Evaporator Woes

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.
High air-flow.
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.
Starving Evaporator.
·        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.
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