An example of a
poor
performing larger residential
air-conditioning system in a new four year old home -
AIR-CONDITIONER
RUNNING TOO MUCH
3.5-Ton system
delivering less than 1.5-Ton of capacity
Technician: I get this
call from a Home Owner selling the home, that the home inspector says
the
delta-T (temperature drop or split) across the indoor evaporator coil
is not in the 15-18
degree
range.
So I get there and it is a 1900 sq. ft. home with a 3.5-ton 10-SEER Goodman. House is
only 4
years old.
Here are the Tech's readings:
Ambient Temperature = 78-F
Suction Pressure = 70- PSIG
R-22
Refrigerant = 41-F Suction Saturation Temperature (SST)
Or, 20-F
Superheat (High)
Suction Temperature = 61-F Superheat -F
Head Pressure = 225-PSIG or, 109-F minus the104-F
Liquid Line
That
Is only 5–F
Subcooling (Low) - (Darrell)
Liquid Line temperature= 104-F
“If
Superheat is high
and Sub-Cooling is low:
Charge must be adjusted. "System is undercharged."
Entering
Dry Bulb= 74-F
Entering Wet Bulb= 64.5-F
Leaving DB= 66-F
Leaving WB= 61-F
Tech: I am at a loss for
the 8 degree (register temperature drop (Delta-T).
"The
Home Owner has never said house does
not
cool well --
just runs quite a bit when above 88 degrees."
Coils,
ductwork, blower, etc.,
checks
OK.
Tech: If
I plug these numbers in my psychometric calculator (which by the way
works very
well) I get these numbers:
17203 "total"
BTU/hr (heat removal)
13306
BTU/hr Sensible
BTU/hr (heat
removal)
3717 BTU/hr latent
BTU/hr (heat removal)
Tech: What is up?
================
Of course, you can
get the delta-T in the lower end of the range by
reducing
airflow; which he did just so it would pass the home sale inspection.
"A lot more testing data was needed for a thorough diagnosis."
Here is only the Sub Cooling and Super Heat diagnosis:
It indicated 20 degrees Superheat with ambient temperatures between 75
and 85
should be 12 to 15 degrees, and 5-F Sub-Cooling that should be 12 to 15
degrees .
Possible
Diagnosis
using Superheat and Sub-Cooling:
If
Superheat is high
and Sub-Cooling is low:
Charge must be adjusted. "System is undercharged."
If superheat is low and sub-cooling is high:
Charge must be adjusted. "System is overcharged."
If Superheat is high and Sub-Cooling is high:
"Could have blockage in coil, orifice, or line set."
If Superheat is low and Sub-Cooling is low:
"Piston orifice could be too big, there is no orifice in the unit or
the
orifice is stuck, or in backwards, and refrigerant is bypassing It."
If
I were buying the home, I would ask for a full disclosure on all of
the
energy usage appliances, water heater, and especially the HVAC system.
A full,
truthful, accurate report on the HVAC system including the ductwork
should be
made available to potential buyers.
=========================
Gurgling Pulsating Sounds at TXV:Low
evaporator heat-loads lead to reduced liquid line mass and increased
evaporator mass could be due to airflow problems. Eliminate low
evaporator heat-loads before looking into adjusting the refrigerant
charge. Gurgling and pulsation noises at the expansion device can
be
caused by low evaporator circuit heat-loads, low charge, and/or
non-condensibles and moisture in the system. Unbalanced airflow through
the various distributor circuits of the evaporator coil will cause the
TEV to close down refrigerant flow starving the coil.
Piston-flow-rators will make it impossible to properly charge the
system and cooling will be greatly compromised unless you eliminate the
cause! "Put your ear on the liquid line at the evaporator coil."
On
every Rheem
condenser cover it lists "non-condensibles and or
moisture" as causes for a gurgling or pulsating noise at the expansion
device. The entire evaporator circuits, may not become active for
various reasons, - "the entire coil must become fully active for
efficient performance."
The
purpose of
these recommendations is to provide liquid refrigerant at the expansion
device and provide efficient operation. Hopefully, this will aid your
research. If I can be of additional assistance, contact me.
-----------------------------------------------------------
I
estimate over
90% do not properly
purge & evacuate contaminated central air conditioning systems.
The
Triple Evacuation Method is normally done on central air
conditioning systems:
First,
remove any valve cores with a special valve core remover this
will
speed up the evacuation time. Back service valves two turns off their
back seat.
