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Suction Pressure Problems

What is meant by suction pressure?

In an air conditioning unit cold liquid enters the evaporator and when warm air blows over it, the liquid absorbs the heat and cools the air. This heat exchange causes the liquid change to a vapor. The vapor pressure must be controlled to allow it to flow properly to the compressor. Maintaining this suction pressure is essential to the properly functioning of the unit. Read below where suction pressure questions are answered by the Experts.

I have been told that my HVAC unit is not working because of low suction pressure and high head pressure. What does this mean and what should I do?

When you have low suction pressure and high head pressure that means the there is a restriction in the liquid line and that the evaporator is getting starved. Check to see if the unit has an expansion valve on the evaporator inlet. If it is adjustable, try to open it more. If it is not, then it is likely that the valve is defective and will need replacement. Another, although less likely cause could be a blocked liquid line drier. Check the temperature at both ends of the drier. If the exit end is colder than the entry, the drier could be clogged.

At an ambient temperature of 90 degrees the suction pressure on my air conditioner is 85 psi. The liquid line is very hot and only half the evaporator coil is cool. What is the problem?

First, make sure that your evaporator coil is clean and unblocked and that all the fans are running. If the evaporator and fans are okay, then it is likely that your TX valve is defective and may need to be replaced. This is a common occurrence.

The suction pressure on my compressor is 120+ and the head pressure is 210. But the unit is not cooling. Is the problem with the compressor?

The auction pressure is high and the discharge not normal. It should be in the range of 350 to 400. It is likely that the problem is with a compressor valve. However, if you have a big compressor fitted with unloaders, it could be that an unloader is allowing the gas to flow from the discharge to the low side.

What are the correct liquid line and suction pressure numbers when using 410-A refrigerant?

The exact numbers will be available on the manufacturers charging chart. Typically for a 40 degree indoor coil you will need 120 psi at the low end. If you want a condensing temperature of 100 degrees you will need 320 psi. Keep in mind that the high side pressure will change along with the outside air temperature. Normally you would want to have about 25 degrees above the exterior ambient temperature as the condensing temperature.

I have a Rheem R22 condenser and I cannot get more than 30 psi of suction pressure. High side is 100 psi. I did a solvent flush on the line and evaporator coil and get free nitrogen flow. I replaced the dryer and charged Freon to 12 lbs. But the suction pressure stays at 30 psi. This is an old model with no TX valve – only an orifice.

You appear to have a clog in the system which would account for your problems. Check the screen by the orifice – it may be blocked. Take the line out and clean it. Also check to see if the condenser is dirty and clogged. Either of these could be the cause.

Can you tell me the right head and suction pressure at an ambient temperature of about 85 degrees for a 2 ton heat pump on a packaged unit?

The head and suction pressure depends on a lot of variable like the manufacturer, ductwork static pressure, indoor temperature and so on. Having said that, for the type of unit you describe, the typical suction pressure would be around 120 to 140 psi and the head pressure would be in the 300 to 320 psi range.

Suction pressure in a HVAC unit is a very complex issue and suction pressure problems can affect how well the unit works. Because troubleshooting suction pressure issues often requires a suction pressure gauge and other special equipment, these are rarely covered in an owner’s manual. If you think you have suction pressure regulator or other suction pressure problems, and your manual cannot help you to resolve them, it is advisable to get Expert help to prevent damaging the unit further while trying to fix it.
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