Condensing Units

The condensing unit is a simple yet technical piece of equipment. It sits out in the back
(or side) of your house and kicks on and off almost by itself. At least it seems that way to
most people. In this box made of sheet metal, is the heart of your cooling system. Or for those
with a heat pump, it is the heart of your heating and cooling. The condensing unit houses the
compressor or the heart of your system. It is the pump that is moving heat to the outside and
bringing the refrigerant (which absorbs the heat) to the inside of your home. Vice versa for a
heat pump in the winter. For those of us inside the heating and
cooling business who work on these units, we understand them to be the transferers of heat.
This transferer of heat, the compressor, is hermetically sealed and non-servicable. There is not
much you can do with a burned up compressor except replace it with a new one. However, there is
much you can do to maintain the equipment to give it a longer than average life and keep it
running as smoothly and efficiently as the day it was new. Other components inside the
condensing unit include the coils, the outdoor fan motor, and several controls.

Checking the Condenser Fan Motor

In late winter or early spring it is a good practice to check the condenser fan motor to make sure
it turns. Unplug or turn off the condensing unit at the disconnect and then set the thermostat to cool.
Go back outside and restore power to the condensing unit. Watch the condenser fan motor to make sure
it turns. The fan should be blowing plenty of air up. If the motor fails to start it is recommended
that it be replaced. There is probably a bearing going bad in the motor. Whatever the reason
there is no need to take chances having this motor fail and cause problems or damage the
compressor. If the condenser fan motor fails on a hot day, the unit stops cooling and the pressures
in the condenser rises until a high pressure switch (not all units are equipped with high pressure
switches) trips or the compressor overload shuts the compressor down. There is a possibility, with a failed
condenser fan motor, that the compressor fails for good never to run again. A condenser fan motor is
a lot cheaper than a compressor. Make sure the condenser fan motor is turning or running before
the hot weather arrives.

The question you are probably asking now is "What can I do to keep it running smoothly and
efficiently?" First you can keep the coils clean of grass, dirt, and mud. Over time, these things
build up inside the coils and block the coils. This accumulation of debris reduces the designed
surface area of the coils. Reducing that surface area causes the compressor to work harder
because there is less heat being exchanged from the coils to the atmosphere. The less heat
being displaced to the outside air the more heat that stays inside the unit and the refrigerant.
This causes the pressures to rise inside the unit. For the typical AC or heat pump the pressures
should not exceed 300 psig on the hottest day of the year. (Recent HFC refrigerants out on the
market operate at higher design pressures.) If your unit has dirty coils and the thermometer
outside is above 70 degrees then your unit is most likely running at a higher pressure than it
is designed to run at. The solution is to clean the coils.

Before you drag the water hose over to the unit and start spraying, you'll want to secure the
unit. This begins at the thermostat and ends at the disconnect box located at the unit. Turn the
thermostat to the off position and pull the plug inside the disconnect box. Some disconnects have
a switch like a circuit breaker located inside them. Turn the power off.

After the power is off and the unit is secure, break out the water hose with a good nozzle
that will allow you to spray water at a high pressure. A little soap will help clean the dirt
and other debris off the coils also. Apply the soap and let it soak for a few minutes. Then
spray the coils. Be careful not to use too much pressure as you may bend some of the fins
that surround the coils. For best results, it will help if you spray the water from the inside
of the coils out. This may require you to take the top of the unit off. If you are not
mechanically inclined do not attempt this procedure. Simply spray all the dirt and debris off
the coils as you can possibly spray off.

Delay Timers and the Compressor

Air Conditioners are big refrigerators. Air Conditioners
use the refrigeration cycle to transfer heat from a place where it is
not
wanted to a place that makes no difference. The heart of any air conditioning
or refrigeration system is the compressor which pumps the refrigerant
through the system. The following diagram will help explain the refrigeration
cycle. When the refrigerant leaves the compressor it is compressed
and therefore a higher pressure than it was before it entered the compressor.
This pressure remains constant (give or take under certain conditions)
until it passes through the metering device where the pressure and
temperature are reduced. This low pressure remains constant (again
give or take under certain conditions) until it again reaches the compressor
and is compressed again. When you turn the air conditioner off these
pressures equalize within five minutes. The equalized pressure is often
referred to as static pressure to HVAC professionals. The high side
pressure on a hot day can be as high as 300 psi in some systems. If your
system is not equipped with a delay timer and you turn the system on
within five minutes of turning it off, the compressor must start against
that higher pressure. That is not good for the compressor and could
possibly damage it. The air conditioner should not be started unless
the system is at the static pressure. I have responded to calls where
the
unit was not equipped with a delay timer and the system was inadvertently
started before the pressures were allowed to become static. Often the

compressor is locked up, the capacitor is shot, a fuse is blown, the
compressor is damaged mechanically or electrically beyond repair, or
a
combination of any of these things mentioned.

Compressor
Mechanics

If the homeowner is lucky, the only thing wrong is the
capacitor or fuse. The capacitor for your compressor can be replaced
for approximately $10.00 to $30.00 (depending on the microfarad rating)
plus the service call. If the compressor is locked up there is a chance
that it can be unlocked (note the word chance). A hard start kit and
a good rap with a hammer can sometimes do the trick. If this doesn't
unlock the compressor then you will have to replace the compressor or
the unit. It is often in the homeowners best interest to replace the
condensing unit. Before this happens to you, call the company you often
deal with to maintain and repair your HVAC system(s). Ask to speak with
the service manager or technician that has worked on your equipment.
Ask them if your unit is equipped with a delay timer. They should be
more than happy to answer your question, and if they are
not sure check to see if the unit is equipped with a delay. You
may want to have the model number, serial number (for both the condenser
and air handler), and type of thermostat you have. Most modern digital
and programmable thermostats have delay timers built into them.