September 28th, 2020
The evaporator approach temperature is the leaving brine temperature minus the refrigerant saturation temperature at the evaporator pressure.
The approach temperature is a measure of how well the heat exchange is happening vs. the design approach temperature.
Approach comes from Q = 1/k * area * (TD), Q is the overall heat transfer, 1/k, the transfer coefficient of the material, and TD, is the temperature difference across the tube, essentially the approach temperature.
For "off design" operation approach must be compensated for change in flow since, Q = Mass flow * Cp * TD, changes in flow change Q, since the transfer coefficient and area can't change in the above equation, TD must change.
Convert the evaporator pressure to a temperature for refrigerant type using a TP chart.
Subtract the found value above from the leaving brine temperature (Should not be negative.)
The remainder is the evaporator approach temperature.
Below is an example of how approach can change with flow:
Outside of getting dirty, evaporators usually don’t have many problems other than what the system inflicts on them. System symptoms seem to like to manifest there.
Evaporators do get dirty, and they get dirty fast, because of the wetted surfaces. They are the hardest part of the system to clean properly and the most labor intensive part to replace.
You must clean the air-inlet side of the evaporator. Cleaning the leaving sides, the sides you can usually see when you take the panel off, is only a ‘feel good’ measure.
There is no magic spray I’m aware of that will clean the back side from the front, and if there was, all that junk would end up in the unit. Cleaning usually takes disassembly, a fin comb, a Shop-Vac®, and maybe some solution, but most of the time, solutions or water must be very limited to protect the surrounds.
Roof top and package units can allow access to the evaporator in some designs, but just as many don’t. You may get away with a water hose on these, but if water runs down the ductwork and into an office, watch the sparks fly!
Never clean a system with the power on. (That goes for condensers too, you know who you are!)
It can be a challenge to just look at the gauges and determine the evaporator needs cleaned. One method that does tell you there’s a problem, is checking the static pressure drop across the coil while the fan is running in cooling mode.
Most A/C coils will display a pressure drop of .1 to .15 inches of water (most low pressure systems to .5 total static), but that design value is always in the installation manual. When the drop is higher, and not due to too much airflow, the coil is dirty.
A dirty coil may also demonstrate some of the following system symptoms:
1. Low airflow, less than 300 cfm per ton of cooling.
2. Low saturated suction pressure.
3. Low superheat.
4. Low capacity.
5. Possible frost or ice on the case and suction line around the coil connection.
6. Evaporator case will sweat profusely in the right conditions.
7. If air is still making it through, it will be colder than usual (less than 50 F).
8. Compressor will run a cold crankcase.
9. Compressor will also have low amps.
But use caution here, these are also the symptoms of a very dirty filter, that’s why the static pressure drop test is recommended to accompany the testing.
NOTE: If you happen to have a high supply temperature, high saturated suction pressure, and high compressor amps accompanied with a noisy air-handler and water in the ductwork, you probably have high airflow across the evaporator.
In the installation manual for the air-handler you will find a table like the one below:
The table tells you the multiple air-flows the blower will produce when opposed by the system total static pressure. If you measure a static pressure of .3 inches of water, and the blower tap is MED-LOW, then the airflow should be 950 cfm, and the coil temperature rise about 37 F.
This type of table is very useful to determine if the system is performing per design capability. Another way this table can used, is if you know the air-flow is 950, but the tap is on MEDIUM, and your static is .3, then you know there’s a restriction in the air system, or the duct is badly designed.
· Unit doesn’t cool very well, or only works at night.
· Saturated suction temperature well below 45 F possibly freezing (32) or less.
· High suction superheat.
· Low compressor power (kilo-Watts).
· Low saturated condensing temperature.
· Temperature drop across the liquid line from metering device to condenser (>3F).
· Sight glass has constant bubbles visible.
· Supply air temperature is high. (more than the saturated coil temperature +25).
· Frost or ice on the suction line.
· High compressor discharge superheat.
