A few days ago a friend presented me with the following problem, how can I install a 24 volt thermostat if my fan coil and my condenser unit are 220 volts that is not possible. I explained to him that, if it was possible that with a transformer and contactors with a 24-volt coil it is enough, but he did not understand me, so it occurred to me to make a sketch and explain it to him.

Ok the first thing that is observed in blue are the 220V lines, which will supply energy to the fan coil and the condensing unit. But previously they pass through the contacts with 24 V coil since the equipment has 220 Volt power, but they do not connect directly because the thermostat supplied its power and output signal are 24V, and this is responsible for the control of the equipment when it is required to cool or stop cooling the environment.

The second thing is to take two lines of 220V and using a current transformer at 24V, feed the thermostat placed one phase in purple to the power of the RC thermostat, and the other line is used as a COMMON phase that is red and segmented, this is important since it will be the one that feeds the coils of the contactors in one phase and the other will be the signals of ignition of the equipment as required by the environment.

In the thermostat the signal of the fans is taken that will be commonly the letter G in the apparatus and the other phase of the coil of the fancoil is connected so that this is activated. And in the phase of the thermostat of the compressor or condensing unit that is the letter Y, this is connected to the contact phase of the condensing unit.

There our air system would work, since the thermostat when it is turned on, will activate the fancoil and the condensing unit and later control the ignition of the condenser when required.