Caleffi Unvented Group Kit 3 Bar 533003CST is also known as Pressure Relief Valve, PRV, Safety Valves, over flow valve and Expansion Relief Valves. The function of this type of Valve is to relieve the system of over pressure in a safe manner.It is set to open at a predetermined set pressure to stop pressure building up and preventing the expansion vessels and other equipmen and from being damaged. Please always refer to Manufacturers instruction, and get a competent person to carry out any repair to you boiler. Make sure your Gas Safe Engineer uses Original Boiler Spare Parts, and they should come packed in the manufacturers box and sealed. When contacting Heating Spares please have your boiler details to hand and if possible have the boiler (GC) Gas Council number ready. Heating Spares 247.com is a an Approved Boiler Spares Stockist, and we Guarantee to supply Genuine Boiler Spares from the Boiler Manufacturers. Services available are trade counter pick up, Next working day, next working day AM and Saturday delivery. We also operate same day express delivery service.