Thermostat is broken but needed to heat the house. I decided to jump R and W wires to get some heat this cold night. The question is for how long I can run the heater (gas)? Is it Ok to leave it on for couple hours running? Is it OK to leave it burning gas over night?
The furnace will be safe running for any extended period of time as long as the only problem is the thermostat. The only question is how much heat and temperature can you take, since the living space will get very hot with nothing controlling the temperature. If it were me, I would get the house warm and shut it off and go to sleep. If it gets cold overnight you can get up and reheat the house.