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The best way to trouble-shoot your cooling system is to go through the following items one by one, even if you feel sure you know what the outcome will be.
1. Thermostat checks
- Is the system selector switch in the "Cool" position?
- Is the temperature set at least 3 degrees below current room temperature?
- If your thermostat is electronic, are the batteries charged?
- Some forced-air furnaces have a switch on the thermostat that allows you to turn the fan on independently of the furnace or air conditioner. This switch will be marked "ON" and "AUTO". Does your thermostat have such a switch?
- If the answer to the above is Yes, listen for the furnace fan coming on. If you cannot hear the fan, go to one of the supply registers and see if you can feel any air coming up. Is the furnace fan operating?
Note: some electronic thermostats have a delay built in to protect the air conditioner from being accidentally switched on while the furnace is running. This delay can be up to 15 minutes.
Summary: If everything seems fine with the thermostat, the next step is to make sure there is power going to the furnace.
2. Electrical checks
(A) Items to check at the furnace
- Your system may be equipped with an emergency shut-off switch for the furnace. If so, make sure it is turned ON. In most homes, the emergency shut-off switch looks just like a regular light switch and will be located close to the furnace. (Trace the wire from the switch to the furnace to make sure you have the right switch.) It is quite common for this switch to be turned off accidentally. If the switch is ON, but the fan still doesn't operate, proceed to the Items to check at the electrical panel.
- Some heating systems also have emergency cut-out switches that are activated when a door or service panel is removed. Often the furnace door is not replaced properly after a filter cleaning and the cut-out switch will prevent the furnace from coming on. Check to make sure all access doors are properly secured and fitted.
(B) Items to check at the electrical panel
- The breakers for the air conditioner and the furnace should be in the "ON" position. If a breaker has been tripped, it will be in a position between ON and OFF. To reset, turn the breaker all the way off and then back on. If you have fuses instead, make sure the fuse hasn't blown. If it has, replace it with the same-sized fuse.
WARNING: If a breaker keeps tripping or a fuse keeps blowing, you should contact a qualified service technician to inspect your system to determine why you are having problems.
Summary: If you have determined that there is power to the furnace but you are still not getting any heat, select your heating fuel type from the list below and continue with the checks. If you have discovered that the furnace not getting any power, call your service contractor.
(C) Items to check at the air conditioner
- Some central air conditioners are installed with a switch, or electrical disconnect, mounted outside to turn off the main power to the unit. Whether or not you have such a switch will depend on the electrical code in your area. If you have a disconnect switch, check to make sure it is in the ON position.
- Is the fan running on the unit outside? (If the fan is running, you have power to the unit.)
iii) Some air conditioners will make a little buzzing sound, even when they are not running. Have you ever noticed this before? Can you hear it now?
Summary: If you have determined that there is power to both the furnace and air conditioner but you are still not getting any cool air, continue with the checks in this guide. If either the furnace or air conditioner does not have power, call your service contractor.
(D) Ice checks
- Is there ice building up on the pipes connecting the furnace to the air conditioning unit outside?
- Touch the plenum (this is the sheet metal box that comes off of the furnace; the evaporator coil is inside). Does it feel very cold? If it does, the evaporator coil may be frozen and you'll need to call for service.
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