Tricky problem - auto fan not engaging


Old 02-13-19, 05:54 PM
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Tricky problem - auto fan not engaging

We have an older (80's I believe) gas furnace. It's a Heil (model : NDGK125DK04) furnace that has been working okay since.

A little background. About 3 or 4 months ago, I decided to do a little repairing on it. It's exhaust blower was leaking a little condensation and noisy as ever. The blower motor was also super loud. So I replaced the blower wheel, blower motor, exhaust motor. I also rewired the thermostat to a better location (using correct gauge, etc.). Everything was great, way quieter, more efficient, etc. I thought all was good...

Fast forward to a few weeks ago and suddenly the fan doesn't automatically start. When you turn up the heat, the igniter goes and starts to heat the burners then after 60 seconds or so, when the fan should engage, it doesn't and the heater shuts down. You can manually turn the fan to ON and just run it constantly while heater burns and all is fine. But of course, you can't have it automatically turn off.

I suspected it might be either the relay to the blower or maybe the flame sensor? I replaced both (pretty cheap), but nope, didn't fix it.

I'm stuck. Any thoughts?? Is it possible that maybe one of the relay wires is bad? If so, any advice on testing that? Any help is MUCH appreciated!

Other info: we were using a Nest thermostat which seemed to be okay. For whatever reason, it now can't find wifi (apparently a common issue). So i swapped it out for a simpler, battery thermostat...same fan issue. Also, I made sure the exhaust was clean, same with condensation trap and I also cleaned the coils. And...replaced the filter.
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Old 02-13-19, 06:01 PM
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There is a fan/limit control bolted to the heat exchanger directly above the burner.
There are one of two systems in use......
1) the burner lights and the heat in the plenum activates the fan switch...... OR
2) there is a heating coil in the fan switch that activates the switch on heat.
The manual below should be for your furnace. Look at page 23 for explanation of the fan limit.
There are diagnostics there too.

Heil NUGK manual (pdf)
Old 02-13-19, 06:13 PM
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Wow, thanks for the quick response!
That makes total sense. I'll switch out that limit switch and give it a go. At least I know I won't be having any problems with the relay, etc...

Much appreciated!!!
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