Even if you have the thing plumbed into the hot water supply you will have to wait to clear the cold water in the pipes before if will even be tepid.
There are ways round this, it is possible to have the hot water from a cylinder circulated around a "ring main" (I'm sure that's not the correct term but I'm not a plumber) with the various hot water feeds drawing off this. This is effective in large buildings like hotels but not cost effective in domestic settings as you have to pay to have pumps running all the time and the cylinder will lose heat quicker.
Even if you have the thing plumbed into the hot water supply you will have to wait to clear the cold water in the pipes before if will even be tepid.
There are ways round this, it is possible to have the hot water from a cylinder circulated around a "ring main" (I'm sure that's not the correct term but I'm not a plumber) with the various hot water feeds drawing off this. This is effective in large buildings like hotels but not cost effective in domestic settings as you have to pay to have pumps running all the time and the cylinder will lose heat quicker.