How do i open the drain out bathroom bottle? Bringing you the top names in boilers-. Turn down your central heating, even by one degree. How do i use drain out bathroom on an extra tough clog or floor drain? Sometimes the boiler may need depressurising to resolve the problem this is a more complex procedure and should only be carried out by a trained gas accredited engineer. What pre-hire questions should be posed to a plumber? Follow steps two through four if drain does not clear,.