DIY Doctor UK DIY Forums

I was wondering if someone could clear something up for me. What is the purpose of the three way valve on the vertical pipe joining the upper and lower pipes feeding the tank? Is it used to bypass the tank? If not, should it remain opened or closed?

It was opened when I purchased the property, but I cant get my head around what purpose it serves.

Thanks.

It seems to be a thermostatic mixing valve with remote sensor, I would guess it controls temperature of domestic hot water. But I am only an electrician.