Valve above boiler

[Mat123]

Hi. Please, can anyone tell me what the valve is on the rightside pipe going from the top of my boiler? (Obviously the opposite pipe on the left has a drain off valve). Out of interest, is there a way to know which is flow and which is return?

For info, my boiler is in the kitchen, with the HW cylinder and pump up in the airing cupboard. Its a Gloworm Ultimate which I believe was fitted when the house was built in 1993, but I've only lived here for 6 years so it may have been replaced before.

Thanks

I am aware that I have already posted this, but I have a feeling my question about this value may have been missed.

 

[gas_man_gaz]

looks like a old style pressure relief, the flow pipe will be the 1 that gets hot 1st wen ya put hw/ch on