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THINGS TO CHECK IF THE HEAD PRESSURE IS TOO LOW:

MOST LIKELY CAUSES:

      
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.

LESS LIKELY CAUSES:

picture of system layout

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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