New Water Heater, Low Pressure at Shower

Just installed a new hot water heater. Hot water pressure is fine everywhere in the house except at the master shower which is the closest fixture to the water heater. We get a small trickle at the shower. We have pretty hard water / calcium deposit in our area and I cleaned alot of the screens at the other faucets, etc after the water heater swap. The shower/tub combo is a Moen so I just replaced the cartridge thinking it was clogged. Hot pressure remained the same.

Could something be blocking the line up stream? How can I clean that out?

I even tried removing the new cartridge and slowly turned the main water valve back on to try and flush anything out...no help.

Thoughts?

Thanks!

That valve may also contain a spool valve (also called pressure balance valve) and it could be stuck. This is not part of the mixing cartridge, but a separate part. Check out the parts breakdown. You may be able to clean and free it up, or may need to replace it.

When the water was off, then turned back on, there was air in the system. This would have created a significant difference in flow to both the hot and cold supplies at the tub/shower until water was available at both sides. This would have forced the spool valve to one side. If it sticks rather than going back to center once things are balanced, I think you'd see what you're seeing.

It also may be that debris, or even a ball of solder, is blocking the hot side inlet to your valve. I have seen this happen few times after hot water heater replacements.
If so, hopefully there is a way to open a line below and let the water carry the blockage out.
Recently i had a hot-side blockage and used a pressure testing cap (one that came with the valve body) to flush out the blockage.
I removed the cartridge and installed the cap, and capped the tub spout and shower head using iron pipe caps, then closed the hot water heater valve, opened the hot side of the basement sink and turned the main water valve on. The cold water came in, through the valve, out the hot side and down to the sink, and lots of old rust and crud came out. (This was very old iron pipe plumbing).
Good luck

Check the restrictor in the showerhead. Unscrew the showerhead from the shower arm to check it.

Is your old water heater from the 90's?
Check this link out....
http://www.rustylayton.com/htmlxtra/diptube.html
You may have some debris to flush out.