NO HEAT !! Pilot on/ Blower on. - http://www.hvacmechanic.com/ Forums

Original message

olior100

68.13.39.154

"NO HEAT !! Pilot on/ Blower on." , posted Thu 17 Nov 00:46

At intial start up ,in call for heat, blower kicks on followed by the pilot but, the burners don't light up. Just found out about the flame sensor. Have not checked milliamps. Did check voltage and it reads 123-128 ac volts. Is that right?
The ignition control unit is a Robertshaw 780-715 U (SP715) Tried cleaning the sensor, no dice.

Replies:

moondog

24.118.198.197

"Re(1):NO HEAT !! Pilot on/ Blower on." , posted Thu 17 Nov 00:53

If the burners don't lite then the flame sensor is not even yet part of the sequence. Understand? On initial start up the furnace blower comes on? Before the pilot? Or do you mean the smaller motor (the inducer exhaust motor)? That has to come on before the pilot. And where did you read the 120volts? Surely not at the flame sensor. When the flame sensor sits in the flame and gets hot, it generates a slight voltage which is 'read' by the module and then keeps the gas valve open to fire the burners. This signal is in microamps.