What is "Super heat"?
You will hear and see this term all the time in reference to refrigeration. Simply put
it is the difference between the temperature of a vapor line in relation to the
temperature scale on a pressure gauge for a particular refrigerant or how much liquid
is feeding the evaporator in relation to how fast it is being boiled off. for example
(R22) if the suction gauge reads 70 psig then the evaporating temperature is 41
degrees but if the tubing is 51 degrees then you have 10 degrees of Superheat. A
typical range for residential air conditioning is 8-18 degrees with some error based on
extreme conditions. Once you understand Superheat you can diagnose obvious
problems. For example a system that is under charged or has a stuck (closed) metering
device will have high super heat (over 20 degrees) at the compressor and a system that
is grossly overcharged or has a dirty indoor coil will have very low Superheat about 3-
7 degrees with low suction pressure and the suction line will be very cold. It is ok and
quite normal for the Superheat to change dynamically while the system is running,
you will have to interpret what you are seeing.
What is "Sub cooling"?
Sub-cooling is similar to Superheat but happens in the condensing portion.
Refrigerant when condensing will happen at a particular temperature which is very
close to the temperature scale corresponding to head pressure for a given refrigerant.
After the refrigerant is condensed it will try to assume ambient temperature but will
never reach it. The difference between liquid line temperature and condensing
saturation temperature is Sub-cooling and is a very good indication of "refrigerant
level", but only when proper Superheat is indicated or you could have a misleading
indication. Typically 20 degrees of Sub-cooling is desirable and the closer the liquid
line temperature is to ambient the better (indicating an efficient system). Checking
Subcooling in the heat mode of a heat pump has to be done carefully because you
have influence of the space between the indoor coil and the point of measurement. For
best heating you will want most of the refrigerant to be condensing in indoor coil
without backing it up with refrigerant.
As a general rule Subcooling = Refrigerant charge quantity, Superheat = Refrigerant
cycle performance. Check both!!!
The following conditions are for R22 in the cooling mode and relate to indoor coil
problems which contrary to human nature we find to be 80% of the reason for poor
cooling.
Human nature says that the Thermostat is "where is all comes from" and if you can
just get a new enough outdoor unit with the right brand name everything will be ok,
B.S!
I will have to make a chart for outdoor coil problems. The goal for outside is to get a
full liquid line and the head pressure as low as possible. On a newer unit 190-225 PSI
of head is normal, on some older units (25+ years) 300 PSI of head on a 90 degree day
is not out of the question and be normal. Anything higher wash the coil and consider
the unit may be overcharged.
*Warning: There are situations where due to the conditions of you system you may
have lower than normal suction pressure and/or higher than normal head pressure.
Usually lower than normal suction pressure due to low airflow (either intentional or
due to restrictions) and any attempts to raise the suction pressure by overcharging will
cause damage by flooding the compressor. The trick is to determine if this pressure is
normal for this system, which is usually caused by airflow problems. For example
some systems 50-55 PSI (30-35 degree evaporating temperature) is normal. The secret
to being a good mechanic it to determine if this is a normal pressure and leave it
alone. See Superheat above.
Suction pressure~65-90
Head pressure~190-250 on Full liquid line, sight
Normal psi
newer units, 300 not out of the glass will be clear,
Superheat 5-15 degrees superheat.
question on an older unit on a compressor will draw
Normal depending on size of
90+ degree day. * Pressure will near rated current
suction indoor coil, air flow and
be a function of outdoor (NEVER charge by
pressure condition.* Pressure will
temperature. current draw).
track indoor conditions.
0-5 degrees superheat suction
pressure 35-40 psi Suction
Low super heat Coil will ice up quickly.
Dirty indoor coil, no line will freeze out to to the
low suction One of the most common
or poor air flow. compressor. Compressor will
pressure problems!
be ruined because of oil
dilution.
Low super heat Over feeding If overfeeding head 65-80 psi 5-10 degrees
pressure will be slightly
higher than
refrigerant or lower than normal if
normal suction superheat.
overcharged overcharged head pressure
pressure
can be sky high.
Refrigerant not Could be slightly
High superheat
feeding properly. undercharged or a problem
lower than 40-60 psi suction. 20-30
Compressor is being with the metering device. If
normal suction degrees superheat.
destroyed by overcharged head pressure
pressure.
overheating. can be sky high.
High superheat Could be almost out of 20-40 psi
very low Refrigerant not refrigerant. compressor Compressor will get very hot
suction feeding at all will overheat and be and shut off on internal
pressure. destroyed. protection.
Compressor will
Low super heat, Compressor is not pumping
Replace compressor. be flooded.
high suction properly. Or possibly metering
Check for open 80-100 psi.
pressure. Lower device is missing or stuck open.
metering device or Pressures will
than normal head A heat pump check valve (indoor
blown check valve. equalize almost
pressure. coil) is blown.
instantly
I Work very long days and sometimes I end up working on this stuff at 3 or 4 am
after having been up since 10 am. So some of the information is incomplete and there
are some errors here which I will try to correct when I get a moment to sit down and
go through this. So I will continue this page when I have more time but for now please
follow the links below. If there are any volunteers out there who would like to help
me write this please knock your self out. In a nut shell don't over charge your system.
Final words: As a general rule Subcooling = Refrigerant charge quantity, Superheat =
Refrigerant cycle performance. Check both!!!
