Very low water pressure in the 2nd floor shower but good pressure every where else

Question

Here are the facts:

I have a 2 or 2.5 inch water pipe from the street and supposedly very high water pressure.
This is the master shower and is located on the second floor.
It was remodeled 11 years ago, we do not know how the pressure was before. 
The water pressure for the kind of shower-head I had purchased was disappointing even at the time the work was just finished but acceptable.  The plumber said something about the restrictor in the shower head, I removed-it did not improve.  I let it be since the water pressure was acceptable.

-Over the years the water pressure got less and less, similarly the water temp was reduced noticeably.

However, the tub in the same bathroom has an incredible pressure and gets very hot.  Sink faucets work well.
Other showers in the house work well.
Brought in one plumber who took out the sensor valve and cleaned it.  It did not improve anything.  
The second plumber took out the same valve and found some debris in the pipe leading into it.  And using the screws next to the sensor it increased the pressure of hot water.  What we noticed is that the hot water pressure coming out of the pipe (without any sensor involved) is considerably low compared to cold water pressure.  
Although both pressure and temp are now better, both are below optimal.  My quesion is why, is it that the hot water going into the shower somehow is clogged up or is it that the plumber used the wrong diameter tubing in the rough work behind the shower wall?

Jerry_Contrary · Answer

You stated that the home was remodeled 11 years ago and you don't know how the pressure was before. It could be that the previous owner installed some type of scald protection device for young children and it has been going out. These range in design and price. Here is more information about Anti-Scald Devices

Chris Taylor · Answer

Check for sections of galvanized iron pipe leading to the shower. Galvanized iron pipe (and black iron pipe) will gradually build up a layer of corrosion over time and restrict water flow, typically more so on the HOT side than the COLD side. The corrosion is due to galvanic action when disimilar metals, such as iron and brass or iron and copper are used in the same line. There are special unions available to prevent the two metals from physically touching, but while this does reduce the amount of galvanic action it may not be a perfect solution either. Galvanized iron pipe was at one time the preferred material for home (potable) water lines until copper came in to common use. By the way, the description of a 2" or a 2-1/2" water supply to the house does seem unusually large. Most homes are either 1/2" or 3/4", rarely 1". Apartment complexes and industrial buildings usually have water supplies larger than 1".

Answered by Chris Taylor on January 1, 2021

Ed Beal · Answer

I have found this problem many times with modern anti scald valves some I have been able to turn up others I soaked in CLR calcium, lime & rust remover. To restore proper functionality. I have had to replace the cartridge where the internal components were damaged. But do expect some pressure loss as even when new there is some because of the way they reduce the temp.

Answered by Ed Beal on January 1, 2021

Very low water pressure in the 2nd floor shower but good pressure every where else

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