As an HVAC tech, there’s a lot of hardware you’ll need to familiarize yourself with. The king valve is up toward the top of the list; we’ll talk here about king valve operation and what you need to know about servicing this component in an HVAC system. 

What is a king valve, and what does it do? 

The king valve is designed to let a technician evacuate all the refrigerant out of the system’s low side; when the king valve is closed, the compressor can run until all (or nearly all) refrigerant is pumped out and into the compressor and receiver. The king valve is typically located on the outlet of the receiver. 


Is the king valve a service valve?

Yes and no. A commercial system typically has three manual stem service valves: 

  • Suction service valve
  • Discharge service valve (both of these are located on the compressor)
  • King valve

These valves will typically be fitted with a gauge service port. It’s important to remember that the only place you’ll find a king valve will be on the outlet of the receiver; if a valve’s address is anywhere else in the HVAC system, it’s not a king valve. For instance, a service valve on the suction side of the system is a suction service valve – not the king valve.

These valves are used to manage flow and separate one part of the system from another. Valves will allow refrigerant flow when the gauge port is closed and the valve is open. The term “front-seated” is interchangeable with “open,” with the valve stem all the way in. To operate the king valve, the valve should be front-seated, then the compressor should be started. A closed king valve will stop refrigerant flow at the receiver, with the refrigerant that was on the low side being emptied out and migrating to the high side. 

Closing the king valve and evacuating refrigerant this way will result in pressure readings being the same on the high side and low side, meaning there will be a pressure differential between the liquid refrigerant in a jug and the line, making it safe to introduce liquid and charge the system. As liquid refrigerant enters the system, the suction gauge pressure will increase and the high side gauge will indicate pressure remaining in the jug — not the actual pressure in the high side. 

Refrigerant added will be relocated in the isolated high side and then changed to gas in the evaporator. From there, it will be in a form where it can safely be drawn into the compressor, then condensed again in the condenser and eventually it makes its way to the receiver which starts filling up. The technician can monitor this process by watching the sight glass and monitoring the weight of the jug to see how much refrigerant has been introduced. Periodically close the red hand wheel on the jug and open the king valve to see how things are progressing. When you’re close to completely recharging the system and reaching target pressure, it’s a good idea to finish the job using the vapor charging method so as to not over pressurize the system. 

Safety tips

  • Never front-seat the discharge service valve and then run the compressor. This would create a situation where discharged gases would have nowhere to go, and is extremely dangerous. Front-seating the king valve, on the other hand, will allow the system to pump down. 
  • When servicing any valves in the system, use a specially-designed HVAC service wrench rather than an adjustable wrench. An HVAC wrench will prevent the valve stem becoming mangled and rounded off, thanks to its more precise fit. 
  • Always wear the appropriate PPE when servicing the system, and use caution to avoid injury from refrigerant burn and pressure blow-off. 
  • You’ll notice that HVAC systems include Schrader valves, which look (and function) a lot like the valves on tires. A Schrader valve includes a poppet-type valve and a spring-loaded stem and can only release or accept gas when the stem is depressed, just like a tire valve. This means a tech can add refrigerant or bleed off excess charge without taking the system offline or having to completely depressurize it. When you’re done using the Schrader valve, remember to always replace the cap on it, just like you would with a car tire; the cap will prevent any refrigerant from migrating out in case the valve starts to fail and seep.        
  • Any valve should be fully back-seated before attaching or removing gauges. 

Additional Help 

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NuComfort LLC is a wholesale distributor of both commercial and residential HVAC systems. We also stock a full line of accessories and replacement parts. We have a long history of satisfied customers, and are ready to serve you from our 3 Chicagoland locations in Glendale Heights, Chicago and Crestwood.