Honeywell Dt200

But what volt free, are they compatible? My system is fully pumped from a condensing boiler

Yes it is a room stat, how your son is old stat? you should have no problem to change them more

Volt-money "is equivalent to" switch "or" relay ", etc. contacts.

The term is used in the controls, where the dry contact is used to indicate the status of a control or a control signal.

Why is "floating" - Well, we also use switches like transistors, which operate a little differently, but in general the same result, and there are other arrangements using switches or transistors together with resistors, so that short and open circuits in son control or supervision can be detected too.

Volt free, means that the thermostat contacts are isolated from the electronics in the stat, which is powered by batteries. If you have two son, connect to the terminals, if you have three son will be a late one neutral and not to him. If you have more than three, after returning with more details.

DT200 is NOT a tester temperature thermostat.That would be your problem right there.Go honeywell.com or hire a pro.