Momentary Cold water

Hi
I just replaced a failed copper re-circ line in a house for a friend. I used Uponor Pex with the 5006 rating for domestic with chlorine. I also installed a uponor demand pump with motion sensors in the bathroom, client likes the Motion sensors rather than the pump running 24 hrs a day. The only problem is after about 5 min in the shower they are getting a burst of cold water not long enough to adjust temp just long enough to be uncomfortable. The pump also has an aquastat does anyone have any thoughts on the cause.
thanks

Do you have a check valve on the re-circ line? If not your getting colder water coming up the re-circ line to the shower faucet.

John

Thanks for the reply, the pump has a built in check valve, have you ever heard of them not being effective
thanks

Not saying they don't. But I never heard of them having a built in check.

John