問個furnace的問題

有furnace，點火1分半鍾，熄火（伴隨啪啪兩聲），再過30秒，風扇停，再過1分種，點火電熱絲發紅，10秒後點火。循環往複。屋裏溫度68，設置72。什麽問題？

所有跟帖： 

• Was it good before? Sounds like not enough return airflow. -borisg- ♀ (0 bytes) () 12/20/2012 postreply 12:52:37

• tenants said it is always like that. today is cold, the heat is -sweetptt- ♂ (0 bytes) () 12/20/2012 postreply 12:57:10

• Then it is either return clogged by junk or the vent layout is f -borisg- ♀ (0 bytes) () 12/20/2012 postreply 13:07:51

• 聽起來像熱傳感器不靈了，一個細細的金屬棒，卸下來 -David2005- ♂ (31 bytes) () 12/20/2012 postreply 14:00:09

• yeah，try clean the flame sensor first -地主老頭兒- ♂ (0 bytes) () 12/20/2012 postreply 14:28:22

• upper limit switch is fault. -jy101- ♂ (0 bytes) () 12/20/2012 postreply 17:01:38