问个furnace的问题

有furnace，点火1分半钟，熄火（伴随啪啪两声），再过30秒，风扇停，再过1分种，点火电热丝发红，10秒后点火。循环往复。屋里温度68，设置72。什么问题？

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

• 听起来像热传感器不灵了，一个细细的金属棒，卸下来 -David2005- ♂ (31 bytes) () 12/20/2012 postreply 14:00:09

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

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