These symptoms are very much like under-charge, but here are a couple problems that can also display these symptoms:
1. Liquid line kinked or crushed.
2. System leaks leading to under-charge.
3. Metering device won’t let refrigerant through. On TXV, the power head may have failed.
4. Metering device is too small. Check design manual for correct size.
5. Head pressure control isn’t working during low-ambient operation.
6. Free water contamination freezing off the TXV. Unit may work for a while after being off for several hours.
Note: If you are having compressor problems, wear, leaking valves, rings, etc., you won’t draw down the evaporator low enough to freeze and pressure ratio will not recover even after adding refrigerant.
The expansion valve can have several problems that can look like refrigerant charge problems. This makes diagnosing an expansion valve a little tricky.
Before you dive into trouble-shooting the TXV, make sure you don’t have conditions the valve is just re-acting to.
· Un-insulated sensing bulb.
· Loose sensing bulb.
· Incorrectly mounted sensing bulb. Usually should be in the 2 or 10 o’clock position.
· Power head is blown. Check this by removing the bulb and warm it in your hands while watching the suction pressure, then return it to the line. You should see the suction pressure change as the valve tries to keep up.
· While you’re looking at the power head, make sure it’s the correct one for the refrigerant in the system.
If you have verified the above the above to be correct, you have eliminated many problems that can occur with a TXV valve. The remaining problems, flooding, starving, and hunting only have a few causes.
Flooding or refrigerant over-feed. If you’re sensing bulb is mounted right and working, then there are only three reasons the TXV will over-feed the coil. The valve is stuck open, the valve is too big, or the valve is adjusted for too low of a superheat.
If the valve is stuck open, the liquid flood back conditions will not respond to any adjustment, charge or setting, superheat will remain essentially zero.
If the valve is too large, there’s usually a load condition that it will work okay, then trips the oil switch at night or low load. The valve will also try to change with the evaporator load, fluctuating the suction pressure, this is called “hunting”.
A TXV comes factory set to some value, I’ve seen 10 degrees on about everyone I’ve picked up for A/C service. That setting is particular to the construction of the valve and power assembly. The setting is independent of the system. So, unless you, or someone else, tinkered with the setting it should be correct.
Note: when installing a new TXV, put a dab of red nail polish where the cap for the adjust meets the body of the valve, that way, you’ll be able to tell if it’s been tampered with.
If the setting is not correct, an adjustment of no more than one turn at a time, with a waiting period of 15 minutes, should be attempted. Do Not force the screw, there is an end of travel in most valves that is easily broken off, you pass it, and the internal wheel falls off the thread, its game over. Caution- if you are working on a valve that runs below freezing -10 or less, the brass may ice seize and you will twist off the adjustment. If it’s hard to turn, something is wrong.
Note on fixed metering devices, only times they over-feed is from over-charge, or, unusually high head pressure.
Starving the evaporator.
Stuck, wrong valve, set wrong, or inlet is plugged. If it’s your TXV valve causing the problem. (Assuming you checked the power assembly and/or the liquid line filter)
If the valve is stuck closed, partially or completely, no changes are going to make that high suction superheat go away. You’ll also have low compressor power, compressor runs hot, and a high supply temperature 60-70 F.
If the valve is too small, there may be times, possibly at night, the system keeps up. You can drop the blower speed a couple steps, and if the system begins too cool at a moderate load, the valve might be too small or stuck.
Once again, unless you know the setting has been tampered with, it’s a last resort.
Plugged? most TXV valves come with an inlet strainer, poor installation/service practices see too it these plug off. You’ll may only know this for sure once you’ve pulled the valve out and looked in the strainer.
Note on fixed metering devices, low head pressure will starve the evaporator.
Hunting. Hunting shows up as a fluctuating suction pressure is caused by either a problem with the sensing bulb, or, the valve is too big for the system and overshoots the process needs continuously. If the valve is too big, you’ll have flood back conditions, if it’s a sensing problem, everything will be moderately okay, just low superheat and poor cooling performance.
Compressors can have a plethora of problems most stemming from what the system is doing to it. In a rare instance, the compressor is just worn out.
