A Pump Down Solenoid and What it Does
Depending on what segment of the business you are in and what area of the country you work, you either work on pump down solenoid systems all the time or YOU HAVE NO CLUE what they are.
If you work on refrigeration or straight cool units up north, you are likely very well acquainted with pump down solenoids. If you do residential HVAC in the south you may have never seen one.
You know that you pump down a system by closing the liquid line? That’s all a pump down solenoid does. It closes when the system is running causing the system to pump all of the refrigerant into the condenser and receiver (if there is one).
The trick is that in order for it pump down the compressor needs to be running and then it needs to SHUT OFF once it is done pumping down. This means to need a good, quality, properly set low-pressure switch near the compressor to shut it off when the suction pressure gets low enough.
There are a few benefits of a pump down solenoid. First, it helps prevent liquid refrigerant migration down the suction line into the compressor. When liquid refrigerant migrates into the compressor it dilutes and can cause slugging on start up.
The other cool thing is you don’t need any LV controls between the indoor and outdoor unit. The solenoid is in the liquid line near the air handler inside so by opening the valve the suction pressure increases and the compressor turns on and when it closes the compressor pumps down and shuts off.
Obviously, this would not work on a heat pump system because in heat mode it would attempt to pump down into the indoor coil which would not work. They also won’t work when you have complex controls of any type.
So… there are places where pump down solenoids make sense and applications where they don’t.