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James A/C Co.
serving Do-It-Yourselfers since 1990.
Satisfying every customer....one at a time!
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Call toll free 1-866-584-0355 or 1-903-759-2160 fax 903-759-4605
309 Patriot Circle, Longview, TX 75604-2240
THINGS TO CHECK IF THE HEAD PRESSURE IS TOO LOW:
- 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.
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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 valves and caps and flare or compression fittings. If you suspect a spot, pour 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.
- Cool ambient conditions will surely cause the pressures to be low. It is much more difficult 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. A good method in the heat mode is to add refrigerant until the HOT GAS line from the compressor is 100 degrees F. above the ambient (outside) temperature.
- Icing of the outside coils will reduce the high side pressure. See the heating section on
coils icing.
Don't forget that icing of the outside coils in the cold and wet weather is normal for heat pumps. A defrost cycle will likely occur every 30 to 90 minutes attempting to defrost this ice, but in really damp and cold seasons may fail to succeed at this. It is especially a problem if the roof drips water directly into the unit.
| Watch for dripping water to freeze on the outdoor fan discharge area. Don't run the unit if the ice is restricting the movement of the blades. If you have this problem, do not build a cover over the unit and restrict the airflow. Move the unit instead. |
If icing persists and your indoor temperature is below the set-point, you may have to switch to the backup heat exclusively by placing the thermostat in the EMER HT (AUX HT or ELEC HT) position. This will TURN OFF the heat pump and you will operate on your indoor heating unit. It will be very adequate.
- 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 heating coil that is not heating well and a compressor that is being flooded with cold liquid refrigerant. It will likely be sweating all over and making strange sounds and begging to die. The superheat will be non-existent or excessively low. You well need to replace the TXV (thermostatic Expansion Valve.
- Leaking check-valve 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 the drawing of this system has two check valves and the arrows show the direction of flow they will allow. In the heating mode, the hot gas will go directly from the compressor through the four-way valve and to the indoor coils. As it leaves the coil the check valve will allow it to by-pass the indoor expansion valve and go through the two-way drier and to the outside unit. The check-valve in the outside unit will force it to go through the expansion valve there and the outside unit will become an evaporator coil.
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