Top of my list of compressor killers is liquid flood back. Liquid flood back, or slugging, is when liquid refrigerant is returned to the compressor from the evaporator. The two significant problems it causes are; it displaces the oil causing loss of lubrication and it won’t compress so, it dead heads the compressor piston.
Those problems lead to the following symptoms:
1. Noisy operation, when the piston hits short of stroke, the head and cylinder walls take the brunt of the force rattling the compressor to the bone.
2. Due to symptom (1), you can probably guess there will be vibration. If the compressor makes it long enough, I’ve seen it snap copper lines.
3. The compressor is going to overheat. The friction goes up because there is little or no oil. The metal to metal contact makes heat.
4. The system suffers capacity problems. Getting liquid into the compressor reduces the head and alters the saturated condensing temperature reducing the refrigeration effect.
5. High saturated suction temperature. All the liquid isn’t evaporated so the temperature of the gas isn’t lowered as much.
6. Superheat will be close to nothing, but when it’s even about 3 – 5 you should worry.
What to look for when you have these conditions might seem obvious, overcharge! And it would be my first guess too.
But there are some other culprits:
1. Evaporator air-flow is too low. Which can be any number of problems itself.
a. Loose or broken drive belt.
b. Plugged air-filter.
c. All the supply registers closed.
d. Duct is blocked with insulation.
e. Duct is crushed.
f. Incorrect speed selection or drive sheave.
2. The expansion valve could be stuck open. Could possibly only freeze at night.
3. On large flooded evaporators, the level could be set too high.
Those are the more common ones. Less common are the design issues. Unit is oversized, or equipment miss-matched.
Next on the list, flooded startup. Causes the same problems as liquid flood back, but it clears up after the unit runs for a bit. Can be tough to spot unless you are there when it starts up and know to look for it.
A clue can be that the system displays oil faults and low oil levels. If you had to add oil more than once, suspect a flooded start.
On hermetic compressors, it harder to catch without a sight glass for the oil, but they will probably be rattling from the damage, just like one that has been overcharged, just no other current symptoms.
There are a few suspects:
1. Bad crankcase heater. The heater comes on when the compressor goes off. It is intended to boil out the liquid that gathers in the sump before the compressor starts.
2. Pump down controls have failed. The pump down solenoid shuts off the liquid to the evaporator just before the unit shuts off. This operation is intended to remove excess refrigerant from the evaporator and suction line to prevent a flooded start up. Often found on outdoor package units.
3. The pump down controls may be working fine, but the solenoid isn’t closing. Refrigerant is leaking through during the off cycle. If this is the case, you’ll be able to watch the pressure rise on the suction line after the unit shuts off. Should rise some, but not equal to the condenser pressure.
Oil loss, oil loss is definitely a compressor killer, and can sometimes be hard to remedy.
Liquid flood back, and flooded starts are the most common reasons for oil loss, but here are a few more:
1. Oil isn’t returning to the compressor due to improper line sizing. The suction line velocity should stay above 1500 foot per minute to carry oil back.
2. Wrong oil with the refrigerant being used. Some refrigerants don’t like some oils and it just won’t carry it back. Sometimes adding small amounts of an oil the refrigerant likes will fix it, but it could also lead to sludge. Check the manufacturer for a solution.
3. Failed separator filter or return trap float. Some oil separators use a coalescing filter to strip the oil out of the gas stream. These materials, over time, get contaminated and quit working. Some also have a float that returns the oil to the compressor, the float can stick or the seat can get blocked off.
4. Bad oil return solenoid.
5. Bad heating element in the oil still. Large system with oil recovery stills use heat to drive out refrigerant from the oil. If the heater goes bad, you get back refrigerant, not oil.
6. Oil skimmers not on or evaporator level to high/low. Also on larger systems, the evaporator can have a series of pipes that should align with the refrigerant level in the evaporator. If they don’t the level control might not be working or needed adjusted.
Running the compressor to hot will lead to a slow death of the compressor. Compressors are tough, but they can only take so much.
