I came across an outdoor AC compressor the other day that was running at full speed. This was not immediately unusual, because it is August. Then I realized that at this early hour of the morning the outdoor temperature was only 50 degrees Fahrenheit! Upon closer inspection I noticed one whole side of the compressor was covered with about a 2 inch layer of ice!!
After I ran into the home and quickly turned the air conditioning off, I took a side bar with my clients and explained how an Air conditioner works: Through the powerful processes of evaporation and condensation the AC unit is able to absorb heat from the inside air and exhaust it outside. Right before the coolant enters the home it has been cooled and evaporated so it is a low pressure gas. After the warm house air is passed over the cool coil the coolant that has absorbed that heat travels outside is compressed and is a high pressure/high temperature liquid. Now the outdoor air which is cool compared to the compressed high temp liquid (100 degrees F is much cooler than 180 degrees F) is passed over the coils and heat is exchanged. Then the liquid is evaporated and it is cool again and cycled back in to the home.
The fatal flaw that can be noted on a cool day, is the inside air is not warm enough to absorb an adequate amount of heat from the gas. This causes ice to form. The ice covers the small metal fins that the air is blown through to exchange heat. If the heat cannot be exchanged the low pressure/low temperature gas cannot continue to drop in temp. Eventually the compressor, which should only compress gas, will try to compress liquid. When the unit tries to compress liquid the AC unit is toast.
The outside temperature being above 65 F is critical to testing AC units. For around 6 months of the year home inspectors in Salem, Oregon cannot test the AC system because the outdoor temperatures are too low. On most units there will be a fuse or switch near the outdoor unit. After September it may be a good idea to turn the unit off to prevent accidental cycles.
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