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Beginning steps to diagnose your HVAC system

Tip: Use safety when around electrical components

Check the breaker panel.

Usually a breaker has popped and is causing your system not to work. If you reset the breaker and the air conditioning system pops the breaker again, there is a bigger problem. What you want to do next is check all electrical components like blower motors, compressor, fan motors and crank case heaters for shorted to ground condition or over amp draw due to a mechanical failure, or compromised motor winding inside these components.

Check the thermostat.

Sometimes the thermostat can be your problem. If the system is not calling for cooling, the system will not work. Also, sometimes the program in the thermostat has not been properly programmed and the default settings are still in use.

Check to see if you have refrigerant in the system.

If you have little or no refrigerant in the system, you can now have the fun of trying to find the leak. There are different methods to this and once you have found the leak, be happy you did because sometimes this can be very hard.

AC Troubleshooting Chart

This AC troubleshooting chart is like a cheat sheet. By following all the symptoms of your AC system, you can pinpoint exactly what's wrong. Troubleshooting chart is by Heat Controller. You can find that chart here.

AC Compressor troubleshooting

This AC Compressor troubleshooting page can help you diagnos your comporessor failures By following You can find that here.

Troubleshooting Motors

Some PDF's that explain trouble shooting motors. Very easy to understand from Fluke®.  Links will require Adobe Reader® which can be found here





Carlyle O6D, O6E and O6CC Video

Troubleshooting Carrier Carlyle O6D, O6E and O6CC video. This is a older video but these compressors are in use today.

HVAC Installation and Air Flow tips

HVAC Duct Design

If you need help designing and sizing your ductwork, provides Manual J calculations and Manual D calculations duct design for your home.

  • 10 ft x 10 ft room requires 6" duct supply
  • 12 ft x 12 ft room requires 7" duct supply
  • 14 ft x 14 ft room requires 8" duct supply

Rooms Larger than 200 SF should have 2 Duct supplies

  • 200 square foot requires 2 6" duct supplies
  • 300 square foot requires 2 7" duct supplies
  • 400 square foot requires 2 8" duct supplies

These are only estimations of duct sizing. Sometimes you have to consider the length of the duct run and if the room is in the sun most of the day. Over sizing the ductwork won't hurt since you can always adjust the air flow using the damper in the collar.

Heating and Cooling Sizing

A few basics when sizing your Air conditioning system. We are not going into the science of sizing the system. I am going to keep it simple, and use a basic HVAC sizing system. This is not the proper way to size your HVAC, but it is a good starting point to get a idea of what size HVAC system might work for your residential application. Click here for a HVAC sizing chart

400 Square feet per ton.

If your house is 800 square feet, you need 2 tons of A/C.

If your house is 1200 square feet, you need 3 tons of A/C.

Sizing a system for commercial applications will be different. Things like computers, copy machines, lighting, people and a consistent flow of fresh air have to be included in your calculations. Over sizing a system in a commercial application will not hurt. I suggest using the basic sizing plus adding tonnage for all the elements or doing it properly.

Basic Troubleshooting Using Superheat And Subcooling

These four temperature differentials are the critical measurements used to determine all refrigerant related problems. Often a manifold gauge set is not even necessary.

  • Superheat is a temperature differential.
  • Subcooling is a temperature differential.
  • Evaporator entering air versus leaving air temperature is a differential.
  • Condenser entering air versus leaving air temperature is a differential.

Critical Temperature Differentials

  • Air temperature drop over the evaporator should not exceed 20 degrees F.
  • Air temperature rise over the condenser should not exceed 30 degrees F.
  • The low side superheat should be between 20 and 30 degrees.
  • The condenser subcooling should not exceed 15 degrees.
  • An air temperature drop over the evaporator greater than 20 degrees indicates low evaporator airflow.
  • An air temperature rise over the condenser greater than 30 degrees indicates low condenser airflow.
  • A low side superheat less than 20 degrees indicates too much liquid refrigerant is in the low side.
  • A low side superheat greater than 30 degrees indicates too little refrigerant is in the low side.
  • A condenser subcooling exceeding 15 degrees indicates too much liquid refrigerant is in the high side.

Comparing these readings will lead to an understanding of what is wrong with the system. For example, assuming adequate airflow over both the evaporator and condenser the following is true.

  • High superheat with high condenser subcooling indicates a restriction. Too much liquid is in the high side and too little in the low side.
  • Low superheat with high subcooling indicates an overcharge. Too much liquid on both sides.
  • High superheat with low condenser subcooling indicates an undercharge. Not enough liquid on either side.
  • Low side superheat and condenser subcooling simply tell us where the refrigerant is located.
  • Too much refrigerant on the high side and too little on the low side indicates a restriction.
  • Too much on both sides indicates an overcharge and not enough on either side indicates an undercharge.