Combination Boilers – Thermostats. Mains voltage 2 cable thermostat All combination boilers should have an area thermostat fitted. Without one, you will have no real means for the atmosphere heat to be managed. Volt terminals that are free Some boilers have actually volt-free terminals. Whilst the title indicates, they are maybe maybe not attached to any energy supply, nor must any energy be reproduced for them. When you look at the diagram, these terminals are labelled A and B, but…