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Evaluating Air Conditioning Problems

Running Systems


The term “running system” means the compressor and blower are running, but a complaint of insufficient or no cooling has been made.  The majority of air conditioning service calls are made on running systems.  Running system problems can be divided into two major categories:


1.  Airflow

2.  Refrigerant Flow


Airflow can be further defined as too much air or too little air.  We will be dealing with a system that apparently ran and cooled properly after the original installation (if it didn’t, the installing contractor should have “fixed” it under warranty) we will make the assumption that it was set up properly and too much air will not be a problem.  Air handling systems do not increase their air volume without some kind of human intervention.  The vast majority of airflow problems are not enough airflow rather than too much.  To properly function, an air conditioning system needs 350 to 450 CFM per ton across the evaporator.


It should be determined if you are dealing with an airflow problem or a refrigerant flow service problem.


Most service technicians use a dry bulb temperature difference (DT) across the evaporator of 20oF as an indication that airflow is correct, about 400 CFM/ton.  The theory being that if the airflow is too low, a greater DT is created because the air is in contact with the coil for a longer time, thus decreasing its temperature when leaving the coil.  If you can compare the measured dry bulb DT with the manufacturer’s specified dry bulb DT for that “wet” coil, you know airflow is okay.  The problem is you probably are not going to have the manufacturer’s specifications.  Don’t despair.  We can use a little judgement to see if our airflow is in the ballpark without having to take wet bulb readings and using a psychrometric chart.


For a constant air entering dry bulb temperature, the DT across a coil increases with decreasing relative humidity.  This is because the coil condenses a decreased moisture (latent) load.  If a coil doesn’t have to condense much moisture from the air, it can perform more sensible cooling.  Sensible cooling is what we are measuring.  Humidity can account for several degrees of dry bulb deviation.  Therefore, on a humid day, the sensible DT reading will be lower as more of the air conditioner’s capacity is used to dehumidify.  Conversely, if the relative humidity of the return air is low, the sensible DT will be higher.  On the average, about a quarter of an air conditioner’s capacity is used to dehumidify at 50% relative humidity.  As an example, a 4 ton air conditioning system uses about 3 tons for sensible cooling, while one ton is used to dehumidify.  This sensible heat factor (SHF) should be taken into consideration when taking the dry bulb DT over an evaporator to determine airflow.  The dry bulb DT could vary from about 15oF, very humid return air, to 25oF, very dry return air.  The 20oF DT is assuming about 50% relative humidity return air.


The air handler formula BTU/HR=CFM x DT x 1.08 won’t work with an air conditioning coil unless a SHF is considered.


BTU/HR x SHF = CFM x DT x 1.08


Examples:  If we try to determine the CFM’s using the formula for a three ton air conditioner on which we’ve taken a 20oF DT without using a SHF, we will get an erroneous answer.


36,000 = CFM x 20 x 1.08


Solving for CFM we’d get 1666.67 CFM.  This indicates too much air.


Inserting a SHF of .75


36,000 x .75 = CFM x 20 x 1.08


Solving for CFM we get 1250 CFM, about the right amount of air for a three-ton system.


If you have the instrument and time to take wet bulb readings and determine relative humidity, do it.  Your judgement of proper airflow will be more accurate.  If you do not, you’ll have to add to your judgement of airflow your judgement of the return air’s humidity.  Remember high humidity = lower DT and low humidity = higher DT.


If the DT is greater than required across the evaporator, we are dealing with an airflow problem.  Too low an airflow makes the air stay in contact with the coil too long.


Some causes of low airflow: Dirty filters, loose fan pulleys, slipping belts, slow running blower motors, dirty evaporator coil, anything that restricts or blocks the duct system.


Many service technicians believe that a frosted-over coil is always an indication of low airflow.  This is not true.  It can be a good indicator of low airflow, but an undercharged or low return air temperature can also cause frosted coils.  In the case of an undercharge, frost will accumulate at the entry of the evaporator due to low pressure of the refrigerant and consequently low saturation temperature.  As the entry portion of the coil freezes over, air is blocked, causing the saturated refrigerant to move further into the coil, freezing it an inch at a time.  Finally, the entire coil will be frozen over--blocking airflow!  A simple test for undercharge is to feel the suction line.  If it is warm, the system may be undercharged, even with a frosted coil.  (An undercharge can eventually freeze the suction line, but this is a long, slow process.)  Low return air temperature will cause coil frosting, even though airflow CFM is okay.


