What are the 3 positions of a service valve?

Many refrigeration compressors incorporate a suction service valve and a discharge service valve. These are used to allow refrigeration gauges to be attached to the system; and, if needed, they are used to isolate the compressor from the rest of the system.Working with these valves is not difficult, but there are some finer points to remember.
A typical compressor service valve is composed of four essential parts:

Line connection.
Compressor connection.
Valve stem.
Gauge port.

There are three positions in which the valve can be oriented. If the valve stem is all the way out, the valve is considered to be in the back-seated position. The gauge port is closed and the valve is open, allowing refrigerant to flow through the system.

If the valve stem is all the way in, the valve is considered to be in the front-seated position. The gauge port is open to the compressor connection and the refrigerant line (suction or discharge) connection is closed.
If the valve stem is between the front-seated and back-seated position, it is considered to be in the mid-seated position. In this position all three connections — the gauge port, line, and compressor — are open. This allows the refrigerant to flow through the system and the system pressure to be read through the gauge port.
When placing the valve in its mid-seated position, it is best to have the valve in its back-seated position first and then turn the valve stem only one or two turns into the mid-seated position.
Points Of Use
When opening a service valve, make sure it is secured by placing it in a vise or attaching it to the compressor.Also, look to see if the service valve has a packing gland nut. Many service valves will have this. The packing gland nut helps to ensure a leak-free seal. It is typically made of brass and is found at the base of the valve stem. It must be loosened by a quarter turn to a full turn before opening the valve stem.
Not loosening the gland packing prior to turning the valve stem may cause the valve to leak. In addition, make sure you tighten the nut when you are finished manipulating the valve stem.
When opening and closing compressor service valves, make sure to use the right tools, such as an appropriately sized service wrench. Do not attempt to open a service valve with an adjustable wrench. You can easily round the valve stem edges, rendering the valve useless.
If the valve stem is difficult to open and appears to be stuck, lightly tap the end of the valve stem with a hammer. This should open the valve. (Remember to loosen that packing gland nut before attempting to open the valve stem!)
Remember to replace the service port cap and valve stem cap after closing the valve port and removing your refrigeration gauges. These caps serve two purposes:

They help prevent a refrigerant leak; and
They prevent the threads on the service valve from being rusted over or otherwise damaged.

When brazing either a suction or discharge service valve, make sure the valve is mid-seated before brazing. If left in either the front-seated or back-seated position, heat from brazing can cause the inside plunger to “weld” to the seating area on the inside of the valve body. When brazing, it’s best to wrap the service valve in a wet rag. This prevents the valve from being overheated and damaged. Be sure not to allow water to enter the system.
Working with these valves is very simple. Just remember some basic rules for trouble-free operation.

Thank's to Joe Marchese from dhilreefer
Joe Marchese is owner of Coldtronics, Pittsburgh. He can be reached at 412-734-4433, www.coldtronics.com, or .
Publication date: 10/06/2003
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This tech tip video comes from my friend Andrew Greaves of AK HVAC and HVAC Comedy on Youtube and the HVAC Vehicle Layouts group on Facebook. Many residential techs get confused when they see these multi-position valves in larger equipment, and Andrew does a great job of demonstrating the basics in this video.

In the video, Andrew describes the following positions:

Back-Seated (all the way out, fully counter-clockwise)

This position provides full operational flow through the valve body but is closed to the access port. These valves have no Schrader port, so there will be no pressure on the port when the valve stem is back-seated.

Front-Seated (all the way in, fully clockwise)

Front-seated closes the valve and shuts off flow through the system at that point while remaining open to the port. Depending on the valve design, the port may be open to the inside or the outside (inlet or outlet) of the valve; this is important to be aware of when closing. Some compressors have suction and discharge valves, and you must not front-seat (shut off) the discharge valve while the compressor is operating, or extremely high pressures will build instantaneously.

Mid-Seated (valve in the center position, clockwise around 50%)

Mid-seating will provide flow in all directions in, out, and to the port. This position is ideal for vacuum and recovery with the system off.

Cracked off the Back-Seat (turned clockwise just a little)

This is a form of mid-seating where you just turn the stem clockwise enough to get a reading on your gauges. This position is used for testing and charging.

P.S. – Many techs call these King valves, but a King valve is technically a specific liquid line valve on the receiver.

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