Hello vented and unvented

[jonboy151111]

Hello is it only unvented that needs a blow of valve if expansion vessel fails of to much pressure is going through

 

[steadyon]

"Vented" and "Unvented" are normally used to refer to hot water system:

A vented hot water system has an unobstructed pipe (the vent pipe) from the top of the hot water storage cylinder up to and above the cold water storage cistern (CWSC), terminating in a loop back down to end below the cover of the CWSC but above the water level.

An unvented hot water system has no vent pipe, but has a number of safeth devices built into / around the hot water storage cylinder, including both a pressure relief valve, and a temperature and pressure relief valve. These cylinders are potentially very dangerous, and may only be legally installed and maintained by a competent person holding a "G3" certificate.

Wet central heating systems may be open vented or sealed:

An open vented system will have an unobstructed pipe from the boiler's to above, and looped back down to the feed and expansion tank which feeds cold water to the boiler.

A sealed system has no feed and expansion tank, but does have a sealed expansion vessel to accommodate the increased volume of hot as opposed to cold water, together with a pressure relief valve to dump water to outside should other components fail and the pressure exceed a pre-set value, normally 3 bar.