Air Conditioner / Heat Pump Diagnostic Procedures

Quick Start to Diagnose an A/C or Heat Pump not working

If The Air Conditioning System Or Heat Pump Is Not Running At All

1. Check the Room Thermostat Temperature Setting:
   
   Set the thermostat to at least 5 degrees below room temperature. Lower is better when we're troubleshooting. You can always turn the thermostat back up later.
   
   My elderly mom has no patience with switches and controls. She regularly calls her air conditioning service company with a service request, sometimes late at night, because she has simply failed to set the temperature on the thermostat lower than the room temperature.
   
   Don't drive your A/C like our mother.
2. Check that the Room Thermostat is set to "Cool"
   
    not "Off" or "Heat". If the thermostat is not set to "cool" it is simply turning off your A/C.
3. Check that electricity is on for the equipment.
   
    Check all switches and controls, including service switches, including outside by the compressor, inside at the air handler, and fuses or circuit breakers in the electrical panel.
   
   For example, condensate spilling into an overflow pan that uses a sensor switch can be enough to shut down your air conditioner.
   
   There are several switches and controls, both manual and automatic that can leave an air conditioner or heat pump turned "off" such as a blower compartment door interlock safety switch, an electric motor overload or overheat switches, and a condensate tray spillage detector switch.
   
   Watch out: See A/C - HEAT PUMP CONTROLS & SWITCHES to be sure you have found and checked every manual or automatic electrical switch on the system. A bad or failed starter capacitor could also be leaving your system shut down, failing to start a blower, fan, or compressor motor. Some hard-to-find switches could be keeping your air conditioner from starting, such as
   
   a Float Switch on Condensate Tray or
   
   
   a blower MOTOR OVERLOAD RESET SWITCH

If after the checks above the air conditioning system is still not working, or if the A/C system is running but cool air is not coming out of the supply registers you probably need to call a service technician, but below are some things to check further yourself.

But first, see

AIR CONDITIONER WON'T START

for more detailed DIY help in diagnosing and repairing the trouble. when your cooling unit or heat pump simply will not turn on at all.

If At Least Some of the Air Conditioner or Heat Pump Equipment is Running but Cool Air is Not Being Delivered

Follow these diagnostic air conditioning / heat pump system checks to get the air conditioner running again.

If you are not sure what A/C or heat pump component is not working or where the trouble lies, follow the steps just below.

But to make any sense of these checks, you need to know the names of the main parts of your A/C or heat pump and the location of those components.

If you have no idea of the names and locations and purposes of the major parts of your air conditioner or heat pump system, first

read HOW an AIR CONDITIONER WORKS - then come back to this page to step through the diagnostics below.

Step 1: Check the Outside Air Conditioning /Heat Pump Compressor-Condenser unit

If your air conditioner or heat pump system is not working at all, that is, it won't start,

the problem could be at the thermostat, or lost electrical power, or something as simple as a switch that's turned off.

For cooling, set the thermostat well below the current room temperature, or for heating, set the thermostat well above room temperature or simply "all the way up" to see if the system will start.

Then check the electrical switches at the indoor air handler or blower unit, at the outdoor compressor/condenser unit, and in your electrical panel to be sure that power is turned on to the equipment.

If the system still won't start, more details of these checks are found at

AIR CONDITIONER WON'T START

where you'll get more help diagnosing and repairing the trouble.

If the outdoor compressor/condenser unit never starts:

If you don't know what or where this component is,

see COMPRESSOR & CONDENSING COIL

You should hear the outdoor compressor condenser motor start and run (you'll hear an electric motor noise) and should see its fan blades spinning.

If not, be sure that the outdoor compressor/condenser unit SERVICE SWITCH [image] and the air conditioner indoor power switch, fuse, or circuit breaker are all in the "on" position - as we suggested earlier

If the indoor air handler blower starts (described in step 2 below) and there is good air flow but the outdoor compressor/condenser does not run,

the supply air at registers will not be cool.

If the outdoor compressor / condenser unit doesn't start

Check the outside compressor condensing coil for damage or blockage.

A blocked condensing coil can cause the compressor to overheat, stop running, or become noisy and work poorly.

See A/C COMPRESSOR PROBLEMS - diagnostic articles for the compressor/condenser unit.

If you cannot find your air conditioner controls and switches

see A/C - HEAT PUMP CONTROLS & SWITCHES - list of controls and switches found on air conditioners and heat pumps

Also check the compressor/condenser outdoor fan and fan motor

COMPRESSOR / CONDENSER DIAGNOSTICS

Condensing unit fan diagnostics:

at FAN, COMPRESSOR / CONDENSER UNIT we explain the diagnosis of problems with the compressor/condenser fan and fan motor.

Also see ELECTRIC MOTOR DIAGNOSTIC GUIDE.

at BURNED-OUT COMPRESSOR we explain that if the condensing unit fan is not working the compressor itself may shut down or even be ruined by overpressure or over temperature.

