Your variable should be available to your Run Lua action code as Reactor.variables.Heat_Day_Sun.
Your action would need to look like this (note correction to your parameter table): luup.variable_set(“urn:upnp-org:serviceId:TemperatureSetpoint1_Heat”,“SetCurrentSetpoint”,{NewCurrentSetpoint=Reactor.variables.Heat_Day_Sun},14)
But you do not need to do this in a Run Lua action. You can also set the setpoint to an expression/variable value directly in a Device Action by choosing the device, Set Heating Setpoint action, and then typing {Heat_Day_Sun} as the value.
I have tried that, and have not been able to get it to work with my Honeywell TH8320ZW1000 thermostat (installed as a generic Z-wave device because Vera does not have a configuration for this thermostat). The list of device actions given in the Actions tab has 2 identical choices “SetCurrentSetpoint”. I have read elsewhere on this forum that one is for heat and one for cool, but I can’t get them to work. Sometimes if I manually trip the sensor twice, the setpoint will get set, but it is not reliable. So now I am trying to use luup to set the variable using the name from the list of variables in the “Advanced” section of the thermostat device.