above: Anatomy of a central A/C system: Condenser coils; Low pressure switch; For the sake of clarity, the wiring has been simplified or largely deleted, as in the cutaway view above.
A typical central air conditioner works like a room unit, except that the condenser coils, condenser fan, and compressor are outdoors while the evaporator coils and blower are indoors. Insulated tubing carries the refrigerant between the two parts.
Indoors, cool air circulates through ducts, most often the ducts of a forced- air heating system. The evaporator coils are usually inside the main supply duct, or plenum, directly above the furnace’s blower, which circulates the air. This arrangement is called a split or remote system (as opposed to a package” unit, which is basically an extra-large room unit connected to ducts).
The compressor and condenser fan generally operate on a 240-volt circuit and the blower on a separate 120- or 240-volt circuit with the furnace. A thermostat controls the system.
A central air conditioner’s cooling capacity is rated in Btu’s per hour. As a rough rule, you need 12,000 Btu’s for 1,000 square feet of well-insulated space or 400 square feet of poorly insulated space. But structure, window area, sun exposure, climate, and other factors also affect a house’s cooling needs. Have the best system size deter mined by a contractor using a standard guide such as the Air-Conditioning Contractors of America’s Manual.
For even cooling, balance the ducts for cooling as well as heating. Seasonal needs vary. A kitchen, For example, may need little heat in winter but extra cooling in summer. Make sure the ducts can carry as much air as the system supplies. Inadequate return ducts are a major cause of poor cooling.
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ENVIRONMENTAL HINTS
To save energy and money, raise the thermostat a few degrees. For each degree, electricity costs drop by 4 percent. Insulate ducts passing through uncooled spaces. Wrap batts, vapor retarder facing out, around each duct, and seal joints with moistureproof duct tape. Also, make sure your home’s insulation is adequate for your region.
Shade sunny windows or cover the glass with reflective film. Make sure all openings are well weatherstripped.
Before buying a new unit, compare the SEER’s (seasonal energy efficiency ratings) of units with the same capacity. The higher the SEER, the lower the operating cost.
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Troubleshooting:
| SYMPTOM | POSSIBLE CAUSE | SOLUTION |
Won’t start Starts but stops Cools poorly Compressor runs non-stop Cycles on and off often Frosted evaporator coils Wet furnace Noisy outdoor unit |
Power off at service panel or at switch Tripped high- or low-pressure safety switch Faulty high- or low-pressure safety switch Faulty transformer Faulty compressor Faulty capacitor Faulty contactor Faulty thermostat Faulty condenser fan motor Dirty filter or blower Clogged or frosted evaporator coils Clogged condenser coils Loose or broken blower fan belt Faulty blower motor or relay Insufficient return-air flow Compressor runs: faulty condenser fan motor Condenser fan runs: faulty compressor Low refrigerant level Faulty thermostat Dirty filter Clogged evaporator coils Clogged condenser coils Low refrigerant level Faulty contactor Faulty thermostat Clogged condenser coils Faulty condenser fan motor Dirty filter Clogged evaporator coils Loose or broken blower fan belt Faulty blower motor or relay Low refrigerant level Insufficient return-air flow Clogged evaporator drainpipe Fan hitting obstruction Loose fan blades Loose screws in housing Unoiled fan motor Faulty fan motor |
See General troubleshooting; check outdoor switch. Press reset button; wait 5 mm. before restarting unit. Test and have replaced. Test and replace. Test and have serviced. Test and replace. I Test and replace. Test; repair or replace. Test; repair or replace. Clean or replace filter. Vacuum blower. Have dirty coils serviced. See Frosted evaporator coils, below. Clean. Tighten or replace. Repair or replace motor; test and replace relay. Open or unblock return registers. Or have return system enlarged Test; repair or replace. Test and have serviced. Check for oily spots on or below refrigerant lines and have serviced. Test; repair or replace. Clean or replace. Have serviced. See also Frosted evaporator coils, below. Clean. Check for oily spots on or below refrigerant lines and have serviced. Test and replace. Test; repair or replace. Clean. Test; repair or replace. Clean or replace. Have serviced. Tighten or replace. Test and repair. Run blower alone to melt frost. Check for oily spots on or below refrigerant lines and have serviced. Open or unblock return registers. Or have return system enlarged Clear. Remove obstruction. Tighten Tighten. Oil motor bearings. Test; repair or replace. |
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Gaining access:
To remove fan grille on outdoor unit, loosen and take out cap nuts securing the grille to the top panel. Then lift off the grille. CAUTION: Turn off the power to the outdoor unit and the indoor blower before opening the unit.
To remove coil guard wrapped around many models, unscrew bolts or screws around base. Lift off guard, being careful not to strike fan or delicate condenser fins.
Other housings are removed by loosening screws in top and removing screws on sides. Be careful not to remove the wrong screws. Wearing heavy gloves, lift up top slightly while freeing base.
To reach controls, loosen screws holding access panel in place. Inside are the capacitor, contactor, and high- and low- pressure safety switches. On some units controls are in a small box with a removable cover.
