Hello,

I have a rebranded ICP gas furnace (NTC7075BFA3) with a SV9501 smart valve and I'm having an issue with the pilot light proving...only when the furnace hasn't been run for a while. If it's been off for ~2-5h it will start up 50% of the time the pilot will always start up fine and stay on but the main burner won't ignite or if it does it will turn off immediately. When it cools down overnight (20C - 17C temp drop) it will fail to turn on the main burner 99% of the time but the pilot will remain lit. If I decrease the temperature and increase it to call for heat again it will work 100% of the time after the failure. This makes it difficult to diagnose any fix as it takes anywhere from 2 to 8 hours to check if it worked. The pilot will always stat on but the main burner won't, the system will think it's heating and just blow cold air until the temperature is lowered and raised

I have cleaned the flame sensor and the pilot assembly. Measured 24V across the igniter when on and 4.3ohm resistance when it's cold. When the furnace is running the VAC from the flame sensor wire to ground is 110V to 108V. If I check for resistance between the flame sensor and ground while it's running the system turns of and will turn on when I stop.

I've ordered a new flame sensor/igniter but I'm not sure this is the problem since it works every time except when it's been off for a while.

Any suggestions?

Thank you

 

Welcome to the forums.

Did you clean the pilot orifice. That's very important.
You need a big bushy flame..... plenty of blue.

 

Thank you!

I did, the pilot is very blue, very little if any orange. I also blew out the tube as well when I removed it to clean the flame sensor and didn't feel any resistance, everything looked clear.