How does a tankless coil operate in the heating season? While a hot water boiler is heating at, let's say 180 degrees, what prevents the domestic hot water from overheating?
Are there different scenarios depending on storage tanks?
Nothing prevents the domestic hot water from overheating, which is the reason why you need a mixing valve on the domestic water system so you don't scald someone, or worse.
Similar situation with an indirect for the domestic. Generally you'll want the hot water temperature at 140, then mixed down to avoid scalding. Controls and an aquastat on the indirect should prevent you from raising the indirect to 180.
I'm guessing the mixing valve is electric. What type of controls? Does the control get wired to an aquastat that is seperate from the aquastat that controls the boiler for hw heating?
Are there mixing valves that are non-electric?
Most (if not all) mixing valves for domestic water temp control are non-electric. They have an internal element that responds to changes of inlet temps, and a manual adjustment knob to set the mixed outlet temp. Nothing electric to fail or stop running when the power is out or a wire gets disconected.