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Old 09-01-2009, 07:12 PM   #1
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a//c pro needed

Have just completed replacement of my A/C roof gaskets and thought I'd run them while the covers were off.

This is a 92 model but does anyone know why the cold lines frost up and why they are not wrapped with an insulation like most central air house units are.

Seems to me this could waste a lot of cold power on very hot days.

would anyone recommend trying small diameter foam pipe wrap.
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Old 09-01-2009, 10:18 PM   #2
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It's possibly low on freon. That's what happens when a car A/C unit starts freezing the lines before the evaporator. After the evaporator is usually a non issue.

No idea about the insulation, it costs money and if the unit met spec who cares according to the bean counters. Go to a plumbing supply store and get regular pipe insulation, cut to length and secure with zip ties.
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Old 09-02-2009, 03:51 AM   #3
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Quote:
Originally Posted by gordon23 View Post
Have just completed replacement of my A/C roof gaskets and thought I'd run them while the covers were off.

This is a 92 model but does anyone know why the cold lines frost up and why they are not wrapped with an insulation like most central air house units are.

Seems to me this could waste a lot of cold power on very hot days.

would anyone recommend trying small diameter foam pipe wrap.
Did you verify that the evaporator coil was clean when you had the unit off of the roof? The unit may need the shroud in place for proper air flow through the condenser coil. Measure the temperature of the air going into the filter area and the air temperature coming out of a nearby vent after about 10 minutes running. What results do you have? They should be 20 to 24 degrees difference.
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Old 09-02-2009, 07:07 AM   #4
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A frosted compressor suction like can be caused either by low refrigerant or restricted air flow. Restricted air flow can be a dirty filter, a dirty coil or bypassing air from the inlet to the supply side of the unit.

It is cheaper and easier to check the air flow and rule that out. If it is low on refrigerant, that will mean adding valves to the sealed u nit and this is not cheap. Personally, I would put in a newer more energy efficient unit before i started spending a bunch of money trying to make the old unit work by adding refrigerant.

I feel a 24 dF temperature difference across the evaporator is a bit to cold. I shoot fro 18 to 20 dF when working with the roof top units.

ken
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Old 09-02-2009, 07:32 AM   #5
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The air temp at the filter entry point and the exhaust cold vent is about 21 degrees F lower.

From feedback, that is about right. Also, having the cover off may have been the cause of frosting lines. Put it back on and by looking thru the vents I could not see any white lines.

thanks all...........
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Old 09-02-2009, 11:21 AM   #6
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The suction line on the compressor should have a good sweat fromed on it.

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Old 09-02-2009, 04:53 PM   #7
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Old 09-02-2009, 05:43 PM   #8
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The cold line (suction) line is returning from the inside of the unit. It is not the line providing cooling to the unit interior, they are wrapped to control sweating in humid areas and the damage from the water due to the sweating. The hot small liquid line (going to the evaporator) is the one doing the cooling inside of your unit due to expansion in the evaporator. Its sort of like a spray nozzel on a garden hose and the cooling effect you feel with a broad spray pattern.
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Old 09-02-2009, 09:29 PM   #9
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Actually the cooling comes from the boiling of the refrigerant in the tubes at the lower pressure.

Back to the original question....there is some heat loss in the uninsulated suction line. Inside it is either over the drip pan under the coil and it catches the drips or iti si insulated. You can insulate the outer portion if you desire. Use a closed cell insulation so that it will not become saturated.

As for superheat, the last portion of the coil is sized so that all of the liquid is vaporized and the vapor continues to heat at a basically fixed pressure and this is called superheat.

Ken
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