A/C TUNE-UPS IN COOL AND COLD WEATHER
When is it safe to conduct a Performance Tune-Up?
Contrary to most people’s belief systems…ANYTIME!
Modern air conditioning systems can have annual maintenance performed on them at any time.
When the weather is cool or even cold, there are a couple of precautions that need to be
addressed.
# 1.
The power to the condensing unit should be "ON".
That will insure that the compressor crankcase heater is energized and the compressor oil
is warm enough to operate the system.
It also insures that there is no liquid refrigerant in the compressor itself.
If the power has been on for at least 12 hours prior to arrival, the system can be operated
safely.
Most homeowners do not turn the power off to the outdoor unit, but, if the power is off, it
should be turned on for a minimum of 30 minutes per pound of refrigerant prior to
operating the air conditioner.
# 2.
If the system needs to be charged with refrigerant, the cold weather charging method
(Superheat) must be used.
Pre - Season Cool Weather Charging
Here are some basic procedures, which will provide a higher level of accuracy while charging
fixed metering device systems by superheat during pre-season tune-ups.
Most manufactures offer superheat charts that go down to 55° outdoor temperature and 50°
indoor wet-bulb temperature.
These charts are designed for charging fixed metering device systems and they work extremely
well.
But there are a few procedures that will help the accuracy of using these charts during the
pre-season cool weather charging.
Superheat charts that do not use wet bulb temperature
are NOT recommended.
Copyright EPL Residential Inc.
1.
The higher the indoor wet bulb temperature, the more accurate the charge. Ideally the
indoor load should be high enough so the required superheat is above 10 °.
Suggestion:
Turn on the heat while doing the tune-up functions not related to the furnace. Try
to get the wet-bulb to the upper 50's or low 60's, If this is not possible, use the
highest wet-bulb temperature attainable.
2. You should see the evaporator coil temperature drop below freezing when the outdoor
temperature falls into the 60's. The actual outdoor temperature this happens at varies
between equipment.
Higher efficiency systems using a larger evaporator coil typically have higher suction
pressures.
Although the evaporator coil will drop below freezing at some point, using the superheat
charging method at these temperatures and pressures is still accurate.
You won't normally be running the air conditioner long enough for a freeze-up to occur.
4.
You should expect to see a drop in the indoor wet-bulb temperature as the air
conditioner is running. Although re-checking the wet-bulb is not usually needed, you’ll
see a few degrees drop in the superheat from the initial readings.
It is acceptable to charge the unit before washing the condenser coil on a cool day, but
only if the condenser is not real dirty. (The condenser coil is sized and is designed to
operate on hot days consequently the entire surface area is not needed on a cool day.
A little dirt won't affect the head pressure like it does on warmer days.)
By doing this, you eliminate the drying time for the condenser and shorten the overall
time needed for charging, which will maintain the indoor wet-bulb temperature during
charging.
4. If the outdoor temperature is lower than the chart goes, make an interpolation to
determine the required superheat. On most charts, each 5° drop in outdoor temperature
increases the required superheat 2°.
By using this method, outdoor temperatures in the 40's
can be interpolated easily.
Copyright EPL Residential Inc.
Cold Weather Charging By Superheat
The following is a method for charging fixed metering device air conditioners and heat pumps in
cold weather.
This method requires the use of a superheat chart or calculator that uses outdoor temperature and
indoor wet-bulb temperature measuring the indoor heat load.
Step 1:
Measure the indoor wet bulb temperature at the beginning of the maintenance call.
A.
If the indoor wet bulb temperature is 60 degrees or more this will provide ideal indoor
conditions.
B.
If the indoor wet bulb is between 55 - 60 degrees. This will provide adequate indoor
conditions, warming up the house would be helpful, but is not necessary.
C.
If the indoor wet bulb temperature is between 50-55 degrees, it is recommended that the
house be warmed up as much as possible during stages of the tune up process that permits
the heat to be on.
D.
If the indoor wet bulb temperature is less than 50 degrees, the house must be warmed up
first.
Step 2:
Determine the outdoor temperature you want simulated.
A.
Using the manufacturers superheat chart, line up the wet bulb temperature measured in
step 1.
B.
Select an outdoor temperature that will yield a target superheat of 10-15 degrees (avoid
simulating outdoor temperatures that drive the superheat below 10 degrees)
Step 3:
Determine the head pressure level that correlates with step # 2
A.
Add the simulated out door temperature and the Condenser T.D. (the SEER of the system
being charged) to determine the desired saturation.
B.
This will be the saturated Condensing Temperature found on the inner scale of your high
side gauge.
C.
The high side gauge can be used to convert the Condensing Temperature into the head
that you are trying to obtain.
Step 4:
Cause the condenser to obtain the desired head pressure
A.
Cover the top of the condenser with a board or piece of metal that is weighted down.
B.
As the head pressure raises, you will need to adjust the board to cause the head pressure
to stay at the desired pressure level.
C.
It is critical that the head pressure stays at the desired level and only fluctuates a
minimum amount.
Copyright EPL Residential Inc.