Restrictive Flow Tube style Central Air Conditioner


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Old 07-29-15, 02:52 AM
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Restrictive Flow Tube style Central Air Conditioner

Hello, my central ac unit utilizes the restrictive flow tube instead of a TXV valve. The unit works fine when outside temperatures are in the 70's. When outside temp. gets to 81-82 degrees and the AC unit running, The inside temp will begin to rise with the outside temp. And as the outside temp grows higher lets say to 90 degrees for example, the inside temp (all the while the AC unit is running non-stop), the inside temp will be 84-86 degrees.

During that time, an examination of the A-frame evaporation coil shows only along the bottom of both sides of the A-frame is frosting (not frozen with ice) over. The rest of the coil above that does not feel cold. Maintenance tells me the refrigerant is filled correctly and the air at the vents blowing into the room is 66 degrees.

I'm trying to learn more about symptoms and causes so if anyone knows why the unit isn't blowing cold enough air to keep up, I'd be grateful.

Items that have been replaced in the AC unit:
1.Capacitor
2.Line filter/dryer
3.compressor
4.freon

Thanks :-)
 
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Old 07-29-15, 04:05 AM
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A simple way to check your overall A/C operation is to calculate Delta T. Delta T is the difference in temperature between the cold air coming out of a duct (pick one close to the air handler/furnace) and the return air (return air duct). Ideally, the Delta T should be ~20 degrees.

As for the system not being able to cope with high outside temperatures, it's possible that your A/C system is undersized for the size of your house and conditions (how many windows, amount of insulation, etc.). How big is your A/C unit (in tons), and how large is your house (square feet)?
 
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Old 07-31-15, 05:59 AM
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Bob14525

Bob14525, Thank you for responding. The Delta-T is around 5 degrees difference after running all day. I've checked into other things like air leaks, windows etc that could contribute to my AC as well as my heaters efficiency in the different seasons and all is fine, not great but fine. I'm calling maintenance again today, but I have little faith in their experience levels.
 
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Old 07-31-15, 06:03 AM
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A Delta T of only 5 degrees is way too low. Something is definitely wrong. My guess (and it's purely that) is that you're low on refrigerant.
 
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Old 07-31-15, 09:27 AM
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An uneven temperature over the evap coil usually points to low/high charge. The inlet and outlet temp of the evap should be pretty close to the same. If the inlet is widely warmer than the outlet then you probably have an overcharge. Conversely if the outlet is widely warmer than the inlet then you probably have an undercharge.

This is all assuming the rest of the system is in good order, because there is a possibility that you have some plugged capillary tubes which would also cause an uneven temp spread over the evap coil
 
 

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