I have installed a Grundfos UP pump with built-in timer, aquastat, and check valve. It does provide hot water at the tap sooner, but when the water is being used, such as during a shower, cold water is bleeding into the hot water. The return line is tapped into a 3/4" line near the kitchen, then brought back to the water heater. It connects to the top of the pump, which is connected to a tee in the water heater drain. It seems the pump is allowing cold water to siphon back into the hot water line, even though there is a check valve in the pump to prevent this. I'm thinking of adding another check valve at the inlet side of the pump, but don't know if it will make a difference. Any ideas are welcome.
If you are (as is common) "returning" via a cold water supply line, rather than a dedicated recirculation line, I think you need a check valve where the hot recirculation joins the cold line, not just at the pump.
Otherwise the cold can backfeed from the cold water line to the hot loop when the hot water is run, without the check valve at the pump being involved. All the system designs which use the cold water line as a return typically have the pump located at the point of joining the hot to the cold, NOT at the water heater, so that the check valve in the pump works for this application.
If you have a dedicated return line, and it has a check valve in the pump, that should be sufficient, and you should complain to the supplier of the pump if the check valve is not working.