Step by Step Approach for A/C Units

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.
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