How many heating systems can be connected to one thermostat?

This is dependent upon the amperage of the units that you are trying to install. Each thermostat can carry a load of 15 amps.

Once this limit is reached, a relay must be installed (and multiple relays after that, dependent upon the amperage of the units). The Relay-25 is a 25A-240V relay used to increase the load-switching capacity of the Warmup thermostats (15A). Together a thermostat and relay can switch up to 800 sqft of heated floors.

Please note that while the relay needs to be accessible, it does not need to be in plain sight. You may hide it in a cabinet to avoid having two thermostats visually available in the room.

With that being said, if you are not interested in having a master and a relay and would prefer to have just one thermostat installed, you may have your electrician install a contactor panel to help carry the extra amperage load.