Cant agree with that. A large capacity cylinder that's been run cold could take longer than that to recover and if the CH is off for 30+ mins, with people coming and going then the air temp could drop significantly.The whole point is that the CH doesn't cool down significantly while heating the cylinder. A reheat on a cylinder using hot water priority should take around 10-15 minutes, during which time you'll notice almost zero difference in the room temperature. A mid position valve would just result in the radiators getting too hot and the reheat time of the cylinder significantly increasing
The radiators wouldn't get too hot if the set temp was say 60 deg for the HW