Compressors need to operate at the manufacturers stated operating temperature. Here is a list of what to run down if it’s running too hot.
1. Dirty condenser, any surprise?
2. Condenser water or air-flow is low.
3. Excessive superheat. Can be set that way, or be a problem with the metering device. Can also be caused by overloading the evaporator with too much heat.
4. Over-charge and under-charge can cause it.
5. Low oil, or wrong oil.
6. Line voltage or current problems.
7. Mechanical wear. Usually from oil loss, but possibly from wear, I’ve seen compressors still functioning after 30 years in a quality design and installation.
Electrical failures happen but not as often as you would expect.
Symptom: compressor has power but will not run
1. Open winding.
2. Open overload.
3. Open thermal overload.
4. Burnt off terminal.
5. Running a scroll compressor in a vacuum lower than 15 inches of water cause a winding failure.
6. Lightning strikes, lol, so I’ve heard.
If the compressor trips the breaker, check for a shorted winding or a grounded circuit, do not try to operate.
Last but not least, contamination. Technicians are usually really good at keeping stuff out of the refrigerant system. Sometimes it happens through. Problem is, it can be hard to spot since it causes symptoms of other more common problems. Regular oil sampling is the best way to catch it. But here is a list of the symptoms.
1. Compressor won’t reach condensing pressure doesn’t move enough refrigerant.
2. Valves stick open or closed.
3. Compressor overheats.
4. Foul odors from the oil.
5. Unusually high condenser pressure but system is still adequate.
6. High temperature difference on the liquid line dryer.
7. Discolored sight glass.
8. Milky oil.
9. Elevated evaporator temperature.
10. No oil return.
Hope this helps prevent a premature compressor change out.
Common Condenser Problems
Number one on the list of common condenser problems is… ITS DIRTY!
Condenser are subjected to air or water that is contaminated with all sorts of stuff. The contaminants hinder the heat transfer and cause condensing pressure to increase, capacity to go down, higher compressor temperatures, high amp draw, etc. in short, everything must work harder if the condenser can’t shed the heat from the system.
On water cooled condensers, the small temperature difference is a good indicator of cleanliness. The small temperature difference is the actual temperature of the refrigerant leaving the condenser minus the temperature of the water leaving the condenser. For example, a unit with a refrigerant temperature of 100 F and a leaving water temperature of 95 F. That is a 5-degree small temperature difference. For copper and copper-nickel tubes, it should run from 5-15 degrees, steel, from 12-22 degrees, after that, clean the condenser.
Air-cooled condensers usually can be visually inspected to tell whether it needs cleaned. (Micro-Channel® condensers can be challenging to tell if they are blocked off, and hopefully you don’t get stuff stuck in between the layers).
If you can’t make a visual assessment, you can compare the heat exchanger thermal effectiveness with the design. Thermal effectiveness is, (Air out – Air in)/ (Air out – refrigerant saturation temperature). For a typical split system, A/C the calculation looks like the following – (130 – 95) / (120 – 95) = 1.4. If the effectiveness is less than expected it likely needs cleaned (or its under-charged). If you can’t measure the air out, a good estimate is the square root of the discharge temperature multiplied by the ambient. Sqrt (190 X 95) = 134
Other conditions that lead to high head pressure:
1. Head pressure control may not be working. Head pressure controls can be bypass or fan speed controls.
2. Bad fan motor.
3. Loose belt.
4. Worn sheaves allowing belt to slip.
5. Wrong sheave causing lower than needed fan speed.
6. Air-flow is blocked.
7. Bent over fins.
8. Damaged pump vanes cause internal bypass.
9. Pump impeller worn causing low volume.
10. Leaking pump discharge check valve.
11. Leaking hot gas bypass valve allowing discharge gas into evaporator when not wanted.
12. Broken/missing fan blade.
13. Plugged strainer.
14. Non-condensable in refrigerant.
15. System overcharge.
16. Condenser air short cycles from poor location i.e. too close to opposing walls.
And, believe it or not, low condensing pressure usually isn’t a condenser problem. You could have a fan cycling control that doesn’t turn the fan off when the head gets low, or, you could have a condenser water valve stuck wide open for some reason. Most likely, the unit isn’t charged correctly or has no load, but its not a condenser problem.
