Low water flow at tub spout

[PatTheRat]

I'm trying to help my sister-in-law with her tub. When i first looked at it, it would run constantly at a low flow no matter what the knob was set to - off, on, or anything in between. This is a Moen single-handle shower valve model 1220 I believe. I removed the old cartridge and replaced it as per Moen instructions. Everthing went well and I thought I'd be done. I turned the water on again and still the same thing. Water just keeps running at a low rate no matter what the handle is set to. So I went back in, removing the new cartridge and took a small bottle-cleaner (made for coffee pots) to the hot and cold channels inside the valve bore. I turned on the water while it was still out and got good pressure coming out of the valve bore. So I put the cartridge back in and reassembled and when I turned the water back on, I still only get a low flow of water out of the spout, but now it does seem to turn off the flow with the handle off. I just don't know what to do next. Could it be a problem in the pipe from the valve to the spout? Pretty frustrating. I called Moen and they had no answers. They just sent me a new valve body. I'm really hoping to be able to fix the old one as the builder didn't provide an access panel to the back of the shower fixture. I'd have to cut a hole in the drywall and install a panel to get to it. It's also PEX plumbing and I don't have the tools or knowledge to repair the connections. Any ideas what could be causing the low flow?

 

[Terry]

I hope you don't have PEX to the spout. That's a know restriction that they warn about.
That should be copper pipe or threaded nipples. PEX and PEX fittings are smaller and they restrict the flow, often pushing water up to the shower head while filling the tub.

 

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[WorthFlorida]

To check for PEX at the tub spot, you can remove the tub spot with a set screw or it unscrews off the pipe stub.

Is this a new problem or has the flow always been slow?
Is this a newer home with PEX or a bathroom remodel?
How is the shower water flow?

To get an idea of the water flow, use a pail and measure off one gallon increments. Then time how many gallons per minute to fill the pail.