20 Most Recent Grohe Integrated 1/2'' Shower Thermostatic Rough-In Valve With Stops - 3412 Questions & Answers

the temp, control valve is getting to hot!!! there for only let so much hot water go thru, did you take the inner parts out when soldering this in, if not you might have damaged it, and if so need to replace the cartdrige

Get a couple of pipe wrenches for parts that you can unscrew, a torch for copper pipes because you can unsweat the solder and pull the pipes apart. You can take these to the recycling center and get a little money back for all your hard labor. Or take a receipt ocarina saw with a few metal blades and cut them into pieces. If you use a torch don't burn down the house, if you use a reciprocating saw don't cut any electric lines. Also if the pipes are insulated there are special precautions you need to remove it

valve should have a gear like deal under handle that may need to be adjusted

Replace the rough in valve

you need to change the cartridge, it is a temp control and is going bad so it keeps you from burning yourself it is like a mixing valve and is givng more cold water, that way you don;t get burned,,

MAy I get the model # of the valve that you need to remove the cartridge from to help you more.

Having looked at the diagrams of the valve I not sure the handle is tight on your stem an is not turning the valve stem.
Look at that and if that's not it I will try to help.

he probably put it together wrong.
there is usually a blue gasket inside for testing purposes, if that is not removed, the valve will not turn off.