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.
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.
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