Hey folks. I have a 2200sqft home with a 30 year old bulletproof
Lennox Landmark 4ton AC system installed. It's about time to look
into replacing this thing. Our condenser is placed a very long
distance away from the coil. I am guestimating about 100 to 120 ft of
pipe roundtrip. I am curious if it can be calculated how much energy
is being wasted in such a long run due to heat gain as well as
compressor working harder and such?
I am trying to decide if I should put the condenser nearer to the
coil. Problem is, that then puts it into direct afternoon sunlight
from noon to 5pm. I've had people tell me that sunlight doesnt make
any difference to condenser operation and efficiency but I dont
believe that.
In this long run scenario, I have thought about using a 5ton condenser
replacement and keeping the 4 ton coil and blower in the attic.
Curious what thoughts others can offer.
On Fri, 27 Jun 2008 09:18:11 -0700 (PDT), kazkotx@gmail.com wrote:
>Hey folks. I have a 2200sqft home with a 30 year old bulletproof
>Lennox Landmark 4ton AC system installed. It's about time to look
>into replacing this thing. Our condenser is placed a very long
>distance away from the coil. I am guestimating about 100 to 120 ft of
>pipe roundtrip. I am curious if it can be calculated how much energy
>is being wasted in such a long run due to heat gain as well as
>compressor working harder and such?
If the suction line is insulated then there isn't enough heat gain to worry
about. And if the installer followed the recommended tubing diameter vs
distance tables then the flow loss is too low to consider.
Be aware too, that the suction line can be TOO large. R22 doesn't mix
particularly well with the oil in the system. The mist of oil that the
compressor makes is entrained in the liquid flow. It coalesces in the
evaporator and collects at the bottom. It is returned to the compressor by
friction with the gas. If the gas velocity is too low then the oil will
collect in the condenser, reducing its efficiency. Eventually the compressor
becomes oil-starved and fails.
The tube sizing tables have been developed to balance all these sometimes
conflicting requirements.
>I am trying to decide if I should put the condenser nearer to the
>coil. Problem is, that then puts it into direct afternoon sunlight
>from noon to 5pm. I've had people tell me that sunlight doesnt make
>any difference to condenser operation and efficiency but I dont
>believe that.
"People" will say all sorts of crazy things. Think about this logically.
Sunlight raises the temperature of the condenser (just like it does, say, a
solar water heater). The higher temperature does two things. One, it
increases the delta-P across the compressor, making it consume more energy.
Two, the liquid leaving the condenser is hotter. When it is sprayed into the
evaporator, it must first cool itself before cooling the air. That is does by
flashing to gas until its temperature is reduced to prevailing conditions in
the evaporator. This flashed gas is lost to cooling your house.
Bottom line, keep the condensing unit away from sunlight and out of the
shrubbery and out from under porches or anywhere else heat is produced or
trapped.
A significant improvement in efficiency can be made by installing a
de-superheater. This can be nothing more than the liquid line running inside
a larger suction line. The cold vapor returning to the condenser absorbs the
heat from the hot liquid, bringing its temperature down to about that in the
evaporator. IOW, it removes the superheat from the liquid. The liquid no
longer has to flash down to the evaporator's temperature and thus that
refrigeration (and energy) isn't wasted.
>In this long run scenario, I have thought about using a 5ton condenser
>replacement and keeping the 4 ton coil and blower in the attic.
Only if you like to waste energy. Part of improving the EER of an AC unit is
carefully matching all the components. The evaporator is sized to use all the
refrigeration available without causing excessive pressure drop in either the
refrigeration or the duct system.
Aside from energy wastage, another problem with too small an evaporator is
freeze-up. The compressor can move enough refrigerant to bring the evaporator
down to below freezing. What that happens, ice forms which insulates the
evaporator while blocking air flow. This is a positive feedback loop and
ultimately the evaporator becomes a solid block of ice. In the worst cases,
the ice exerts enough force on the silver soldered "U" tubes on the end of the
evaporator to jack them out, breaking the solder joint and creating lots and
lots of leaks.
I've fixed a number of evaporators in which that has happened. If I were
charging the industry labor rate (I don't, as I do this part time for
enjoyment), it would be cheaper to replace the evaporator. "Repair" consists
of extracting the evaporator from the unit, standing it on end and using
several burners, heating each U-joint enough to silver solder it back into
place. A major job.
Icing can be controlled using a "back pressure regulator valve" but that
results in the compressor running underloaded with a relatively low flow of
refrigerant at a very low pressure. AC compressors rely on the refrigerant
flow for cooling so unless additional steps are taken such as suction line
liquid injection (energy wasteful), the compressor will fail early.
I like to slightly OVERSIZE the evaporator (or depending on how you look at
it, undersize the condensing unit). I can then turn the fan speed down to
restore proper conditions in the evaporator. The result is a LOT less air
flow noise and a little less energy use.
A complete new unit could pay for itself in as little as a year or two. New
ones are that much more efficient. Some high EER units draw as little as a
third of the power that a 20 or 30 year old unit of the same capacity did.
Get rid of that dinosaur and enjoy your "raise". :-)
John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
What do you call 10 blondes standing in a row? Air line.
On Fri, 27 Jun 2008 13:31:18 -0700 (PDT), kazkotx@gmail.com wrote:
>What would you say to installing a ten year old "second hand" 15 SEER
>5 ton unit with dual stage pump, 5 ton coil, but leaving the current 4
>ton blower. In this scenario, how would one go about allowing for
>dual stage control when the blower is only a single speed?
I'd have to see the details of the unit and the installation to give any
advice. Did the original installation have a dual speed blower? It's easy to
convert a single speed blower to dual speed - just install a dual speed motor.
Do you know what the original blower looks like? Are the dimensions similar?
If so, it might not matter about the blower since capacity tends to scale with
size.
In any event, you'd need to do some careful data gathering after installation.
This is always true whenever you deviate from a factory-designed setup. In
particular, I'd measure the evaporator suction line temperature and the air
temperature. This is best done on a cool evening after a hot day where the
house is heat soaked and presents a good load but ambient is cool which lets
the condensing unit be most efficient.
If either temperature drops below about 35 deg then you'll have to do
something to protect against freeze up. A suction line pressure switch made
for the purpose will do the job, as will a clip-on Klixon thermostat, also
designed for the job. Also, depending on the control system, the dual speed
setpoint might can be adjusted to reduce the capacity before freezing occurs.
Like I said, I'd have to look at the specific unit to know for sure.
John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
There is much pleasure in useless knowledge. —Bertrand Russell
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>Lennox Landmark 4ton AC system installed. It's about time to look
>into replacing this thing. Our condenser is placed a very long
>distance away from the coil. I am guestimating about 100 to 120 ft of
>pipe roundtrip. I am curious if it can be calculated how much energy
>is being wasted in such a long run due to heat gain as well as
>compressor working harder and such?