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Brian HVAC Guy
Brian HVAC Guy, HVAC Technician
Category: HVAC
Satisfied Customers: 852
Experience:  I have been an HVAC/R mechanic/technician for 30+ years.
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Rough winter! I havea Heil NUGK100DH07, a 1987 80,000BTU gas furn

Customer Question

Hi,
Rough winter!
I havea Heil NUGK100DH07, a 1987 80,000BTU gas furnace which went out at the end of the heating season.
Actually, the exhaust inducer fan doesn't spin so, I suppose the pressure switch doesn't stay on. I thought it was the fan blower as it went out about 5-7 years ago, but I may be wrong and it may be something else which is depriving it of line power. I tried sucking on the pressure hose and the unit ignites and gas will flow but without the exhaust fan, the unit won't work.
I replaced the exhaust blower fan because I tried to apply voltage directlyand it wouldn't spin so presumed the motor was bad. But, that may not be the case. :(
The blower motor will run and run and that's how I knew there was a problem when I woke to a running blower motor that wasn't putting out any heat at the registers.
Any ideas? I haven't been able to detect 120VAC at the system relay (I think that is what that is -- I've looked at the connection diagram ).
Submitted: 1 year ago.
Category: HVAC
Expert:  Brian HVAC Guy replied 1 year ago.
Hi, I'm Brian. I'm here to answer your HVAC question to your satisfaction!
With the symptoms you describe above, especially the part where you say "I tried sucking on the pressure hose and the unit ignites and gas will flow" that tells me the system relay is energizing. If you have 120 volts to ground on terminal 5 coming into the system relay but don't have 120 volts to ground on terminal 3 of the relay going to the inducer fan, then you do indeed have a failed system relay and you need to replace it.

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