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
Some PDF's that explain trouble shooting motors. Very easy to understand
from Fluke®. Links will require Adobe Reader® which can
TROUBLESHOOTING SINGLE PHASE MOTORS
TROUBLESHOOTING THREE PHASE MOTORS
TROUBLESHOOTING RELAYS, CAPACITORS, TRANSFORMERS, COMPRESSORS
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
If you need help designing and sizing your ductwork,
Perfect-Home-Hvac-Design.com provides Manual J
calculations and Manual D calculations duct design for your
- 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
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.
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
- Condenser entering air versus leaving air temperature is a
Critical Temperature Differentials
- Air temperature drop over the evaporator should not exceed 20
- Air temperature rise over the condenser should not exceed 30
- 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 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.