Arrived Onsite to perform diagnostic on 20 year old Lennox systems. Homeowners recently purchased the home and are still in the moving process. He changed out 2 nest thermostats for 2 ecobee thermostats last night and now the systems won’t turn on and the screens are blank. Checked thermostats for power and found that they did not have 24 volts due to no common wire being wired in on any thermostat. Existing wire does have enough wires to put a common wire in. Rewired both furnaces as well as both furnaces to now have a common wire wired in. Secured all connections, turned furnaces on. Thermostats are now recieving proper power and are turned on. Set up system settings. Homeowner will be setting up WiFi and room sensors. Cycled through modes. Systems are operational upon departure.