How does this thing work? (Symmons Temptrol cartridge)

So, if you only turned on one of the supplies, that would mean zero pressure on the other, the balance spool valve would try to compensate by cutting back the good supply's output, and when new and working well, that will often mean NO volume when only one supply is turned on.

On an older valve, when that happens, it can jam the spool valve to one end and it may not move back once both supplies are restored. Now that it's working, open each valve to the shower control slowly the first time, and equally, and it should be fine.

When the water PRESSURE coming in is identical, the sliding piston is centered and lets the water flow freely. If, say, the cold water pressure dropped (someone flushed a toilet for example), that COULD reduce the cold water pressure which would make the outlet of the shower hotter, since there was less cold available. THe sliding valve compensates by decreasing the opposite water supply so the relative BALANCE between the two supplies' pressure stays the same, keeping the water TEMPERATURE fairly even. It's all part of the mandated (since the 80's) anti-scald tech that needs to be in any shower valve assembly. THere's more than one way to do this, but a pressure balance spool valve is the most common.So, if you only turned on one of the supplies, that would mean zero pressure on the other, the balance spool valve would try to compensate by cutting back the good supply's output, and when new and working well, that will often mean NO volume when only one supply is turned on.On an older valve, when that happens, it can jam the spool valve to one end and it may not move back once both supplies are restored. Now that it's working, open each valve to the shower control slowly the first time, and equally, and it should be fine.