Water coming from shower head when filling tub
We've recently renovated a 2nd floor bathroom in a rowhouse we just bought -- all new plumbing going up from the basement. We installed Grohe Atrio shower fixtures with a port diverter at the shower control. With the tub spout turned on, after a few seconds water begins to come from the shower head. When the diverter is engaged, the shower works fine.
I saw some answers to this same problem on this site where the response was that water pressure may be too high. However, our plumber is blaming the problem on pressure being too low, saying that the diverter doesn't "catch" or something. Unfortunately the wall is tiled and finished. I would say that, while not a problem elsewhere, it sure doesn't seem like we have a ton of extra water pressure. Also, we have a 40 gallon water heater and basically everybody has said that we should have a 50 gallon for our house. Unfortunately I don't know what the water pressure coming in from the street is. What are our options? Thanks in advance.