Author Topic: airflow problems  (Read 3150 times)

Offline vinnyg

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airflow problems
« on: July 23, 2007, 02:57:30 PM »
I hope someone can help me. I am working on a 2 year old Ruud gas furnace with a 2 year old cased coil and a 2 year old condensing unit. My problem is that the unit suddenly does not cool in the heat of the day. I have a 30 degree split at the unit, and I pick up between 10-15 degrees at the supply vents. During the heat of the day the house wont cool below 84 degrees. Using a anemometer and calculating, the unit is moving approximately 1300 cfm. My pressures are 64/200 psig with a TXV. According to the unit charging chart, the charge is right on. The ductwork is metal with thin insulation. I can understand if the ductwork is heating up from the thin insulation in a hot attic, but I dont understand why I have a 30 degree split at the unit.  Any advic anyone can give me would be appreciated.

Offline TECH X

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Re: airflow problems
« Reply #1 on: July 23, 2007, 05:19:20 PM »
Is it a 3 ton unit?  How many square feet is the home?  Has it worked without problems in the years past, or has it never been able to satisfy the thermostat?  What is your liquid line temperature?
A 30F temperature drop at the furnace seems too high, and unlikely.  I'm assuming your ambient is at least 84F, in which case those pressures seem low.  Your in Fort Lauderdale, Florida?  It's probably even hotter then that.  I would expect a discharge pressure of around 235PSIG.  A suction pressure above 75PSIG. You should calculate your subcooling.  At your current discharge pressure your liquid line would have to be 90F to have 10F subcooling.  The liquid line temperature is probably higher then that, which means low subcooling, indicating a low charge.  What's the actual compressor RLA?  Your missing a lot of variables.

The easiest way to solve this problem is to weigh out the charge to see if the unit is undercharged.  If not look at the TXV as being the problem, possibly a damaged bulb.

Offline HVACJOE

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Re: airflow problems
« Reply #2 on: February 24, 2008, 10:45:00 AM »
I'd definatly put my money on undercharged, as tech said, your backpressure is to low with an 80 plus degree return air, and that head pressure is a little low as well for the ambient temp.