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R4795 & RM7895 RELAYS

There are two types of R4795’s. The R4795A is recycling and the R4795D is non-recycling. (Honeywell has informed us that the R4795D will no longer be available by about the end of 1995. (Note: R4795A are still available as of 5-5-00.) The R4795’s were superseded by the R7795 series (discussed later) and now the RM7895 series is the latest upgrade.

 

You may be asked how long purge timing should be for the plug-in purge card. Purges are supposed to be long enough to make four air changes before lighting the burner. If you knew the CFM rating of the blower, and the volume of air in the boiler and chimney, you could calculate the purge timing needed to charge the air four times. Since most of the R4795’s will be replacements, use the same timing as the existing card.

 

Low voltage controllers cannot be used with R4795’s. The T & T terminals on an RA890 are now 6 & 7, where an airflow switch is connected.

 

Purge timing does not start counting until the airflow switch closes these contacts. Once the purge has timed out, the lighting sequence is the same as the RA890’s. The amplifier circuit is energized during purge so we have safe start check. If a flame simulating condition is present during purge, the flame relay coil, 2K, will energize preventing ignition, but the burner motor will continue to run. This will give continuous purge, a “safe” failure condition. The relay will not “lock-out”. If the flame simulating condition, or real flame, goes out, the start-up will proceed. If a purge card fails or is not installed correctly, the burner motor, on a call for heat, will run but pre-purge cannot be completed so ignition cannot occur resulting in a continuous purge.

 

If the airflow switch doesn’t close, or opens during pre-purge, the purge cannot be completed, and once again, the burner motor will run but no ignition can take place.

 

If the airflow switch opens during the run period, terminals 3, 4, and 5 will be de-energized, dropping out the main valve, pilot valve, and ignition. Terminal 8 will remain energized so the burner motor will continue to run. If the airflow switch closes, the purge timing will start and the start-up sequence will begin again. Note that no lockouts have occurred which have to be manually reset. Lockout requiring manual reset happens when no flame is detected after purge. Flame relay, 2K, will not energize and the safety switch will heat and lockout the control in about 15 seconds. If there is a flame failure during run, terminals 3, 4, and 5 are de-energized; pilot, ignition, and main valve. If airflow is still proven, an R4795A will begin purge timing and attempt to re-light. It will make only one try. An R4795D will not recycle. An R4795D will lockout on flame failure during run.

 

An R4795D differs from the A series in safe start check. If a flame is detected during pre-purge (2K relay energizes), the purge will stop and safety lock-out will occur in about 15 seconds—the time it takes the safety switch to heat up. These two things are the only differences between R4795A and D.

 

The next upgrade of the R4795’s was the R7795 series. The R7795 series used more solid-state technology. The R7795’s still used plug-in purge timers, ST795A’s, but the amplifier is not plug-in or interchangeable. Therefore, an R7795 has to be selected with the correct amplifier to match the scanner. R7795A’s are used with UV detectors and B’s are flame rectification. A’s and B’s are intermittent pilot models. R7795C’s and D’s are interrupted pilot models, the C’s for UV detectors, the D’s with flame rectification detectors. R7795’s require a Q795 sub-base. Their operation is the same as the R4795’s.

 

In light of the RM7800 series, do not upgrade a customer from an R4795 to an R7795. Always upgrade to the 7800 series. Honeywell is only keeping the R7795 available due to O.E.M. demand. To an O.E.M., the R7795 is less expensive than the 7800 series and OEMs are very, very price conscious. With the demise of the R4795 series, the 7800 series will be the service industry’s control of choice.

 

To select an RM7895 system to replace an R4795 system, some decisions have to be made. All R4795’s were intermittent pilot. We can now choose intermittent pilot, the RM7895A or B, or interrupted pilot, the RM7895C or D. Intermittent pilot means the pilot is on during the run period. Interrupted pilot means the pilot is shut off during the run period. All RM7895’s have an initiate sequence that lasts at least 10 seconds on initial powering of the relay. During this ten seconds, the relay is checking that the line voltage is within 132 VAC and 102 VAC and line frequency is within plus or minus 10%, or 66 HZ and 54 HZ. If any of these tolerances are not met, the 10 second initiate sequence will go into a hold condition until the tolerances are met, and if not met the RM7895 will lock-out in four minutes. If, at any time during this hold period the tolerances are met, the 10-second initiate sequence will restart checking voltage and frequency again.

 

After passing the initiate sequence, the relay goes into stand-by. Stand-by can be any length of time. Stand-by simply means the control is waiting for a call for heat. On a call for heat, terminal 4 is powered; the blower motor and pre-purge begins. Pre-purge timing is whatever ST7800A plug-in card was selected, from 2 seconds to 30 minutes. The airflow switch (AFS), installed between terminals 6 and 7, must close within the timing of the short timing purge cards, 2, 7, or 10 seconds, or within 10 seconds for longer timing purge cards. The purge timing does not start to count until the AFS closes. Should the AFS not close within the specified time or 10 seconds, whichever is shorter, the control will recycle or lock-out, depending on jumper 3 being intact; recycle or cut; lock-out.

 

All RM7895’s have three configuration jumpers. Jumper number 3 is the jumper that governs what happens if there is AFS failure. If the AFS opens at any time after it has been made, that is in pre-purge, ignition trials, or during run, the RM7895 will recycle if jumper number 3 is left intact or if the jumper is cut the control will lock-out.

 

All RM7895’s have three jumpers that can be cut or left alone. They are labeled JR1, JR2, and JR3. Cutting a jumper enhances the level of safety. Cutting a jumper never makes the control inoperative! Jumper number 1 configures the PFEP (Pilot Flame Establishing Period). Left intact, terminal10 will be powered for 10 seconds, the terminal that the ignition transformer is connected to. If this jumper is cut, terminal 10 is powered for only 4 seconds. Jumper number 2 configures the control to be a recycle or lockout control. If left intact, the control will recycle on flame failure. If cut, the control will lockout on flame failure. Just like the RM7890, this jumper must be cut if an amplifier with 3 second flame response timing is used. Jumper number 3 has been discussed.

 

The RM7895B and D have a feature the A and C series do not; an air flow switch check. What this means is that on a call for heat or in stand-by, the control checks for a closed circuit between terminals 6 and 7. If this circuit is closed, the RM7895B or D will lockout in 2 minutes. Remember this: In the “old days”, to check R4795 nuisance shutdowns, we often jumped out the AFS for a while to see if bouncing contacts in the AFS were causing the problem. Obviously, you can’t do this when dealing with the RM7895B or D.

 

The block diagram, Fig. 6 on page 11 of Honeywell’s form 65-0086 on the RM7895 has an error. The “Airflow Interlock” is shown as a closed circuit. It should be shown as an open circuit. Another error is on the top of page 4. For the RM7895B under “Flame Establishing Period” “main” it says “yes”. This should be “no”. Under “AFSC”, it says “no”. This should be “yes”.

 

Attached is the Gordon Piatt diagram that shows the results of converting from the T3 or T4 timer system to the R4795

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