[Princeton_MN]

Hello,

My low efficiency Heil furnace will intermittently fail to turn on the main burners. The exhaust vent fan starts, the glow plug lights, piolet light ignights, the main blower motor turns on and remains on, but the main gas burner never fires. It seems like the gas never turns on and the blower will run until power is turned off. Sometimes we have to cycle the power a few times to get the main burner to fire and then all is well for a few days.

Two days ago I replaced the old main timer board with one that has a status light (Honeywell ST9120U). While behaving as previously described, the st9120U status light is a dim-blink-dim-blink pattern that is defined as a heartbeat (no error code). Any ideas?

[hvac1000]

Exact model number of Heil furnace?

[Princeton_MN]

Model number NTC6075FBA1

[hvac1000]

Here is your manual

http://icpindexing.mqgroup.com/docum...4101230604.pdf

Look over the pilot flame picture and see if it looks like the flame on your pilot. If the pilot flame is incorrect it will not hit the flame sensor portion of the pilot and the furnace will not activate the gas and lock out with blower staying on.

[Princeton_MN]

Thanks for the manual link.

The pilot flame is good. The temp sensor glows red hot. I cut the wire to the temp sensor. If I make a break in the wire the pilot goes out. I also pulled 1 wire at a time on each of the temperature sensors and watched the pilot lite go out each time. I did this again for the exhause blower switch with the same result. Any other ideas?

[hvac1000]

Make sure the ground from the electric breaker panel to the unit is good and making a good connection. That is the bare copper or green wire. That control you are using likes a very good ground. NOW I am not talking about the white neutral wire but the ground.

Hook your meter up to the gas valve and make sure it is getting the proper voltage when the unit should be running. If no voltage to the gas valve it will not open for gas flow. Also if voltage is going to gas valve and it will not open it is a bad valve.

[Princeton_MN]

There have been no problems in over a week since the electronic gas valve was replaced. In hindsight, I should have verified there was voltage at the valve wiring harness when the gas burner was not working prior to replacing the control board. However, the control board was less then $100, easy to replace, and over 10 years old. Electronics tend to fail between 10 and 15 years anyway.

Thanks for the help.

[hvac1000]

At least you now have some heat and that is all that counts.