Heil NT series horizontal mount - frustrated !!

Hi there, my home has 3 Heil NTG3 / FBF 80% Eff units - two have never had issues, but one I am ready to take a hammer to.. LOL


Here is the scenario - 1> thermostat calls for heat, and unit does not come on , so I open the blower door, activate the interlock 4-5 times and unit comes on works fine until it sits for a while then it does not come on again. I figured it was a sticky relay on the fan / control board.

After about a week of on/off/on now its dead and no amount of interlock on/off will bring it to life.

So I did some troubleshooting - thermostat is good, so is the interlock switch as is the pressure switch and any limit switches I can get to - when I trip the interlock the transformer hums and applies 24V to the control board so I deduced that it was a nearly 100% chance the fan control board was bad.

I ordered a new one, installed it and SSDD - dead. No difference.

For the hell of it I tried the Air Conditioning - worked fine.

The process of firing goes like this:

1> Thermostat calls for heat
2> Fan control board fires the draft induction motor
3> Pressure switch closes, telling Ignitor module to fire ignitor
4> Once ignitor / pilot is lit ignition module opens gas valve
5> Once temp sensor opens, blower starts

Simple stuff. I have worked on far more complex systems than this, unfortunately, the schematics we can find are rudimentary and not very accurate.

I have not replaced the ignition module, but from what I understand it has nothing to do with staring the draft motor - maybe I am wrong.

I short circuited the pressure switch to "fake out" the ignitor into thinking the draft motor was running and still nothing ...

I found three limit switches - all 3 are fine. Unless there is some hidden I cannot find. No loose connections, draft motor works fine, cannot believe I've wasted as much time on this as I have.

From what I understand about these systems they are pretty simple... if the ignition module is bad the furnace should still start it just won't ignite - ditto for the ignitor itself... usually starts, then shuts off. Never seen anything like this before.

Any ideas ??

Thanks.