A faulty furnace is believed to be the cause of a fire in Plymouth Friday morning.The call came in around 7 a.m. from the Lower Detroit Road.The house was already up in flames when fire crews arrived.Assistant Fire Chief Arrin Farrar says they do not believe anyone was home at the time of the fire.Farrar says one firefighter was sent to the hospital, but he is unsure of the reason.He says the frigid temperatures always pose a challenge.” Extremely cold conditions radios die. Hoses freeze up, water freezes. So we get many problems. The pumpers have froze up. They had to leave the scene and go back to the station.”The Fire Marshal’s Office is on scene investigating.