temperature difference between outside and in

lbdoveJuly 2, 2009

Ok I live in Louisiana and during the summer the temp is 95 - 100 with a heat index of 110 +. I have a 2400 sq ft. manufactured home with no trees or anything so the roof is exposed all day to direct sun. If the temp is 100 and the heat index is 110 should my AC be able to keep the house at 71 or is that unrealistic for this kind of heat? I would really hate to pay $70 for the AC tech to tell me there is nothing wrong with my unit it's just really hot.

Thank you

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That's unrealistic. Properly sized and running non stop, mid 70s is more like it. Industry standard is to size cooling to 75 inside on the hottest typical day.

    Bookmark   July 2, 2009 at 5:49PM
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I live in Atlanta. When my home was built the builders documents stated that the cooling system is designed to maintain 78 degrees. It also said that when the temp goes above 95 degress it would only guarentee a 15 degree between in and out. We've had days when it was over 102 but I never noticed a problem, however I don't keep it set for 71. Daytime it goes to 82 at night we keep it at 76.


    Bookmark   July 2, 2009 at 5:50PM
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The best thing you can do, first, is to make sure your filter is clean. Then, I would go to Auto Zone or a similar place and get a thermometer which looks like a pencil with a round indicator on the other end. Stick the thermometer then in your return air (where the air sucks in) and write the number down (after letting it sit there for several minutes). Then take that same thermometer and put it in a supply vent (where the air blows out) fairly close by. Leave it there for several minutes. (Those thermometers can be hard to read so make sure you are looking at them closely from straight out).

Subtract the supply air from the return air measurement. This difference should be 14-20 degrees.

The other thing you can do is make sure your blower wheel and blower compartment are fairly clean. Also make sure your indoor coils are clean. Too, you can check your outdoor condensing unit and make sure the coils are clean. There is a nice video on youtube.com that shows how to do these things. You MUST kill the power to the respective units before doing any of this. If you are unsure of how to proceed, call a technician.

If all of this checks out, you can conclude that your unit is not sized for these extreme conditions.

As far as maintaining 71 degrees, it is impossible to determine if this is realistic. There are too many factors to consider. These factors mostly involve whether your unit was sized for design conditions in the first place. The unit should be running continuously with an outdoor temperature of 95 degrees plus.

You can reduce the heat load on your unit by shading your home, shading your unit (nothing within 18 inches from the sides and nothing affecting the airflow), pulling heavy drapes or blinds on your windows on the hottest part of the day. (Your south and west exposure are going to HOT!) Adding insulation is another thing you can do that may help. In a manufactured home, this can be pretty difficult.

These factors aside, there is nothing you can do other than to reduce the heat load or having a technician check the refrigerant charge. Checking the air-flow temperatures is an indirect approximation of checking your charge.

Good luck!

    Bookmark   July 2, 2009 at 6:08PM
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I have a friend who lived in a manufactured home in Texas and was hot even with the AC going full blast. He took aluminum foil and covered every window. It was dark as a cave and cold! A bit extreme, but it worked. Shows the power of eliminating heat gain and loss.

    Bookmark   July 2, 2009 at 6:40PM
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Ditto here in Dallas. My upstairs unit is on all day till about 8PM or so. Very typical of builders under sizing units and using subpar coils-which mine are leaking apparently. The last AC guy charged me $85 to show me.WOW! Anyhow, my unit can only produce 80 degrees when 100. I typically set at 79. Maybe I should set at 80 so it may cycle.
I am more concerned now with reducing the heat in my attic. It reaches 133 degrees when 100 outside. Guess I could use more insulation, but not completely sold on spray radiant barrier. Must be lucrative b/c a company advertises on every AM radio staion in the metroplex!!!

    Bookmark   July 2, 2009 at 11:33PM
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A manufactured home has heat coming at it from ALL SIX surfaces. If you have a tall ceiling in the living area, as most double wides do, there's not alot of insulation up there. 71 is a tall order.

    Bookmark   July 3, 2009 at 5:41AM
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Thanks everyone I appreciate your feedback. I do have fans that are running all day, I also have all the blinds closed, but the foil is a great idea. At least I won't have to waste the money for a tech to tell me that there is nothing to be done. I'll just set the thermostat higher and sit very still under a fan :+)

    Bookmark   July 3, 2009 at 9:37AM
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