I've been trying to think of a way to make this setup cover a power outage situation as well. You could use a NO zone valve, but that would dump the water whenever the power failed, regardless of water temp. That would be a bad thing if there was a fire roaring in the boiler at the time. You could have it on a battery backup, but that starts to feel pretty complicated. You'd need some sort of bi-metal controlled zone valve, or perhaps some variation of a thermostatic mixing valve like many showers use.