furnace runs for 5-8 min, shuts down for 3-4, runs, shuts, runs and so on..... mod #GSU 75 MHN cannot locate, looks like an old Janitrol ??? VERY COLD w/ KIDS.

 

It sounds like a heat anticipator problem. That is a device inside the thermostat that gives off heat to stop the furnace slightly short of the desired temperature. That way the residual heat in the heat exchanger of the furnace can be pushed into the house and the result is temp being exactly what you set it for. If you have it set wrong, it may give off too much heat and stop the furnace way too soon. If you have a thermostat that is not digital, pull off the front cover and look for a small rotary slide resistor or a metal pointer with numbers near it. You should see the word LONGER and an arrow somewhere. Move the pointer a couple numbers toward longer and see what happens. If you need further asistance, post here again.