Friday, January 7, 2011

Measuring Air Conditioner Performance

Almost a year ago, in the message about picking up our LD I mentioned how our A/C froze up and how it didn't really have enough capacity. During this summer the problem reoccurred while we were at the Life On Wheels conference in Moscow, Idaho. Up to then, I had accepted the way the A/C worked as a fact of life. An instructor at the conference assured me that freeze-up was abnormal so I asked Steve at Lazy Daze what to do. At his direction, I contacted Dometic and got the name of a local dealer from its automated phone system.

It turned out to be cool, about 70-75 degree day. The local dealer noted that it was freezing up as A/C's may on a cool day and that the split temperature (the amount the inside air gets cooled by a trip through the A/C with the blower on high) was 17 or 18 degrees. Everyone said that the split temperature is supposed to be 19 to 21 degrees and the A/C was drawing about 11 amps. The temperature was almost OK, Dometic was slow answering my dealer's call and I needed the coach the next day so I settled for paying for (it was outside of warranty) a thermal switch to turn off the A/C compressor when it froze up.

I am a professional electronics engineer, but don't know that much about A/C. But after the dealer visit I understood what measurements to make and what they meant. So a week or so later when a hot day came I repeated the measurements. On a 90-95 degree day the split temperature had fallen to 15 degrees and the A/C was still only drawing 12 amps instead of the 15 it should have (with a 115V line).

The temperature measurements are easy to do. I made them with a Radio Shack automobile inside/outside electronic thermometer. The current draw measurements are harder. They were made with a clip-on ammeter in the coach's line cord with all other AC appliances turned off.

The ammeter has a set of jaws that close around the wire to be tested. It is intended for AC lines only (not DC) and is nothing special, you can get them at many hardware stores. It measures the NET current passing through the hole in its closed jaws so you must be able to clip it around just one of the wires. If you close them around the entire cord you will measure zero since there is as much current going as coming in the cord.

To make measurements possible I constructed an adapter consisting of a short extension cord that has the outer sheath removed. That way one can close the ammeter's jaws around just the hot, typically black, wire. My adapter was made out of 15amp hardware store electrical fittings, male on one end, female on the other connected together with a short piece of #12 three wire cord. I cut away the sheath in the middle of the cord, being very careful not to nick the insulation around the conductors and bent out the black wire so I could clip the ammeter around it.

AC units are specified at an outside temperature of 90 to 95 degrees so when the split temperature is specified as 20 degrees and I measure only 15, that means I was getting only 3/4 of the cooling capacity I had paid for. With the data in hand, I asked Steve at Lazy Daze to help me contact Dometic which he did. I talked to two people at Dometic the rep. Roger, and then the tech Al. They thought the most probable problem was that there was insufficient refrigerant in the A/C's system. The upshot was that Dometic authorized replacing the A/C, which was done last week by the same dealer as before at no charge.

Contributor: Linley

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