<chuckle>
Oh dear, here we go...
"Competent to comment on all matters in relation to buildings" is what I claim.
I'm very wary of claiming expertise above and beyond that of a competent practitioner in my profession.
[I also worked as a plumbers mate at one time but I tend to stick to what I'm good at these days].
Hot water cylinders may have two means of heating the domestic water supply; -
i) an electric element, the so called immersion heater
ii) a coil fed by the boiler - the indirect heating method
The water in the coil doesn't mix with the water in the cylinder.
The water in the cylinder is for domestic use - HW taps, showers etc.
A separate boiler pipe feeds the radiators - the Primary circuit - and the boiler/rad systems is kept topped up by, and expands into, an expansion tank.
This is smaller than and often separate from the main tank.
You can sometimes see the smaller tank perched above the larger one on older indirect systems.
The hot water cylinder cylinder therefore regularly empties, refilled by water from the main attic tank, with hot water being drawn off to the HW taps throughout the house.
The HWC will eventually empty if the main tank is drained and the main tank supply cut off and people continue to use the hot taps in the house.
The Primary circuit passes into and out of the HW cylinder at top and bottom.
Like the immersion, it is designed to heat a cylinder full of water, not full of air.
I have seen HWCs fail at these connection points.
Thus my concern that I would be wary of running a heating system for any length of time when the HWC is empty.
I am happy to stand corrected or receive some wisdom on the matter.
ONQ.
PS I'm assuming this is the method of heating used.
It may be an older house with a totally separate heating system in which case no problems may arise
It may be also possible to shut of the branch to the presumably empty HWC.