General Perimeters for Typical A/C
What should it be? I hear that question quite often. But the reality is, there is no simple answer. If you look over my posts, you’ll see I rely heavily on AHRI, design data, and pressure-enthalpy charts because manufacturers can build just about whatever they want so long as it functions as stated and meets regulations.
There are some typical aspects of air-conditioning design that are dictated by materials of construction and good practice such as line size and sub-cooling. For instance, lines are sized to keep pressure drop low (larger lines) but promote oil return (smaller lines). The sub-cooling will probably fall around 9 – 15 degrees on a split system because they can’t control how much liquid line (added pressure drop) the installer will add. But on a package unit, where they control the entire design, it may operate just fine with 5 degrees of sub-cooling, which reduces the total charge and cost of the unit.
Here I’m going to outline a few aspects of typical air-cooled single stage air-conditioners that won’t get you into too much trouble. (heh, heh, heh).
EER rating: This is actually a pretty useful piece of information if you find it on the unit. The EER is the ratio of the Btu per hour to the Watt per hour energy it takes to achieve capacity. If you know it’s a 5000 Btu/hour unit, and has an EER of 3, then 5000/3 = 1666.7 Watts are needed, divide by 120 volts, 1666.7/120 = 13.9 amps. You now know the total amp draw of the unit.
The EER was tested when the condenser was seeing 95 F air, and the evaporator was getting 80 F air at 50% relative humidity. If your conditions are different you will have a different current draw, but it will be close.
The EER is also NOT the SEER, seasonal energy efficiency rating. The SEER is an average Btu/Watt hour for an air-conditioning season of 125 8 hour days of who knows where. The SEER is used to sell units and appease regulators.
AMPS: For the compressor allow 1 horse power per ton (12000 Btu/Hour). Multiply the horse power by 746 Watts, then divide by the appropriate single phase voltage (120 or 220 volts). For three phase, multiply the volts by 1.27 first, then divide. (it’s 1.73 X .92 X .8 for the factor, efficiency and power factor.)
For total unit, 1.5 horse power per ton of cooling may be more appropriate.
Superheat: System with a thermal expansion valve, TXV, will run 7-10 degrees of suction super-heat. Units with electronic expansion valves, EXV, will run about 4-5 degrees.
If the unit has a capillary tube or fixed orifice it can be anywhere between 5 and 45 degrees depending on the operating conditions. Use the manufacturer tables or a generic superheat chart at the very least.
Sub-cooling: The sub-cooling varies with design but will fall somewhere between 5 and 15 degrees. Check manufacturer data. Usually on one of the panels of the unit you will find a chart like the one below. Use your phone to snap a picture and begin a library of these charts by model, you’ll soon be prepared for the unit that the chart has faded.
Condenser/discharge pressure: For air-cooled units, take the outdoor air temperature and add 30 degrees. Say its 90 outside, add 30, to get 120 degrees, look this valve up in the pressure temperature chart for the refrigerant in the system. For R-22 its 260 psig.
For water cooled units, measure the inlet and exit water temperatures and add them together, divide by 2, then add 10 degrees. For instance, the inlet water is 68 F, and the exit water temperature is 81 F, 68+81 = 149, divide by 2, 149/2 = 74.5, then add 10 to get 84.5 degrees, for R-22 that’s about 155 psig.
Suction/low-side pressure: Measure the air leaving the evaporator. Take several measurements and average (add them up and divide by the number of readings you took). Subtract 15 degrees from the average and look up the pressure from the temperature you got. Say the average was 57 degrees, subtract 15 degrees to get 42 degrees. For R-22 that pressure would be about 69 psig.
