What should be the maximum distance between hob and hood and still maintain efficiency

Ay least 24 inches

Customer always needs to follow the manufacturer's instructions about cleaning and maintaining the appliance. If there are some ambiguities then you should contact an authorized service center. I personally believe that it is not a good idea to clean hob with steam cleaner.

6.3 Cleaning the hob
• Remove immediately: melted plastic,
plastic foil, and food with sugar. If not,
the dirt can cause damage to the hob.
• Remove when the hob is sufficiently
cool: limescale rings, water rings, fat
stains, shiny metallic discoloration.
Clean the hob with a moist cloth and
some detergent. After cleaning, rub
the hob dry with a soft cloth.
• To clean the enamelled parts, cap
and crown, wash them with warm
soapy water and dry them carefully
before you put them back.

Dado

In my experience, the hood should be just above head height, that will avoid banging your head, and at least 33 inches or 8ocm above the hob.
Hope this helps.

The minimum distance is 18" and the max is 36"

Maintaining the lowest acceptable setting and leaving it at that temperature is the most economical way to run the unit. If the space is going to be unoccupied for an extended time (several hours), lower temperatures are okay. This minimizes the need to burst the heat to warm the space quicker. As I understand your unit, it minimizes electrical & gas consumption by basing the need on the temperature difference of actual space temperature and setpoint. The larger the gap, the more power and gas is consumed.
In summary, when the space is occupied, maintain a consistent temperature setting. When the space is unoccupied, maintain a lower setpoint.

Hope this helps. Good Luck. :-)

The poupose of either is to eliminate any smoke created by the range or grille and move as much of the cooking odors to the outside. I wouldn't use both at the same time, but if the hood is vented to the outside, it would be just fine to use only the hood. Aregular cleaning will be necesssary. The downdraft has limits as to the distance it will efficiently exhaust air. Flex hose should not be used. And excessive bends or vent length, cuts into the efficiency.

Minimum above a gas burner should be 750mm and for electric hob should be 650mm.

Normally the rangehood should be at least 700mm from the top of the cooktop.