Zip Unvented Water Heater Acessories - Zip Unvented Water Heater Accessories Will the pipework accommodate expanded water without hot water entering any supply pipe connected to a cold water draw-off?If expansion in the cold water main is restricted, e.g. by a valve or meter, an expansion vessel and other relevant accessories MUST be fitted. The Zip Unvented Acessories are specially designed for those situations.