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AC low air flow inside

Posted by planewood (My Page) on
Sun, Jul 31, 11 at 14:47

Location Katy TX (it is HOT)
My son has a Rheem F22 unit that is 10 years old. Probably about a 4 ton unit. The house is 10 years old (~1800 sqft) and is well insulated with double pane windows throughout.

He's getting low air flow inside the house and it is taking a LONG time to cool the house down. The low air flow was first noticed about a month ago. I've looked into the blower unit and the fan is running full speed and the fan blades have minimal dust coating.

We have taken the air filters in the ceiling out and removed 6 of the ceiling registers with still no appreciable increase in air flow. Air filters were fairly clean and have recently been replaced. We have examined the attic ducts and found no loose connections and no signs of air loss there.

I have measured several system temperatures and the one that sticks out is the outside suction line temp is showing 48-50 degrees. That seems way low to me. The evaporator coils are not frozen over. The suction line is sweating good just up to the case of the compressor. The compressor case is not sweating. I'm guessing that something is blocking the air flow over the evaporator coils. The only problem is that when the unit is opened we can only see the downstream side of the coils. These are the rectangular type coils (not the "A" frame type).

I have no way to measure the air pressures on either side of the evaporator coils.

Any suggestions? Am I on the right track? (mike@mikeinkaty.com)


Follow-Up Postings:

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RE: AC low air flow inside

Mike..I know I'm asking a lot of questions and they are not intended to be antagonistic but, the answers will help myself and other Pro's here help you narrow things down.
Are the return registers clear, no furniture or curtains blocking the supplies? The temperatures and condensate descriptions you give indicate the charge is most likely ok. No way of knowing precisely without actually checking the pressures. A big help if you can do it...what is the temperature of the actual line(smaller line) going into the evaporator coil and the temperature of the suction line (larger line) coming from the evaporator coil? It would be best to use 2 dial thermometers taped to the lines and left there for 10 minutes or so. At the same time, the temperature of the air going through the evaporator is necessary. Knowing those numbers can tell a Pro whether the unit is close to full charge or not.

How about the air temperature differential between the air going into the evaporator coil and the air coming out headed back into the house? Knowing that would be a big help.

When you say it takes a long time to cool down the house, what temperature is he cooling it from and to? What is the temperature outside when the unit is turned on? What area do you live in? Is there a LOT of condensate draining from the system?

Are all the registers opened all the way? If so, this can be a problem too. You really need an air flow meter to accurately determine the fan is moving enough air.You would use that to do an air flow balance where each register is supplying a minimum of 500fps. Then you would go back and open and close registers to assure all registers are supplying the same output. You also need a manometer to determine whether the static pressure is to manufacturer specs.

As for the fan motor, if you measure the amperage, you can get an idea if the fan is pulling too many amps. Generally that means the run capacitor could be bad, the bearings are dry and adding resistance. If the unit uses a belt to drive the fan, the bearing on the motor or fan pulley could be dragging. If the fan (or pulley)is not making noise, oiling them would be a good idea and then again, once every 2-3 years.

Here's something I have run into on a few rare occasions...With the larger units, a mat is laid inside the main duct to help minimize air and vibration noise. I have had occasions where a piece of this has lifted up and partially blocked the main run.


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RE: AC low air flow inside

The condensate draining from the system is all enclosed and I cannot tell how much is draining. But in this humidity I imagine it's like a water faucet!

The temperature of the liquid line as it comes out of the house at the back of the outside unit is warm to the touch; not hot and not cold. Probably in the 110 degree range during the hottest part of the day (102 degrees). As I said earlier the suction line is showing 48-50 degrees at that same location. (as a comparrison, my two twenty year old Carrier units usually run about 65 degrees at the outside suction line)

Measuring the air temps at the ceiling registers shows about a 12 or 13 degree difference between the supply registers and the intake registers. As I said earlier we removed half the register grills and all the filters during testing. All the supply register gates were fully open.

Still haven't figured out the reason for the low air flow? I'm going over tonight to clean the fan blades on the blower.


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RE: AC low air flow inside

In a previous home I owned I had similar problems. The coils and blower were caked with animal hair from previous owner not using filters.


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