I think there are two varieties. The first is a tempering valve that works on the outlet of the water heater. The water heater works off engine coolant, and could heat the water to 170F-180F - hot enough to cook you or a lobster! The tempering valve mixes outlet (hot) with inlet (cold) to bring the temperature down to 120F or so - you can set it with a knob. Do you have a tempering valve on your water heater? These work on a thermostat and work pretty well until they pack up. Then you replace them.
The ones on the shower mixer are more a pressure thing - I don't think they use any thermostat. I believe they sense a drop in cold water pressure (someone flushed a toilet) and then back off the hot water flow to compensate. I'm only familiar with the household Moen valves, and they make two varieties. The cheap one doesn't pull in and out - just turn it to the right position for temperature. These are quite prone to freezing up in the cold position. The other variety pulls in/out for volume and turns for temperature - usually work pretty well. At least, I haven't had any problem with mine. Your Scandvik may be the same type.
I'd suggest rapping it with something non-metallic (but harder than your fist) to see if you can loosen it up. Otherwise, you're into a teardown, clean, and lube - recommend silicone grease such as Dow #111 (McMaster 1204K32).
Good luck!