Air flow: Air flow should be about 350 – 450 cubic foot per minute per ton of cooling, with 400 cfm per ton being the most common rating when the fan is set to run on medium-high speed (blue tap). So, a 3-ton system will come set up to deliver 1200 cfm at about .3 inches of static pressure.
All that said, none of this will hold true on an inverter drive system unless its operating at 100% speed.
Troubleshooting the System Charge
The system refrigerant charge is either over or under charged if it isn’t correctly charged.
For a system undercharge, depending on the severity, you will have most or all the following symptoms:
1. The saturated suction temperature is less than expected after converting the suction pressure to its saturated temperature with the PT chart. For instance, common coil temperature for A/C would be 45 degrees F. On a system using R-22, your gauge reads 50 psig, you’re only running a 36 F coil.
2. Your system has a high suction superheat. After measuring the suction line surface temperature to be about 58 degrees, subtract the saturation temperature found in symptom one. In this example 58 – 36 = 22 degrees of superheat. Too high for a low saturated temperature.
3. Low liquid sub-cooling. Find the saturation temperature of the condenser pressure and subtract that number from the actual liquid line temperature. Less than 5 is low, could be zero.
4. Low saturated condenser temperature. Its 85 F outside so you add 30 degrees to get 115 F. Looking up that pressure on the PT chart you read 242 psig, but your gauge reads 190 psig, which is 98 degrees F, low.
5. Low system capacity. Unit won’t cool the room or takes a very long time to do it. Perhaps only works at night.
6. High compressor discharge superheat. Measuring the discharge line about 6 inches from the compressor you see 138 F, subtracting the saturated condensing temperature found above, 138 – 98 = 40 F. That’s above the 35 recommended.
7. Compressor body is warm, not sweating, or even hot.
8. The system could be cycling on the over temperature protection or low pressure cut-out.
9. Oil may smell burnt if running in this condition long enough.
10. Evaporator frosting or ice on the suction line for a very low charge.
Symptoms of overcharge, depending on the severity, you could have most or all the following:
1. High saturated condensing temperature. When you converted your condensing pressure to temperature it was 40 degrees higher than the ambient. Too high.
2. High saturated suction temperature. You were expecting a 45-degree coil, but after converting the pressure with a PT chart you find its running 50 degrees.
3. Low system capacity, here again the unit will not do the job because over or under charge, the system doesn’t function correctly.
4. Compressor amps will be high. This is usually from liquid getting back to the compressor.
5. Liquid sub-cooling (the actual liquid line temperature minus the condenser saturation temperature) will be greater than 15 degrees.
6. Low suction superheat. More noticeable on a system with a fixed metering device it will also cause a drop for a TXV if its severe enough.
7. Compressor may be noisy and vibrate.
8. Compressor starts with a lunge due to partial seizing caused by liquid damage and oil loss.
The above numbers were for example purposes and not from a real unit although the limits mentioned are good practice.
If you approach the diagnostic steps of troubleshooting in the same fashion every time you will be way less likely miss something. It may seem that collecting all that information is wasting time, but trust me, it will save time, money, frustration, and possible embarrassment.
First, make sure you have all the basic data.
A. Who is your contact at the job?
B. What kind of unit is it? Size/make/model?
C. How long has the problem persisted?
D. Where is the unit location? Roof/basement/attic/crawl space?
E. What is the specific complaint?
With this information, you will be better prepared to investigate the problem. It would be prudent to obtain a copy of the unit manufacturer information prior going into the field. (There may not be any cell service or you may not be allowed to use your phone.)
Once you’re on the job site, do a general inspection with the power off. Be on the lookout for:
1. Missing insulation. Can lead to unnecessary load on the unit. Missing insulation on TXV sensing bulb causes hunting.
2. Kinked or damaged lines. Poor line conditions can cause higher pressure drop or even restrict refrigerant flow.
3. Oil leaks. Units need oil to work properly for sure, but an oil leak is also a refrigerant leak.
4. Damage to condensing coils. Flattened coils reduce condenser capacity 20% is too much. Missing fins derogate condenser efficiency. Half the fins missing will affect capacity by about 20%.