To open compressor junction box, pry away clip with screwdriver; lift off cover. Inside are compressor motor terminals. Some models, like the one shown here, also contain an overload protector.
Condenser fan and fan motor:
The condenser fan dispels heat from the compressor and condenser coils. If it slows down or stops working, the sys tem will cool poorly and the compressor may overheat and shut down. A fan needs attention if a clicking or grating
noise indicates that the blades are striking an obstruction or some part. If a blade becomes bent, replace the entire blade unit. A straightened blade may become unbalanced and can loosen the fan motor or hit the condenser coil.
Fan motors, usually split-phase, run on the 240-volt circuit that powers the compressor. To service a faulty motor, see General troubleshooting. CAUTION: Before testing, discharge the fan and compressor capacitors.
Tighten loose fan blades, using a wrench for bolts or a hex key for setscrews. Make sure fan rotates freely and doesn’t wobble. It fan loosens often, apply a drop of thread-locking compound.
Test fan motor with VOM on RX1. Free and label motor leads at control panel contactor and capacitor. Probe common lead and other motor lead(s) in turn. Look for reading of low ohms on all.
To replace motor, record positions of motor lead connections; unfasten leads and any clips holding them. Remove fan; loosen motor bracket with wrenches. Lift motor free, pulling leads behind it,
Compressor and refrigerant lines:
A central air-conditioning compressor is a larger version of the one in a room unit. Some have a heater that runs constantly or on cold days to keep the compressor oil flowing freely. The heater also keeps the refrigerant from mixing with the oil in the compressor when the unit is off for a long time.
In addition to the compressor motor terminals, the compressor junction box may also contain an overload protector; to test it, see above. If you find a refrigerant leak, have the compressor professionally recharged. If your system has a high- or low-pressure safety switch and it tests faulty, have a pro replace it. CAUTION: Before testing, discharge the compressor and fan capacitors.
To test compressor motor for continuity, label and disconnect its leads. Set VOM on RX1; touch probes to all possible pairs of compressor motor terminals. All readings should below ohms.
Test motor for ground fault with leads disconnected. Set VOM on RX1 00. Touch one probe to bare metal on compressor, the other to each terminal in turn. Look for a reading of infinity on all.
To test high- or low-pressure switch, let compressor cool. Push switch reset button; disconnect one switch lead. With VOM on RX1, probe both switch leads. Look for zero ohms reading.
Testing the contactor:
above: Control panel on an outdoor unit typically contains the contactor, fan relay, and transformer.
The contactor starts the compressor and the condenser fan. Before testing it, turn off the power to both the indoor und outdoor units, and discharge the capacitor. Then open the control panel , and label the wires by tracing them to their sources (this may require removing the fan grille and coil guard). Check for loose or corroded connections first. Clean any corrosion from the terminals with an automotive- point file. Clean the switch contacts with electrical contact cleaner.
1. Test contactor through terminals. Set VOM on RX1. Remove leads from one terminal and touch probes to both terminals. Look for a reading of zero ohms, indicating continuity. 2. Test contactor switch terminals with switch contacts open. Remove leads from one terminal. With VOM on RX1, clip probes to both terminals. Look for a reading of infinity.
3. Leaving the V0M clipped to the contactor switch terminals, test them again while pressing button to close switch contacts. Look for a steady reading of zero ohms. 4. Test contactor coil. Remove a lead from a coil terminal. Set VOM on RX100 and probe both coil terminals. Look for a low reading approaching zero. If contactor fails any test, replace it.
Evaporator drainage:
Warm indoor air contacting the evaporator coil condenses, causing moisture to collect and drip. In most units, a plastic pipe carries the water outdoors or to a floor drain. Algae and bacteria growth can clog the pipe. When this happens, backed-up water can puddle, causing rust or shorting the blower.
To check the evaporator drain, look for water dripping from the lower end when the air conditioner is running. Also inspect beneath the coil, if possible, to detect puddling.
To clear evaporator drain, remove plastic tape holding trap to elbow. (If necessary, saw through pipe at elbow.) Flush trap with hose; pour in 1 tbsp. chlorine bleach. Reattach with new tape.
= = = USE AND CARE = = =
Clean or replace air filter every month during cooling season. Don’t run the unit with the filter removed. Let a filter you’ve washed dry completely before reinstalling it.
Before restarting unit in the spring or after a long shutdown, restore power to the air conditioner for 24 hours first. This produces the heat needed to separate the oil from the refrigerant in the compressor.
Before restarting after a brief interruption, turn the thermostat off for at least 5 minutes.
To avoid damaging the compressor, don’t operate the unit when it’s below 60-deg. F outside.
Lubricate fan motor yearly if it has oil ports (they’re usually plugged with rubber or metal caps). Use non-detergent lightweight SAE 20 oil; add no more than 10 drops per port.
Clean condenser coils before cooling sea son. Shut off power; uncover coils and gently brush dirty side. Then hose from inside unit, using plastic bags to protect other parts.
Prune vegetation away from outdoor unit by 2 ft. on each side and above. Spray grass clippings away from unit when mowing lawn. During fall, keep fan grille free of leaves.