Also see NOISES, COMPRESSOR CONDENSER where some noise problems are traced to the cooling fan .

Step 2: Check the Air Conditioning /Heat Pump Indoor Air Handler Unit

With power on to your equipment and the thermostat calling for heating or cooling the indoor blower or air handler fan should start - you'll hear it if you go to the unit or you'll feel air coming out of the air supply registers in various rooms;

If the A/C or heat pump unit is a wall-mounted "split system" there is no duct work but you should hear the indoor unit fan run and should feel air coming out of the unit's outlet opening (usually at its bottom edge).

If the indoor air handler/blower unit never starts

check the electrical switches as we said above, and be sure to check that the covers or doors on the unit are fully in placed and latched.

Often a blower door safety switch will keep the air handler turned off if the door is not fully seated in place.

Is the indoor blower unit running?

If not be sure that the electrical power switch at your furnace or air conditioner air-handler is in the "on" position.

Typically the air handler or blower unit is indoors inside the basement, crawl area, or attic.

Sometimes there is more than one switch, such as one right at the unit and another at the entry to the room where the equipment is located, or even upstairs on a higher floor if your air handler is in a basement or crawl space.

Watch out: it's embarrassing and expensive to call an HVAC repair technician if the only problem was that somebody left a door off of the air handler - causing it to remain "off" for safety reasons.

Make sure that the blower compartment cover or door is properly closed or an interlock switch may be keeping the system "off".

If the indoor air handler blower/fan won't start when your thermostat is calling for cooling and the room temperature is above the "set" temperature on the thermostat, then

see BLOWER FAN OPERATION & TESTING

Step 3: Weak air flow: Check the Air Conditioning /Heat Pump Air Filter(s)

If your air conditioning equipment is running but little or no cool air is coming out of the supply registers, check that your air filter(s) have not become blocked with dirt and debris.

Usually the air filter is right at the air handler or blower unit, or your air filter may be installed behind a grille covering a central warm-air return that sends air back to the air handler.

See A/C FILTER PROBLEMS for details as dirty air filters are a common cause of weak air flow and inadequate cooling.

If the air conditioner or heat pump is running but air flow is still too weak continue at step 4.

If the air handler unit blower starts but air flow is weak,

the problem could simply be a dirty air filter, or collapsed ductwork, or an ice-blocked cooling coil (illustrated above). Go to

A/C FLOW TOO WEAK

where we describe an ice-blocked or dirt-blocked cooling coil or problems with the return or supply air ducts themselves.

If the blower runs normally but air flow is warm or weak,

remember to check for a dirty, blocked COOLING COIL or EVAPORATOR COIL.

Also see ELECTRIC MOTOR DIAGNOSTIC GUIDE if you suspect a blower motor problem.

Step 4: Weak air flow: Next Check the Air Conditioning /Heat Pump system duct work & air supply registers

In ceilings, walls, or floors, where cool air is supposed to be delivered to various rooms in the building, be sure that the register is in the "open" position (you will be able to see through it into the ductwork) and be sure you haven't blocked the supply registers with a carpet or furniture.

Look for a duct damper or register that has been closed; look for a flex-duct section that has become disconnected, bent, or crimped or squashed. Remember that a duct may have become disconnected in an attic or crawl space.

Look for an air filter that has come loose and blown into the ductwork, clogging it.

In our photo at above/left you can see a ceiling air supply register that has leak stains around its opening - further investigation for a condensate leak or roof leak is needed.

Also see DUCT & AIR FLOW PROBLEMS for a complete diagnostic guide to inadequate air flow problems

Also see RETURN AIR, HVAC - inadequate return air will reduce the supply of cool air in cooling mode or warm air in heating mode.

Step 5: No air flow at all: Check the Air Conditioning /Heat Pump indoor air handler blower fan assembly

If the indoor air handler blower fan itself won't start,

see BLOWER FAN OPERATION & TESTING.

Check for lost power, a fan motor that wont' start or has tripped a motor reset button or needs a start / run capacitor.

Check for a blower fan that is loose on the motor drive shaft or for a broken or lost blower fan drive belt on pulley-driven blower systems.

Also see ELECTRIC MOTOR DIAGNOSTIC GUIDE for details

Step 6: Air flow is OK but air is not cool: check for lost refrigerant, outdoor compressor/condenser not running, or a stuck refrigerant metering control

If your air conditioner's supply output air flow feels normal or strong enough but the air temperature doesn't get cool enough or even blows out warm air when it should be cool, the problem could be lost refrigerant but it's also possible that the outdoor compressor isn't running or we have still another problem.