1)
Re-claim unit charge (Recover all the refrigerant)
2)
Charge
system to 150 PSIG with dry nitrogen and leak test
3) On
contaminated systems replace the filter dryers. Then Repair all
leak(s)
4)
Evacuate system to 500 microns valve off & see if it holds 500
microns for ten minutes, if it holds, break
the vacuum with
dry nitrogen
5)
Evacuate system to a deeper 300 microns, valve off vac pump, & again break the vacuum with
dry
nitrogen
6)
Evacuate system to 300 microns and charge unit (Recharge with fresh
clean refrigerant)
7) Check
to see if the Supply and Return air ducts were correctly sized &
sealed by the original installer.
The
“micron” is a metric unit of measure for
distance. The micron is a unit of linear measure; one micron equals
1/25,400ths
of an inch. Modern high capacity vacuum pumps help speed up the
evacuation process.
===============
For many
good reasons, Inspectors' need to get very tough on homes and
commercial buildings that are being sold.
http://www.udarrell.com/ac-trouble-shooting-superheat-subcooling.html
http://www.udarrell.com/ac-trouble-shooting-chart.html
__________________
udarrell
The easy
check is to add
refrigerant until the suction superheat
comes down, then recheck the temperatures and pressures and airflow
through the evaporator coil, which should
make for an easy diagnosis.
The pressures will indicate whether the compressor is okay, --it is
possible that the system is imbalanced possibly due to, low evaporator
heat loading, charge level, air in the refrigerant system, and/or wrong
orifice size. (An oil-clogged system would be another factor to
consider
in inadequate evaporator loading providing similar conditions are found
on a system.)
c.) Higher than expected heat removal by the condenser is an evaporator
too loaded up -- not a factor in this troubleshooting illustration. I'm
including it to complete the possibilities for AC systems in general.
It's a good idea to check if the piston/orifice is correctly sized
according to the manufacturer's literature. Charts for orifice size are
only useful when you have the outdoor and indoor unit model numbers,
--leaving supply duct problems or an overcharged system for these
temp/pressure test results. The piston must be installed so that it
seats and seals when te liquid flows against it otherwise the
refrigerant will bypass the orifice hole!.
It is possible to find three or more things wrong on one system, which
is what separates the real Techs from the dudes.
Normal Condenser Split for 10 SEER 3.5-ton A/C condenser can be, depending on
its CFM, as high as 29ºF, 12-SEER would usually be between 20 to
24ºF -- usually
ends up about the manufacturer's design Condenser Delta T (Saturated
Condenser Temp minus outdoor ambient).
Checking the condenser discharge air temperature split is an effective
performance test measure that should be part of any maintenance check.
Since there are no duct variables to contend with, the condenser is
easy to inspect, thermometer calibration isn't an issue using the same
thermometer for both in and out air, wet bulb temperature doesn't
matter, and the compressor is
closely matched to the condenser.
This is merely an illustration of some useful diagnostic procedures.
========
One Research Performance Assessment revealed:
Several recurring factors that accounted for inadequate
flows:
* Return ducts and return grills were often
undersized
* Blower Fan set to wrong speed for
cooling operation
* Filters blower wheel blades and cooling coils were
dirty
* Duct system static pressures were elevated due to
circuitous runs, pinched ducts etc.
* Larger outdoor units were installed without
changing the indoor unit and duct system
* Devices had been added which increased system
static pressures.
Darrell Udelhoven
Take the condenser entering air
temp and leaving air
temp, subtract for the temp-split. As a double verification: You can
use
the
high-side (SCT) Saturated Condensing Temperature minus the
outdoors-ambient temperature; the difference gives you the condenser
temperature-rise or temperature/split. There is NO excuse for not
utilizing this important diagnostic check. Always
use an accurate volt meter and amprobe to make sure you are not
overloading the compressor's Wattage Service Factor and check the
compressor discharge line to see that it is under 225-F.
CONDENSER
TEMP-SPLITS - My Brother's Heil 12-SEER Condensing Unit
1.5-Ton - Rated at 18,400-BTUH, Condenser fan CFM 1400
(Total Cond. Watts 2221 X's power Factor 0.88 X's= 1955 X's
* 3.413 = 6,673-BTUH Motor Heat +18400= Condenser's "Rated Gross
Heat Ejection" is 25,073-BTUH / 1400= 17.9-F X's 1.08 = 19-F Temp Rise Cond/Split.
His condenser only gets a 10 to 13 temp rise split, the evaporator
appears to be under heat-loaded or, an unbalanced
heatload on
the DX coil's circuitry.