Air conditioning systems are designed for maximum load, and as the return air temperature drops, the load drops.  Capillary tube systems have very little load adjustability and even TXV’s lose their load adjustability at low load.  A standard TXV loses load adjustibility at about 30 percent of its rating. Balanced port TXV’s lose load adjustibility at about 15 percent of rating.  Therefore, if the air conditioner is forced to run with very low temperature return air (why hasn’t it shut off?), the suction pressure will drop until the saturated refrigerant temperature goes below freezing and the coil will freeze over.


If frost is present on the suction line, it should be present on the evaporator.  If not, suspect a restricted suction line rather than low airflow.  If low airflow is a problem, you should notice uniform frosting of the evaporator.  Irregular frost patterns indicate other problems.  Uneven frosting indicates poor refrigerant distribution or poor air distribution.  Use a good electronic thermometer.  Check the temperature of each evaporator circuit just before it ties into the suction return header.  Any deviation of 5oF indicates uneven air or refrigerant distribution.  Even temperatures with low DT indicate low airflow, or excess oil in circulation.  Oil is an insulator, and blocks the flow of heat.


To check for excess oil in an evaporator, turn off the blower or block off completely the indoor airflow with the condensing unit running.  Watch the evaporator.  Uniform distribution of frost and slow frosting indicate excessive oil.  Uneven frosting indicates poor refrigerant distribution.  Do this test quickly, because you are causing flood back to the compressor.


If the system passed all the above tests, but we still have poor or no cooling, we have a refrigerant flow problem.  If, while doing the suction header and blocked airflow tests, you found poor refrigerant distribution in the evaporator, you better hope it’s a cap tube system.  Cap tubes each feed their own circuit.  If one is restricted, completely clogged, or mis-sized (yes, sometimes manufacturers will make a mistake), it can be replaced.  Some manufacturers have gone to using a simple metering orifice.  With a metering orifice, if one circuit doesn’t get its share of refrigerant, the excess simply goes through the other circuits.  With a TXV, the same thing happens.  With either metering method, orifice or TXV, the problem is the evaporator circuiting or distributor which will require an evaporator charge. 


Note:  If a compressor was ever changed on the system, there may be excess oil in the system.


The system should be purged of oil when a compressor is changed out.  On systems with semi-hermetic compressors, it is possible to watch the compressor’s oil sight glass and remove excess oil.  With hermetic compressors, this is next to impossible to do.  To repeat, excess oil is evidenced by the reluctance of the evaporator to frost in the blower off/block airflow test, or with cap tubes indicating partial clogging (they are clogged--with oil!).  Other symptoms are low suction pressure, loss of capacity, low suction superheat.  Improper suction line piping can cause oil-logged evaporators.  Excess oil must be removed from the system and piping corrected to prevent oil logged evaporators.  (Suction line piping will be discussed in a future Info-Tec.)


The most common refrigerant flow problem concerns the proper charge of refrigerant.  The vast majority of running system service calls for poor or no cooling are caused by the system having leaks and therefore, the system is undercharged.


There are only two ways to properly charge an air conditioning system.  The best and most accurate way is to weigh in the exact charge.  How do you know what the exact charge is?  Look up the manufacturer’s model number of the condensing unit and coil in your library of specification sheets for every unit ever made to find out the proper charge for that unit.  Then measure the length of the refrigerant lines and add the amount of extra refrigerant per foot of line length.  Don’t forget to add any extra refrigerant for the addition of or change in the size of a dryer.  This is usually printed on the dryer.  Add all these numbers together and weigh into an evacuated system that exact amount.  This leaves no gray area to contend with; the charge is correct.


Since hauling every specification sheet around on every job is not practical, you’ll have to use the only other way to properly charge an air conditioning system: Superheat and subcooling.


As we’ll see, superheat and subcooling measurements are also excellent diagnostic tools too.  In order to fully utilize subcooling and superheat readings, a full understanding of each is necessary.


Superheat is the temperature of a vapor above its saturation point.  Superheated gas can only be found where no liquid refrigerant is present.  Superheat should only be found in two places on an operating refrigeration system: leaving the compressor to the first one or two bends of an air-cooled condenser, and leaving the last bend or two of an evaporator back to the compressor.  In short, entering and leaving a compressor.


To get superheat and subcooling readings, you need a good gauge set, and preferably, an electronic thermometer.  When attaching a thermometer probe to a line, always clean the line of any dirt or corrosion where the probe will be affixed.  Make sure the probe is in good contact with the line.  Insulate the probe so the temperature readings cannot be influenced by ambient temperature.