5. Water leaks from line, unit, or ducts. Water will slowly destroy a unit is left free to roam. Water in the unit can lead to mold issues or worse.
6. Dirty filter/s? Dirty or plugged air filters in the unit reduce capacity and can lead to unit freeze up.
7. Bent, cracked or missing fan blades. This condition reduces the air-flow, capacity, cause vibration. A fan blade can damage a condenser beyond repair.
8. Missing unit panels. Missing panels or large gaping holes allow air to bypass the coils. Bypassing condenser coils can be enough to trigger high head pressure cut-outs.
9. Wiring condition, low and high voltage. Cracked frayed wiring is dangerous and usually means high heat. Loose connections can lead to this.
10. Damaged duct work or disconnected supply runs. The air has to get there to work.
11. Are the supply and return open and un-obstructed? Closing of half the supply is usually enough to freeze a unit.
12. Do you see ice on anything? Unit icing is low refrigerant, low air-flow, or a restriction. This has to be addressed first. Completely thaw out the unit before proceeding. Don’t immediately add gas, the coil is cold, suction will be low. Unit can be hiding a problem, check the static pressure from return to supply across the evaporator > then .15 inches of water column means the evaporator is dirty.
Any one of the above items could lead to improper operation of the unit. To what degree can vary greatly.
Now power up the unit and make a quick round to see if you hear any problems, like rattles, banging, vibration, things out of balance. (If the system won’t power up you need to check power and controls to determine why.)
Once the unit is operating gather the following data:
1. Indoor air wet and dry bulb temperatures. WB_____ DB_____
2. Outdoor ambient air dry bulb temperature. OA_____
3. Supply airflow.
4. Temperature of return air. RA____
5. Temperature of supply air. SA____
6. Temperature of air discharged from the condenser. CDA____
7. Suction pressure. SP____
8. Look up the saturated suction temperature. SST____
9. Condenser/discharge pressure. CP___
10. Look up the saturated discharge temperature. SCT____
11. Suction line surface temperature 6 inches before compressor. SLT____
12. Discharge line surface temperature 6 inches from compressor. CDT_____
13. Liquid line surface temperature just before the metering device. LLT____
14. Check the temperature difference of any liquid line filter dryers. Tin____ – Tout____ = Tdiff____
15. Amp/volts draw for the compressor. CA____ CV____
16. Amp/volts draw for the condenser fan. OFA____ OFV____
17. Amp/volts draw for the indoor fan. IFA____ IFV____
18. Check voltage of low-voltage supply. LVV____
Perform the following procedures/calculations:
1. Calculate the air-handler temperature rise (subtract the exit air from the inlet should see 20 – 25 degrees F).
2. Calculate the air handler BTUH. (Plot the indoor air wet and dry bulb versus the unit discharge conditions on a psychrometric chart and calculate the unit load. If the unit is seeing 80 F and 50% RH there is 6.7 Btu/lb of air, when the RH rises to 75%, the load on the unit doubles. The unit must overcome some of this load to become stable. Total load is the enthalpy difference multiplied by the cfm and 4.5 factor).
1. Calculated the condenser temperature rise (subtract the outdoor ambient from the exit of the fan should see 30 – 35 degrees F).
2. Calculate the condenser heat-rejection. (Multiply temperature rise by cfm and 1.08. Should be around 30% more than nameplate i.e. 24,000 Btu A/C should be rejecting about 31200 Btu from the condenser when fully loaded).
3. Determine if the unit air-flow is within the range of 350-450 cfm per ton of cooling. (This is best done with an electronic meter, but a pitot tube and calculation work fine).
4. Calculate suction superheat (Subtract saturation temperature from actual line temperature. TXV should be about 10, fixed metering use chart).
1. Calculate sub-cooling (Subtract the saturation temperature from the liquid line temperature. Should see 9-15 degrees or use charging chart if available).
2. Calculate the compressor superheat (subtract the saturation temperature from the line temperature about 6 inches from the compressor. Check against manufacture data but much higher than 35 needs looked into).
