Noticed some questions back up this thread about controlling Ferroli boilers, and the meaning of "voltage free contact"
Basically, the Ferroli (like many boilers) has a pair of screw contacts on the edge of the controller PCB, that are shorted out by a copper link.
If you remove that link the boiler will not come on, unless the frost stat trips.
If you short that link, what happens depends on the settings of the internal controls on the boiler.
In effect the internal timeswitch and internal thermostat are in series with that link, so the boiler will only come on if all those other control sources are also asking for boiler on.
What the manufacturer means by voltage free contacts is in effect making or breaking that short.
Very specifically not applying a voltage of any sort to those pins.
A set of relay contacts for instance can be arranged to make of remove that short controlled by an external control system.
I'd avoid using a solid state relay for this, they have a small leakage current when closed, and the how the boiler senses the open/short state of that link will determine if that leakage is sufficient to confuse the boiler into thinking the link is always closed.
If the internal boiler timer is set to be always on, and the internal thermostat is set to max, the boiler is effectively under control of that relay.
With those internal controls enabled, your external control circuit can only force the boiler off, not force it on.
Sensible people wire this up so that the first thing the wire pair from those contacts meets is a two pole, three position, on off on toggle switch.
The boiler is connected to the common pins of the switch. One pair of pins is shorted out, the other pair of pins goes off to your control system.
Thus with that switch in the center position, the boiler is forced off. With the switch set to the shorted pair of pins, this is equivalent to the manufacturers default shipping position with the pins shorted, and the boiler operates as if your external control does not exist. With the switch set the other way the boiler can be held off by your control relay.
At no point should any voltage, either low voltage from the pi, nor mains voltage from some of the available thermostats, be applied directly to the boiler control pins. It 's a bad idea to connect any of the pi's circuitry to those pins, the voltage and current that the boiler controller uses to sense the presence or absence of that short is undefined, and very probably enough to fry your pi.
This is a common scheme, and I know the scheme in use by some of the Ferroli boilers (I have one) but close inspection of your boilers handbook is called for before making any connection. If in doubt, call a boiler engineer.