Water pressure low due to small pipes - any options?
It sounds like your shower valve does not have an anti-scald valve installed to protect people from this problem.


You could replace the shower valve with an anti-scald valve.
You could add an anti-scale valve in the lines going to the upstairs.


These solutions will not bring more water to the bathroom though. When someone flushes the toilet you will still get low flow to the shower. To solve that you need bigger pipes or raise the existing water pressure in the system. So...


You could replace the main lines feeding the upstairs with larger pipes. The small pipes in the walls feeding each fixture could be skipped. It is the main supply that is mostly restricting your flow. Especially if it is old corroded galvanized pipe. The corrosion could seriously restrict the flow.
You could increase the pressure if you are on a well. (This brings with it the hazard of creating leaks where there were none before) If you are on city water then you don't have this option.
You could do a combination of some of the above to achieve your goal.


I would recommend adding the anti-scald valve in a convenient location. Adding it to the shower may require removing a tile wall if you don't have an access panel, this would be very expensive. In that case I would add an in-line valve in the supply lines. I would also increase the size of the supply lines to increase your supply flow. After both of these, your problem should be nearly eliminated.

Good luck!