For discharge superheat, convert the head pressure reading to temperature by using a temperature-pressure (T&P) chart.  Take the temperature of the discharge line within the first six inches from the compressor.  Subtract the temperature taken from the T&P chart from the discharge line temperature.  The answer is discharge superheat.


Example:  Refrigerant 22, head pressure 260 lbs.  Using the T&P chart, 260 lbs. is 120oF.  Actual temperature of discharge line, 180oF.  180oF - 120oF = 60oF discharge superheat.


For suction line superheat, convert the suction pressure to temperature as before.  (On cap tube systems, the pressure should be taken at the suction valve of the compressor.  On TXV systems, the best place is as close to the bulb of the TXV as possible.  If there is no port, a line tap on the external equalizer line could be installed.  Otherwise, if the suction valve of the compressor is used, a pressure drop factor will have to be considered in the gauge reading.  On close coupled packaged units, this won’t be necessary, but if the suction line is long, say 20 feet or more, add two lbs. to gauge reading to account for suction line pressure drop.)


Take the temperature of the suction line.  (On cap tube systems, measure as close as possible to where the pressure reading was taken.  On TXV systems, measure at the same location where the TXV bulb is strapped to the suction line.)  Subtract the temperature found from the T&P chart from the actual temperature of the suction line.  The answer is the superheat of the suction side.


Example:  Refrigerant 22.  Suction pressure 68 lbs. = 40oF.  Measured suction line temperature 60oF.  60oF - 40oF = 20oF superheat. 


Subcooling is the measure of difference between condensing temperature and liquid line temperature.  To determine subcooling, again convert head pressure to temperature, using a T&P chart.  Take the temperature of the liquid line at the entrance of the metering device, or as close as you can get.  Subtract the liquid line temperature from the temperature obtained from the T&P chart.  The answer is the temperature the liquid refrigerant has been subcooled.  (Forget taking subcooling on TXV systems using a receiver.  Subcooling will be minimal if any.  Vapor and liquid are always present in a receiver.  As a result, there is no room for a temperature drop to subcool.)


Example:  Head pressure on R-22 system 260 lbs. = 120oF, liquid line temperature 110oF.  120oF - 110oF = 10o subcooling.


Now that we know what subcooling and superheat are and how to get them, let’s use this knowledge to charge a cap tube air conditioner.


Because a capillary tube has very little load adjustability, the charge will be, at best, optimum for one set of conditions.  The cap tube is a fixed metering device; therefore we must duplicate the conditions we want to have the optimum cooling capacity.  This is usually on a 95oF day.


How to charge a cap tube air conditioner:


1.         Install your thermometer on the suction line within a few feet of the compressor’s suction port.

2.         Install high and low side gauges and turn on the unit.  (If all the charge has leaked out, you may have to charge in a static charge to close a low-pressure control).

3.         If the ambient temperature is below 95oF, block off the condenser airflow to simulate an ambient of 95oF.  This will be about 280-lbs. head pressure

4.         Slowly charge refrigerant vapor on the suction side until the low side gauge reads about 65 lbs.

5.         Let the unit run, about 10 - 15 minutes to stabilize.  Take a superheat reading.  On a unit with line lengths of up to 25 feet, a superheat of 15oF to 25oF is acceptable.  On lines of 50 ft. or so, add about 5oF.

6.         If superheat is too high, add a little refrigerant.  If too low, remove refrigerant.  A double check of your superheat is to measure subcooling.  Subcooling should be between 5oF and 10oF.  Be very careful adding or removing refrigerant.  On a cap tube system, only a quarter ounce of refrigerant can make a big difference! 


The system is now optimally charged for the conditions existing at the time of charging.  It will be overcharged when the outdoor ambient is over 95oF, and will be undercharged at a lower ambient, but unless there is a gross charge in any condition, the unit’s performance should not be so adversely affected as to cause a problem.


An extreme example of the value of the superheat charging method, is an installer who put in a flat coil in such a way that air entirely by-passed the coil.  He charged by superheat method.  There was, of course, no cooling effect, but the equipment wasn’t stressed or ruined because the charge was tailored to existing conditions--zero airflow!  Had he weighed in the charge, liquid refrigerant would have returned to the compressor and damaged it.


