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• Refrigerant undercharge is probably the most common cause of low head pressure. Refrigerant does not get used up. If the system is checked by subcooling and superheat measurements and indeed found to be low on refrigerant, then there is a LEAK IN THE SYSTEM. Someone has left it with a low charge, or someone or something has let it out. (Don't forget that some people "sniff" anything to get a feeling or bodily reaction. This does occur, too.)

  Household refrigerant is usually refrigerant-22. It is not likely to be dangerous unless inhaled in large quantity. It has the characteristics that allow it to displace oxygen in the air and thus in the body. It is not flammable. In fact, since it displaces oxygen, it retards fire. If breathed in pure state, it will make the voice very low and will give you a faint feeling and later a headache. Enough of it will suffocate you. If a refrigerant is in the presence of an open flame or an extremely hot surface, it will turn into a potent acid. The fumes from this acid are very harmful to humans and will cause violent involuntary defensive reactions by a person coming in contact with the vapor.

  Several clues may guide you to finding the leak. Look for oily spots at joints and fittings. About 15% of the systems oil will circulate with the refrigerant. If it leaks, then likely there will be some oil residue at the area of the leak. Check especially the service valves and caps and flare or compression fittings. If you suspect a spot, poor or spray some soapy solution on it to see if it bubbles. Don't necessarily expect large bubbles or a lot of them. Sometimes the leak will only produce a tiny white pin-point size spot. Of course, the technicians will use a very fine and expensive leak detection meter.

  Some leaks are difficult to repair and others are a waste of effort. If the leak is in the body of the evaporator coil, for example, you might as well just buy a new coil and be done with it. If it is in the end-loops of the coils, you may repair it today and the heat around the perimeter of the brazing may later cause several additional leaks to develop. If, however, you are ambitious and willing, then you can braze every joint that is "soldered" and you run a pretty good chance of having a permanent repair. Even this will only give you about a 50% chance, in my opinion, of it being a long lasting and profitable effort. But, if you are good at it and want to try, then go for it.

• Cool ambient conditions will surely cause the pressures to be low. It is nearly impossible to check the refrigerant system in cool weather. Try to fix it when the outdoor temperature is above 70 degrees F. and the indoor temperature is never below 70 degrees F. If you add refrigerant to a system in too cool conditions, you will invariably overcharge the system.
• Water on the condenser coils will also result in abnormally low pressures. Let it dry out and avoid even putting your hoses on the equipment in the rain. Even if you don't get moisture in the system, the rain will increase the chances that you might electrocute yourself. Please read the safety precautions from the home page. Always be cautious.

• Inefficient compressor valves, particularly the discharge valves, inside the compressor will produce a reduced head pressure. There is nothing you can do but change the compressor or the condensing unit.
• TXV stuck open (see explanation in orifice-flow control device in Refrigerants and Charging) will cause an unusually high flow of refrigerant into the low side of the system and will of course reduce the head pressure in the process. Look for excessively high suction pressure and a cooling coil that is not cooling well and a compressor that is being flooded with cold liquid refrigerant. It will be likely be sweating all over and making strange sounds and begging to die. The superheat will be non-existent or excessively low. You may as well replace the TXV (thermostatic Expansion Valve), but you may also take it apart and lightly clean all the moving parts. Don't forget the "push-pins".
• Iced evaporator will also contribute somewhat to lowering the head pressure. Look for sweating compressor, low or non-existent superheat and lack of adequate airflow from the supply grills from the indoor system. It may, however, seem to have good airflow while not being iced completely. You will likely have to visually examine the evaporator coils.
• Leaking check-valve (on the heat pumps) will also cause the condition. You can feel the leakage by checking the temperature of the lines as the refrigerant goes through. No repair is likely possible. Change the valve.
Notice that this drawing of this system has two check valves. One is in the evaporator section and the other is in the condensing unit. The one we are checking now is the one in the evaporator section. Notice the direction-of-flow arrow. In the cooling mode, it is not supposed to let the liquid refrigerant by-pass the expansion valve (TXV) or the capillary tube if you have one or more of them. The check-valve in the condensing unit will be feeding refrigerant through in the correct position during the cooling cycle, so it is supposed to by-pass the flow control device in the condenser during cooling.

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