New house with tub/shower assembly that first kept dripping before we moved in. Call co. and they sent new cartridge assembly. Installed it and then we got hot water but as soon as the single handle was turned down to blend cold, it all goes cold and stays. Called again, new assembly again. Same results! Even had city check water pressure in the house. Today PP's service techs tell us that this shower assembly should not have been installed on FlowGuard Gold piping for it could case a backflow that would cause incorrect blending. Is this for real???? They say it should only be installed on copper or iron piping.

Type of piping should have nothing to do with it. All the pipe does is deliver the water to the valve.
Could they have warped the valve if they held it incorrectly with pliers when they tightend the fittings into place?
Is the plastic stop that limits the amount of hot water to prevent scalding installed correctly or at all?
I had the same issue with one of these faucets myself.
Pay attention to the cartridge. They sell many types all of which fit the valve and many look very much alike. After three changes we finally took the valve apart and studied it. Once we did this we found that we kept getting the wrong cartridge. Even though the cartridge may have a serial or model number, check it carefully.
Just because the valve came new does not mean that someone at the store did not replace a part and send it back as a return. Happens a lot at the big box stores.

Thanks for the ideas! The cartridges came from PP so we just assumed they were correct-will check. Also plastic stop has been turned to full up and we have no problem getting very hot water just blending the cold goes to very cold. Think we will have to call in another plumber to see if he can tell if the valve was warped on insulation. Thanks again for your thoughts.

Hello, Best thing for you to do, is to get ahold of price pfister as I have done in the past as they gurantee there faucets for life and will completely replace faucet or send repair parts for whatever they feel is needed. They stand behind there products 100% Happy Plumbing