Novice Question: When to use check valve

Hi,

I noticed a check valve installed on my old circulating pump for the water heater. I'm curious why this is necessary? I'm also interested in knowing for what other uses check valves are necessary.

Thanks. I appreciate the info.

A check valve allows fluid to move in one direction, but blocks it from flowing the other way.

In a recirclating system, if you did not have a check valve, then when the pump was not running you would tend to have circulation of COLD water back into the HOT side of the WH. Not a good thing, so the simple check valve completes the system

I get it now.

thanks for the info!