Shut down one of two unvented cylinders to match usage?

[the real mr p]

We have two unvented cylinders in parallel to serve a large property. Occupancy most of the time is low so this is massive overkill but necessary when the property is full. By installing two time clocks I can see how we could stop one heating but the system would still draw from both. Is the a way to close one cylinder's output without having to use the manual valve.

I am aware of the need to periodically heat both to avoid legionalla and am concerned that there could be pressure issues.

 

[Johntheo5]

If its radiation losses you mean then, if you fully reheat both cylinders, then shut off the cold feed to the cylinder you want to keep in use, use/draw off all the HW from the "standby" one, then shut its cold feed and reopen the cylinder "in use" you will have no radiation loses for say a week from the standby cylinder until it requires a antilegionella re heat, savings depend on the present cylinders insulation/loses/day.