Boilers don't run continuously--when the thermostat calls for heat, the boiler fires up. 

WiFi thermostat.

So, we know that we want to at least add some zoning control so that both tenants have some level of control over the thermostat.

Hire a good house keeper and between VRBO and the house keeper 85% of your running a short term rental are taken care of.You could hire out the lawn care too if your brother doesn't want to deal with this.I use wifi thermostats to save on electricity when no one is at property and a lock box for keys.I see you say you don't have a pool.

Rently can be nice if you set it up to include the smart lock and smart thermostat.

@Abigail NittleMy only addition to what’s been said is the huge advantage of a remote location to add “smart” features like thermostat and WiFi door locks that allow you to manage the space at a distance.

Before making a no heat call please check that the batteries in your thermostat are properly installed and have a charge.