Does anyone have a good explanation on why some units with undersized returns give trouble often and fail prematurely while others seem to run trouble free for years? I just replaced a Goodman package unit that was 12 years old and still running untill lightening zapped it. It had a 14'' flex return about 30 feet long and homeowner said it never gave trouble and cooled great.?
I would think some are more undersized than others,
and also some may also have other problems besides
undersized returns, such as poor duct volume.
I agree ed but am amazed that three ton units can run trouble free for 12 years with a 14 inch flex return 30' long. Not to mention a 12'' supply takeoff.
People often forget about a failure years down the road.
And as another poster has stated, there are often many other factors causing the equipment to fail prematurely.
IE, no maintenence, dirty filters, to restrictive of filters for system design, running with no filters, enviroment surrounding the equipment, seasonal temps and weather conditions, stat settings, how long the equipment runs per cycle, etc., etc...
I don't have the specs for a Goodman,but I can tell you thaat many package units design for mobiles(higher fan pressure),work fine with 14" for 3 tons.
Check The Blower
A lot of the difference is in the strength of the blower.
I have seen some systems run a total external static pressure of .8" W.C. & still provide proper airflow due to a strong blower.
Then you have the other situation,I have seen some manufacturers who put weak fans in their equipment.
By the time you add a coil & filter to it there is no available static left for the duct system.
Next time you have a chance take a look at the fan performance data for the equipment you are working on.
This along with proper static pressure readings will give you the answers to a lot of your questions.
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Thats not undersized duct, depending on the unit, as stated earlier, units made for trilers have high static pressure blowers.
Check coleman, miller, they use small ducts also for 3 tons.