The next section covers charging expansion valve (TXV) systems with or without a receiver.  All TXV systems should be charged, using the superheat method, and where there are no receivers, subcooling can be checked.  A sight glass can be an aid in charging a TXV system, but is not the final indicator of proper charge.  To even be a good indicator or proper refrigerant flow, a sight glass should be installed right at the entrance to the TXV.  Many sight glasses are installed at the condensing unit, far away from the TXV.  In that position, they are a poor indicator of a proper charge.  To function properly, a TXV requires solid liquid refrigerant entering it.  Any flash gas (bubbles) at the entrance to a TXV will cut its capacity dramatically, and if severe enough, cause the valve to hunt.


A sight glass not installed at the entrance to a TXV can be cleared of bubbles but still have flash gas at the TXV due to the pressure drop in the liquid line from the sight glass to the TXV.


A TXV is a constant superheat valve.  Its sole function is to try and maintain a constant superheat sensed by its bulb, the superheat setting being set by the service tech.  Each system has variables that require the proper superheat to be set by the service technician.


Unlike a cap tube, a TXV is very load adjustable.  Metering the refrigerant flow to the evaporator is the sole function of a TXV.  It must meter the refrigerant at the same rate it is being vaporized by the heat load.  To do this, the TXV keeps the coil supplied with proper amount of refrigerant to maintain superheat of the suction gas leaving the coil.  Let’s look at how a TXV does this.  See Figure 1.




At A hot, high pressure liquid refrigerant enters the TXV.  At B cold, low-pressure liquid, plus flash gas, enters the evaporator.  At C, the entire liquid refrigerant has been boiled off, or vaporized by the heat load (latent heat).  Between C and D, the vapor temperature increases dramatically as further heat load is applied (sensible heat).  At this point, the gas is superheated, above its saturation temperature.  At D, suction line temperature of the superheated gas is monitored by the sensing bulb, which signals the TXV to open or close accordingly.


Ideally, the superheat reading should be taken at the bulb of the TXV.  Superheat should be 10 degrees F, and steady.  If the load changes, you will see a small variation of 1 degree or 2 degrees in superheat as the valve adjusts to the new load.  On initial start-up, a valve may “hunt” for a short time, showing wide swings in superheat.  This is a normal occurrence.  A conventional TXV does not have any anticipation or compensating factors.  There is a time lag between demand and response.  This causes the TXV to overshoot the control point.


After a few minutes of running, a properly sized valve will settle down and hunting will stop.  On sudden, large load changes, a TXV may hunt. 


NOTE:            A TXV that does not stop hunting, and cannot be adjusted for the proper superheat, is probably oversized for the system. 


It was either sized wrong, or has worn out (wire drawn) and either floods the evaporator and then starves it as it tries to adjust.


A TXV power element that has lost its charge can no longer put pressure on the diaphragm in the power head to open the valve.  The TXV fails “safely” on a loss of charge and closes.  You cannot adjust the superheat on a TXV that has lost its charge.


After adjusting the TXV superheat, it is a good idea to check the superheat at the compressor’s suction port.  A long, or poorly insulated, suction line can pick up more superheat.  A range of superheat at the compressor from 10oF to 20oF would be okay.  (An increase of more than 5oF superheat in a suction line, even a long one, is unusual.  It indicates a poorly insulated line, or poor piping.  The line may run through a hot space, accounting for the gain in superheat.  It is a good idea to account for an unusual gain between the superheat taken at the TXV bulb and superheat at the suction port of the compressor.)


On TXV systems without receivers, the subcooling can be checked.  It should be 5o to 10oF.


If airflow, superheat, and subcooling have been checked, and the adjustments made to get the correct readings, the system is performing as it was designed to perform.


One question that has plagued service technicians since the first heat pump was installed is, “How do you charge a heat pump in the heating mode?”


The charge in the heating cycle can be checked in the following manner:


1.       The system should be run in heat mode 15 to 20 minutes.

2.       Verify 350 to 450 CFM per ton airflow.

3.       Attach your electronic thermometer probe to the hot gas discharge line half way between the compressor and where it enters the reversing valve.


NOTE:            Make sure the probe is firmly in contact with the line and very well insulated from ambient temperature influence. 


4.       Wait five minutes.  During that time, determine the outdoor ambient temperature.  The hot gas line temperature should be 100oF plus the outdoor ambient temperature.

5.       If it is higher, add refrigerant.  If it is lower, reduce refrigerant charge.

6.       After each change in charge, wait 10 minutes before taking another reading and doing any charge adjusting.  Add or remove a little refrigerant at a time.  Example:  On a 25oF day, the discharge temperature should be 125oF.


