently available Honeywell literature on RM7890’s: Terminal 3, L1, must be the hot line. The G terminal must be earth-grounded, a good ground.
If converting to a newer RM7890, remember that there are no low voltage circuits in an RM7890. If the old RA890 used a low voltage controller, change to a line voltage controller. If you must use a low voltage controller, a relay will have to be used and a transformer added (see Figure 2).
If you use a 3 second FFRT amplifier, you must cut jumper 2. Built into the RM7800 Series are the UL codes that do not allow a 3 second FFRT as a recycle control. If you want recycle, you must use a .8 second FFRT amplifier and leave jumper 2 alone.
There is no jumper to clip for standing pilot applications. A relay will have to be added. It must be a relay with pilot duty contacts since we are dealing with very low current, microamperes. See Figure 3 for wiring in this relay. The relay coil is powered from terminal 8 and the N.O. contacts are in series with the F terminal and the scanner.
The RA890G is very similar to the RA890F. The G series is used with UV detectors. Unlike the F, which has only one, there are two transformers in a G. One of them is always powered and energizes the amplifier network. Consequently, the safe start check differs from an F in that during "off" if a flame indication is picked up and a low voltage control is used, the control will lockout in about 15 seconds. If a line voltage control is used on a call for heat, the control won’t start and lockout in about 15 seconds. RA890H, J, and K (K’s are obsolete and no longer available. If you ever run into one, it would be replaced with an H), are self-check versions of F & G’s. They do not self-check the scanners, but only their own electronic network for failure. They are distinguished by a green blinking light on the front of the relay that blinks about three times a second during proper operation indicating that it is checking the flame and its own components. If the blinking stops, it means either the flame is no longer being detected or a component has failed. The light glows slightly when the power is on and the burner is off but doesn’t blink.
RA890H & J are equipped with a test button that must be depressed when reading the flame signal. The green light will not blink when this test button is depressed. There is not that many H’s and J’s currently in service.
I have detailed the basic RA890F, as it is the building block for higher safety relays and then programmers. Also, there are many RA890’s in use.
A FAQ on RA890 troubleshooting:
Q: What is the required voltage is on the F & G?
A: There is no reason to measure the voltage. (Certainly to have current we need to have a potential or voltage.) But, to answer the question, anything from 100V to 260V A/C is normal for an RA890.
Remember that this is flame rectification. The circuit rectifies A/C to D/C, so the A/C voltage is measured at F & G. The A/C is turned into a pulsating D/C by the flame rod/ground, area/ionizing gas flame. Measure the current in microamperes with a micro ammeter. Without going into all the technical reasons, digital meters are not good flame meters. You might say they’re too good. They can pick up the pulsation and will usually jump around, making a steady reading impossible and lead to false readings. An analog meter is best, like our TSA6A and 117053 test lead. Be suspicious of flame signal readings taken with a digital meter.
In the case of a negative flame signal:
1. Reverse the meter leads. We are reading D/C current now, a definite polarity.
2. Check the flame signal-if it is above 12 microamperes the diodes in the meter will become unbalanced and the meter will go negative. This situation can be observed on some burners on start-up. During P.T.F.I., the flame signal will be normal and when main flame comes on, due to the position of the flame rod relative to the pilot and main burner, the main burner can become a greatly increased ground area, thereby increasing the flame signal. The meter will show a rapidly rising signal to 12, 15, or more micro-amps (varies by meter quality) but at some high micro-amp reading will suddenly reverse and go negative. This is not a real negative reading or the burner would shut down.
An RA890 recognizes a "real" negative flame signal that can be caused by a reversal of the flame rod/ground ratio, such as would occur on a slowly dying flame that would cause the rod area to get larger than the pilot ground area and consequently reverse the current flow. Since the RA890 recognizes this, the burner would shut down. This phenomenon can sometimes be seen on burners with long gas lines between the gas valves and burners on shut down. If a flame meter is plugged in at shut down, as the gas bleeds out of the gas line, the running micro-amps reading slowly goes to 0 and then may go negative until the flame is completely extinguished
In future articles, we will progress through all the commercial combustion controls, so you will become an expert on the old and the newest controls, including the Fireye.