Possible causes to be checked when air flow is warm include:

• The compressor/condenser is not running at all
  
  see COMPRESSOR / CONDENSER DIAGNOSTICS
• The compressor runs but refrigerant has been lost due to a leak
  
  see REFRIGERANT GASES & PIPING - home
  
  and GAUGE, REFRIGERATION PRESSURE TEST
• The compressor runs, refrigerant charge is OK but the refrigerant metering device or TEV is jammed
  
  see s Step 7: REFRIGERANT PRESSURE & TEMPERATURE CHECKS TO DIAGNOSE LOST COOLING CAPACITY below.

If both indoor air handler and outdoor compressor are running and there is good air flow but the air is not cool, you need help from a service technician.

Details for diagnosing this warm supply air trouble are

at AIR CONDITIONER BLOWS WARM AIR

Watch out: don't confuse weak air flow with "too warm" output air from your air conditioner or from your heat pump when it's in cooling mode.

A coil blocked by ice or dirt will not produce cool air but in this case the air flow is probably weak.

see details at A/C COOLING COIL ICING.

The coil may need cleaning, or low refrigerant or a bad expansion valve may be causing ice blockage on the coil.

If the air conditioner or heat pump runs but in cooling mode the air supplied is too warm there are several

things that will cause warm air discharge from the supply registers of an air conditioning system or from a heat pump system in cooling mode.

Step 7: Refrigerant Pressure & Temperature Checks to Diagnose Lost Cooling Capacity

Photo: a frost-blocked cooling coil in the air handler of an air conditioning system will stop the delivery of cool air to the building. Most common cause: low refrigerant or a dirty air filter.

Dirty air filter

Believe it or not something as simple as a dirty air filter (or dirty blower fan itself) can cause the cooling coil (evaporator coil) to become blocked with ice. That's because anything that cuts the air flow across the evaporator coil can cause ice and frost to form over the coil (since warm air is not warming the coil enough). (A little frost as long as the coil isn't blocked isn't a concern - Ed.)

Blocked coil, filter, iced coil

Look at the cooling coil itself for dirt, damaged fins, or frost or ice that block the indoor cooling coil (at the air handler) or the outdoor compressor/condenser coil.

Technician's additional tests (not for the homeowner):

A blocked coil is indicated if suction side pressure is abnormally low combined with low superheat and high sub-cooling

Watch out: tests of refrigerant pressure are not something that a typical building owner can or even should attempt. Even if you have HVAC test gauges, if you don't have a supply of the proper refrigerant to purge and connect the pressure gauges you can contaminate and damage your equipment, leading to even more-expensive repairs.

Refrigerant leak (low refrigerant)

Technician's additional tests (not for the homeowner):

• Indicated if suction side pressure is abnormally low combined with high superheat and low sub-cooling.
• These conditions may be found also if outdoor air temperature is at or below the normal operating range of the compressor/condenser

Low temperature drop across the cooling coil

Technician's additional tests (not for the homeowner):

• Normally we see 15-20 °F temperature drop across the cooling coil.
• If the temperature drop across the coil is below 15 °F check for a TEV that's stuck open OR a refrigerant leak (low refrigerant).
• If the temperature drop across the coil is above 20 °F check for a blocked coil, filter, return duct, or other air flow obstruction.

The steps given here will be taken by your trained HVAC repair or service technician who will use gauges and temperature measuring equipment to check the system's operating temperatures and pressures at key diagnostic points.

Stuck Thermostatic Expansion Valve (TEV)

Technician's additional tests (not for the homeowner):

Bad TEV

• If suction side pressure is abnormally low
  
  the expansion valve may be stuck closed. Confirm this by finding the combination of low suction-side pressure with both high-superheat AND high sub-cooling.
• Normal Superheat = 8-10 °F.
  
  Find superheat by subtracting the evaporation temperature from the suction line temperature measured at the condenser unit. Normal superheat will be between 8 and 18 °F.
• Normal Sub-cooling = 3-12 °F.
  
  (and no bubbles in the sight glass if one is installed). Find sub-cooling by subtracting the liquid line (high side) temperature from the saturation temperature for the refrigerant.
• If suction side pressure is abnormally high
  
  the expansion valve may be stuck open.
  
  Other causes of abnormally high suction side pressure include the wrong TEV model or wrong TEV adjustment, or an internal leak inside the compressor motor, or a defective or misadjusted hot gas bypass regulator.

See details at THERMOSTATIC EXPANSION VALVE TEV INSTALL, DIAGNOSE, REPAIR

Step 8: Air Conditioner or Heat Pump Runs but Makes Strange Noises

If the air conditioner or heat pump system is running and the air is cool (in cooling mode) or warm (in heating mode) but the system is making strange noises, go to

NOISE AIR CONDITIONER / HEAT PUMP

Step 9: Call your HVAC Service Technician

If after checking the switches, controls or other easy diagnostics in the steps above you the air conditioning system or heat pump is still not working, or if the system is running but cool air is not coming out of the supply registers you need to call a service technician.