Not all air conditioning systems are the same.  That is, there are no fixed splits, coil operating temperatures or pressures, etc., even for small residential units.  Referring to the manufacturer’s specifications for particulars is always the best place to get the correct information.  In actual practice, this is not always possible.  We can use some guidelines that will get us “close enough” to specifications to service most units.  For instance, the EER rating of a unit will make condenser splits vary.  Manufacturers are making coils larger and more efficient.  The larger the coil, the smaller its split.  You should be aware of the energy efficiency ratio (EER) when servicing a system.  For the last four to five years, all condensing units are supposed to have a label showing their ratings.  Older units were not labeled, and their EER ratings were probably seven or less.


The governing formula to determine the condenser temperature and pressure is:


Condenser entering air temperature (CEAT) plus the split equals condensing temperature (CT). 



The split will depend on the air conditioner’s rated EER.  The following chart can be used to determine the condenser split.  Remember, the higher the EER, the lower the split.

EER Condenser Split (degrees F)
Up  to 7 30
7 to 8 25 to 30
8 to 10 20 to 25
10 & up 15 to 20


Example:  The CEAT (ambient) is 90oF.  The unit’s EER is 8 1/2.




CT = 90 + 20 to 25


CT = 110 to 115oF


The high side pressure gauge (from T&P chart) should read from about 227 to 243 lbs.


The old assumption that all condensing units operate at the same split is false.  Condensing temperatures and pressures depend on the design split, air entering temperature, and the unit’s EER.


Since many service calls on running systems are due to leaks, the undercharged system is a common occurrence.  All refrigerant effect is obtained by circulating so many pounds of refrigerant in a specified time.  Undercharged systems mean there is less mass flow rate.  Low suction and discharge pressures, with high superheat, are indicators of an undercharge.  Way undercharged systems will have low or no subcooling because there is no refrigerant to subcool.  If there is a sight glass, bubbles will be seen in it, as hot gas from the condenser will start to move with the liquid refrigerant.


Compressor amp draw will be low because of decreased flow.  The compressor does not have to do as much work.  Liquid line restrictions can give similar symptoms, but the system would have a lot of subcooling, and undercharge will not.  Elevated head pressures alone do not indicate a restricted high side.  With a liquid line restriction, the evaporator will be starved.  The evaporator will not be able to absorb as much heat as normal for the condenser to reject.  If it is a cooler than design day, the head pressure may actually decrease.  Most of the refrigerant will be in the condenser due to the restriction, so subcooling will increase.  An undercharge will have low subcooling.  High superheat and high subcooling indicate a restriction.


Excessive superheat tells us there is not enough refrigerant in the low side of the system.  That means either an undercharge--or the refrigerant has been “borrowed” by the high side of the system.  A restriction can be identified by trying to charge the system until the superheat is normal, but getting excessive subcooling.


Not enough superheat tells us there is too much refrigerant in the low side of the system.  Either the system is overcharged or the refrigerant has been “borrowed” from the high side.  If we remove refrigerant until the superheat is normal and the subcooling is normal, we have been dealing with an overcharged system.  A lack of subcooling indicates oversized cap tube(s) (rare, but possible), a TXV overfeeding, or the compressor is inefficient.


High suction pressure along with low discharge pressure is a sign of an inefficient compressor.  Amp draw will be low because the compressor is doing less work.  Discharge superheat will be high on the inefficient compressor.  Discharge superheat is the indicator of compressor operating conditions.  Discharge superheat is created by suction superheat, heat of compression, compressor friction, and motor heat.  Generally accepted discharge superheat is in the range of 40oF to 80oF.


Example:  Air conditioner, R-22, head pressure 250 psig.  According to the T&P chart, the corresponding saturation temperature is 120oF.  The temperature of the hot gas line leaving the compressor is 180oF.  180 - 120 = 60oF superheat.  The hot gas line temperature should never exceed 225oF.


Charging a system to normal or near normal suction superheat and still having a high hot gas line discharge temperature or excessive discharge superheat is an indication of a compressor with worn and leaky valves, or worn rings, allowing blow-by.  Some refrigerant is being “recompressed”, which raises its temperature as it picks up more superheat.


A compressor may be so worn out that you cannot charge to normal suction superheat.  If the evaporator and condenser are clean and you have proper airflow, the compressor may be to blame. 


The major symptoms of compressor inefficiencies are:


1.      High suction pressure

2.      Low head pressure

3.      Low compressor amp draw

4.      A very hot discharge line.


Future Info-Tec's will cover these theories in greater detail.


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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