3. Check the that the voltage is not off by more than 10% (if 120 is typical line voltage then range is 108 – 132).
4. Check current draw against literature. If not available, the following is usually okay: For the compressor allow 1 horse power per ton (12000 Btu/Hour). Multiply the horse power by 746 Watts, then divide by the appropriate single phase voltage (120 or 220 volts). For three phase, multiply the volts by 1.27 first, then divide. (it’s 1.73 X .92 X .8 for the factor, efficiency and power factor.)
5. For total unit condenser and air-handler, 1.5 horse power per ton of cooling may be more appropriate for checking amps. (Package unit).
6. Filter driers with a temperature difference of 5 degrees F or higher are plugged.
7. If all this is in line with expected results, plot the system on a pressure enthalpy diagram for more detailed look at the performance.
So, the above procedure could be tailored to meet your needs, but I wouldn’t leave out too much of it. Though, not always obvious, after all these checks, the problem will emerge.
Gathering System Data for Use
When collecting real world data to analyze system performance nothing beats a good old fashioned pencil and paper.
You can use a form to keep it organized, or just a list if that works for you. Below I provided a pictogram for up to three stage system.
This can be used for up to three stage refrigeration cycle by marking N/A (not applicable) if the system doesn’t have a particular feature.
The PRESS is short for pressure, and TEMP is short for temperature. WB and DB, are wet bulb and dry bulb temperatures of the air entering and leaving the evaporator.
Each valve has an O or C beside it to indicate whether it’s open or closed.
I’ve posted the Excel file so it can be printed on one page or altered to meet your needs.
Hope you find it helpful!
Using Charts with Caution
There are numerous trouble shooting charts and diagrams to help diagnose system problems. Probably one of the most familiar ones I’ve seen, posted numerous places is below.
The chart points out one thing clearly, you can’t diagnose a problem without multiple pieces of information about the system. I’m not sure who really gets credit for developing this chart, but the first place I came across it was in a very old Carrier training manual, and my guess is, they should get credit for it.
Another type of performance chart is a unit specific charging chart. Using field acquired pressure and temperature of the liquid line, the chart shows you what to do to get the unit performing at design. Below is an example of that type of chart.
Both approaches seem simple enough to use, but neither will work if the system is dirty or improperly installed.
With almost any set of charging instructions you will find the following two lines;
1. Never charge liquid into the suction port of the compressor.
2. Add small amounts of refrigerant at a time.
Why do you suppose that is?
Liquid into the compressor will damage it. It also will cause significant swing to the system pressures and temperatures. It can also quickly overcharge the system provided the compressor can take it.
As for small amounts of charge at a time, that is because when the charge is incorrect, the system is unstable. It will take as much as 20 minutes for the change you made to be realized in the readings you are observing.
To illustrate the system performance, I recorded a dataset for an air-cooled unit cooling a water stream to 45 degrees. Below is the system performance from starting, and running to set point.
Nice stable performance, the suction pressure follows the water temperature right to the set-point. Discharge pressure slowly lowers as the system is cooled, and the pressure ratio is essentially constant.
This system, by the way, used a capillary tube as a metering device. But you will see similar characteristics, such steady motor current, in a properly charged system.
Now, let’s look at a system trying to perform with about a 20% undercharge.
As might be apparent, system stability, or lack thereof, is questionable.
The water temperature warms up some during the initial operation. The discharge pressure climbs as the system can remove all the excess heat from the unit and the process.
Pressure ratio is all over the board for almost two hours, and suction pressure climbs until the system reaches a somewhat stable operation at 4:17.
So, when you are trying to interpret the information from your gauges and temperature probes, keep in mind, the system is always trying to adapt to the process conditions and its problems all at the same time.
So, use some patients and this knowledge to proceed with care when using charts to diagnose or charge a system.
I worked for over thirty years in the HVACR industry. I have designed, installed, serviced, and trouble shot units of various types throughout the years. The posts here are information based on that experience, I hope you find them useful. If you have a different experience, please comment.