The
probable cause is "an
unbalanced airflow heatload through the
evaporator coil. "It's a (Thermo Pride OL
11 oil
furnace). Those oil
furnaces have a very large round heat exchanger that goes to near the
top
of the furnace, --due to a low basement ceiling the DX coil sets
perhaps illegally close to the heat exchanger causing a few of the
coil's circuits to be under heatloaded. Since the liquid refrigerant is
not completely evaporated it will cause the outlet line that the TEV
sensor bulb is on to be too cold and the TEV will shut-down the flow, which greatly reduces the BTUH
capacity of the DX coil and the system. On piston refrigerant
control systems, they may flood back liquid which could damage the
compressor, unless the system is way under-charged. Thermo Pride could
install airflow turning vanes just above the heat exchanger to funnel
the air directly into the DX coil, instead of most of the airflow
hitting the bottom of the DX's drain pan causing extreme turbulence back-pressure
and an imbalanced DX coil circuitry heatload!
The
chart split listed below is at Condenser Design conditions: Indoor Return
Air 80-F dry bulb 67-F Wet Bulb or 50% Relative
Humidity (RH) as you
go up to 99% RH the condenser split could increase by up to 6-F; down
as much as 4-F at a low humidity of 55-F Wet Bulb.
Do
your own figuring
based on this
formula. Motor BTU/hr additive = Watts X's PF x's 3.413 for Btu/Watts
additive added to rated BTUH, divided by condenser fan CFM X's 1.08
=
condenser Temp-Split. Get
the Motor Power
Factors (PF) of the compressor and fan motor from the manufacturers.
(A 0.80 factor could be close.) Some of the temp-split figures need
correcting, will do ASAP. Most Splits rounded off.
CONDENSER
TEMP-SPLITS - Comfortmaker® 12-SEER units - used 0.80
Motor Power Factor
1.5
T 18,400 19
Split
Cond.
CFM
1400
WATTS 2222
2-Ton
24,800
24-F Temp-S Cond. CFM 1400 WATTS
2659
2.5-T
30,200
21-F Temp-S Cond. CFM 2000
WATTS
3404
3-Ton
35,600
18-F Temp-S Cond. CFM 2800
WATTS
4117
3.5
T
42,500 21-F Temp-S Cond. CFM
2800
WATTS
4554
4-Ton
48,500
19.5-F Split Cond. CFM
3400 WATTS
4761
5-Ton
59,000
25-F Temp-S Cond. CFM 3400
WATTS
6969
Page 618,
Refrigeration
& Air-Conditioning (ARI) Second Edition, C 1987
Those
lower
SEER
units
had higher condenser splits than 12-SEER and higher units.
Sorry, I
defiled
the
graph, 90-db outdoor, 80-db indoors with 67 wet bulb or 50% RH
represents
the condenser splits shown above.
The graph should use 78-F indoor dry bulb
(DB) and show the correlative Relative Humidity (RH) to the Wet Bulb
(WB)-F readings. Equipment owners could then use an accurate Relative
Humidity indicator so they could perform the temperature split
diagnosis. Graphs would be specific to the equipment. Condenser CFM
should be shown on the metal equipment specification tag and in all
spec literature and in the home owners manual.
The condenser fan speeds are slower on several of the 10-SEER Tonnage
Models.
We
are only trying
to get a figure to go by for a comparison. When new condensers and
Evaporator-coils "are installed on older air handlers" the new,
or old,
evaporator coils are usually under heat-loaded.
The Base
Spec
sheets
12-SEER part no. 421 41 33301 03, Feb 2001. These
are the Comfortmaker® units, which are nearly identical to
Heil® units. [I used the first rating
on each tonnage class.] While the "Performance Cooling Data" is
listed at a 95-F outside ambient temperature, you can adjust the indoor
airflow to get the Nominal BTUH Rating at the customer's normal indoor
stat' temp' setting and the most outside temperature/degree operating
hours.
Take
the "listed watts" of
the compressor and Condenser fan and multiply that wattage by 3.413 to
get the BTUH heat
additive of
the
motor then add the listed BTUH of the condenser to it, and then divide
by the condenser's CFM. I'm now using the altitude multiplier additive
to the initial temp split figure, look up the additive for the height
you are above sea level.
With
a properly sized system
and proper evaporator airflow you will have consistent optimal nominal
capacity heat absorption and removal coupled with the requisite longer
run-time cycles. I believe that optimal efficiencies, with variable
latent/sensible heat
loads, could be effectively achieved through the use of computerized
control system components.
-------------
Getting
it right makes all the difference in the world.
In
the Prairie du Chien Crawford county area Contact:
RICKLEFF PLUMBING HEATING & AIR CONDITIONING
PRAIRIE DU CHIEN, WI http://www.heildlr.com/rickleff/
Darrell's
Refrigeration Heating and
Air Conditioning -
Federal
Refrigerant Licensed - Retired - Licensed Contractor
If
you want to talk, I'm in the
Lancaster, WI - Phone book - Beetown exchange.
To
Contact me: udarrell@pcii.net
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