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What are the causes of insufficient air flow?

The cause of inadequate air flow for the outside unit or condensing unit is primarily due to dirt on the coil. This dirt can be entrapped in the coil and you may not even see it. Take a flashlight and shine through the opposite side of the coil to examine the coil correctly. This dirt blocks the air flow and lowers the heat transfer. This condition causes the compressor to work even harder. The energy consumption of the compressor increases dramatically as the pressure in the outdoor coil in the refrigerant increases resulting in increased stress on the compressor. The compressor motor temperature increases dramatically. This of course can cause the compressor to have a very short life span as the windings of the motor of the compressor begin to deteriorate. Other less common problems that can cause the condensing unit to have insufficient air flow is the outside fan motor not operating or not operating at the correct speed. Also something placed against the condensing unit or the condensing unit being located too close to a wall results in poor air flow. For this reason the condenser coli should be cleaned thoroughly annually using high pressure but being extremely cautious of not bending the fins on the coil. If you should find the fins bent over for any reason take a knife or a fin comb and straighten them immediately. 

The biggest single cause of insufficient air flow on the indoor unit coil or air handler is due to a duct system that is undersized or poorly distributed. This is generally caused by poor design of the duct system and/or not enough indoor registers or grilles or diffusers. This is a problem that should be addressed immediately and corrected immediately. The outdoor unit should be turned off until the problem is corrected. To determine if the duct system is incorrectly sized request our Duct Design Form by email and you will be able to analyze the duct system and see if it is designed adequately with large enough ducting and sufficient diffusers or wall or floor registers.

The second biggest cause is oversizing of the heat pump system. For example let's say your house had a 2 ton heat pump. Somebody decides that maybe it would be better to install a larger replacement system like a 3 ton system. It's the typical American attitude if the right amount is good then a whole lot more should be tremendous.  The duct system remained the same and was never changed. The 2 ton duct system was marginal to circulate the 800 cubic feet of air per minute.

The new 3 ton system now requires 50% more air than the 2 ton system or 1200 cubic feet of air movement through the duct system in comparison to 800 cfm from the 2 ton system. Because the duct system was never examined for it's capacity before the change out occurred, the new 3 ton system begins to slug the compressor from the first day it is installed. A qualified technician can take temperature and pressure and air velocity readings and find this as a problem from the beginning. If the contractor selected the system and made a mistake he's likely to own up to it. So what the technician may attempt to do is increase the air speed on the indoor fan to try to compensate for the undersized duct. What will most likely happen then is the indoor fan will start huffing. Huffing is a condition that occurs when the fan capacity is increased beyond the capacity of the duct system and results in a build up of air pressure in the duct system. The amperage detector on the electronic fan controller senses this increase in current draw and slows up the motor. So the noise in the air handler and duct system will increase for a second or two and then get quiet repeatedly. This will continue to do this until the problem is corrected. If your duct system is huffing most likely this is the cause of the problem. This problem is easily avoided if the duct system is analyzed before the new system is selected and installed.

Poor maintenance on the system such as dirty air filters, dirty indoor coil or blower in the indoor unit will result in inadequate air flowing over the indoor coil. The indoor coil should be inspected as well as the blower and cleaned at least every 2 years. Another problem is closing of the dampers at the registers or grilles in the house for most of the diffusers in the wall or floor. The result is improper air flow and slugging.

A system that has too much refrigerant or overcharged will also slug the compressor.

The proper method of checking the system operation is to have a certified technician check the superheat of the compressor. This is the first test that will determine if in fact there is an air flow problem. Similar to taking your blood pressure and your temperature at the doctor's office this would be the first diagnostic test performed by a technician.

Let's examine more closely the results of slugging or liquid coming back to the compressor instead of gas. Over 95% of compressors returned under warranty to Copeland and other manufacturers are not defective due to poor workmanship or defective components. The compressors failed because they were slugged and the liquid literally killed the compressor. That translates into over 80% of compressor failures were due to slugging because of improper installations of either oversizing the equipment or a poor duct design. Every week the top trade magazines post problems with improper duct design, poor installation and leaky duct systems and oversizing systems as now in epidemic proportions in the HVAC industry. Epidemic meaning the vast majority. Your only remedy to stopping this problem happening to you is education. This problem is not usually due to contractors trying to gouge the public as much as it is due to ignorance. Our technical support deals with contractor questions everyday and we know this problem is not due to anything more than ignorance. Our tech support always asks the contractor in their first question when they have a problem is did they read the installation instructions that come with the equipment. 98% never even opened the installation instructions that came with the equipment to know what's happening.  That problem alone could be discussed for many pages here but the only solution is for you to do your own installation following the guidelines we present or making certain the installer you select is following those guidelines.

Otherwise you'll become part of the epidemic problem of statistics of improperly sized and installed HVAC equipment and systems. 

How does the manufacturer know what causes the compressor failure? Is this a new problem?

HVAC and compressor manufacturer's such as Copeland will randomly cut open returned compressors to investigate the reason for the compressor failure. They will perform what is called an autopsy. When a compressor has failed due to slugging there will be specific damage to certain areas such as the valves, piston walls and crankshaft and other areas. My own experience working in York International at their Advanced Engineering and Research department provided me with a wealth of information on HVAC systems and particularly the cause for their failures. Any compressor can have an autopsy performed to see what caused the compressor to fail. As it was true 36 years ago and as it is today, there were very few times a compressor failed because of defects in materials and workmanship. A compressor failure is the result of another problem in the system, not the cause of the problem.

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