Do you always need a check valve?

Do you always need a check valve, no, you do not always need a check valve, its necessity depends on system design and whether backflow would cause problems, but there are many situations where it is required or strongly recommended for safety and efficiency. You need a check valve when backflow would cause contamination, damage, or loss of function, such as in sump pump discharge to prevent pumped water from flowing back into pit and causing pump to short cycle, in well systems to keep pump primed and prevent water draining back, in water supply to prevent contaminated water from flowing back into clean municipal supply, which is often required by plumbing code in form of backflow preventer. You also need it after sewage ejector pump to prevent sewage backup. You do not need a check valve in simple gravity drain, where water flows only one way and backflow is not possible, or in open vented system where backflow is not harmful. Installing unnecessary check valves can cause problems like water hammer, pressure drop, or trapping water that could freeze. Plumbing codes dictate where check valves or backflow preventers are mandatory, particularly for potable water protection. So evaluate whether reverse flow would cause contamination, equipment damage, or inefficiency, if yes, install check valve, if no, it may not be needed and could add unnecessary restriction.