I know there are zone valves on the market with manual levers, but this one doesn't have them. It seems that a manual lever for these isn't really required. The ones that have the manual lever I believe are spring closed. Therefore, the lever will just simply open them allowing heating. I don't live in the tundra, so immediate heating if it goes out isn't the end of the world. Any other reason to have it?
The manual lever is so you can bleed the system, so it depends if you can bleed the system without the lever.
Zone valves are really something from the past, they are still used with under floor heating, but the temperature is a lot lower with underfloor heating.
Short history lesson, to get the boiler to use the latent heat the return water temperature needs careful control, so there has been a move from digital (on/off) to analogue, the TRV slowly opens and closes and so does the by-pass valve.
Where a wall thermostat is used it connects to the ebus and modulates the boiler it does not switch it on/off. There is an exception, the boiler will never turn off without either manual or thermostats intervention, it would first of all modulate then the boiler would start cycling, so fitting a thermostat in the coldest, down stairs room with no outside doors or alternative heating will turn off the system as it gets warmer and stop is cycling.
With a non spring return motorised valve you could in theory allow it to part open, not seen one, but that does not mean they don't exist. With eTRV heads they do open a bit at a time, you can hear the motor.
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