Just had a recirculating pump installed. I have only a trickle of hot water at the single handle control faucet where the return line is attached when I turn off the cold water supply. What could be the cause? Is the faucet bad or do we need some kind of valve? Cold water mixes at the faucet and sends cold down the return line no matter if pump is on or not.
"A trickle of hot with the cold turned off" is utterly normal for any modern fixture with a built-in anti-scald valve arrangement, which responds to low cold water pressure by blocking hot water flow.
If cold water is flowing from the faucet to the hot water circulating loop, it sounds like you need a check valve, or if you have one it's backwards, in the wrong location, or not working.