Thank you for your question. you are experiencing "shower-rise" where the water can't get out of the spout as fast as it flows through the valve. This causes the water to back up and come out the showerhead.
There can be one of several reasons for this:
The valve could be upside down. The valve will have the word "UP" on it with an arrow.
The length from the valve to the tub spout is either less than 8" or more than 18".
There is more than one 90-degree angle.
The elbow going to the tub spout is a drop-ear elbow.
Something other than copper or galvanized pipe was used going to the spout, such as PVC or PEX.
Sharkbite fittings have been used with the copper pipe, rather than sweat soldering.
The most common reason is that there is something restricting the water flow to the spout, e.g. solder or something lodged in the pipe. Something is impeding the flow of water to the spout and causing it to back up and go out the showerhead.
If this is the cause of the shower-rise, then it is possible to remove the blockage by removing the tub spout; then, feeding a plumber's snake or a speedometer cable up through the tub drop. The act of feeding it causes it to twirl and loosen the blockage.
Be sure to flush the valve before reinstalling the tub spout.
Also, please be sure to check the tub spout itself for blockage.
If none of these help, you may need to call a plumber. After the shower has been installed and the wall has been closed, there is little that can be done to remedy this situation yourself.
Date published: 2016-10-10