Hi All, First post, glad to become a new member. Stonemason by trade but need some advice on a digital thermostat replacement. Currently have an old honeywell system from 2007 and want to upgrade to a digital d90. However, I've got issues understanding the wiring set up. Looking on you tube etc it only shows a 2 wire set up and I have a 3 wire set up. See attached photos. 1 and 2 N and L seem a straight swap, just need to know where the 3rd wire goes into the new set up photo attached, any advice ?? Thanks