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HEIL gas furnace made in 1990 model NUGE075CH02, older Honeywell dial limit/fan switch, LUX digital thermostat. Daytime temps around 50F, night temps in 30s.
THE PROBLEM: Furnace runs fine all day and evening. In the early morning hours (3 to 5am) we wake up to a constantly clicking ignitor, a running vent blower, a pilot light and no gas to the burners. Sometimes the main blower is running. If I turn it off and let it sit for an hour it will work fine afterwords. Sometimes have to wait several hours. Through some research it appears it may be overheating but I have yet to catch the reading on the limit switch dial quick enough. Observing it during the day and evening it climbs to around 170-180 deg. but nowhere near the 210 deg high limit setting.
WHAT I'VE TRIED: Changed the filter, run it without a filter and removed and cleaned the combo ignitor/flame sensor. Still fails. Last night I discovered if before going to bed I unplug it a few minutes AND leave the door cracked open (it sits in hall a closet) it will run through the night. Not sure which of those is "curing" the problem. Thanks in advance for any advice for a permanent fix.
On a side note, for several months I've been hearing a distinct thump usually when the main blower turns on and always around 3-4 minutes after the burners turn off as it's cooling. Not absolutely sure but it sounds like it's coming from above A/C evaporator or even in the attic. It's more pronounced with the filter removed. I don't see an easy way to get in there and find out what's up.
Just to make sure I've got a grasp of how this thing works it's the high limit switch that cuts off gas to the burners and the pressure switch that kills the ignitor correct? Thanks again.
THE PROBLEM: Furnace runs fine all day and evening. In the early morning hours (3 to 5am) we wake up to a constantly clicking ignitor, a running vent blower, a pilot light and no gas to the burners. Sometimes the main blower is running. If I turn it off and let it sit for an hour it will work fine afterwords. Sometimes have to wait several hours. Through some research it appears it may be overheating but I have yet to catch the reading on the limit switch dial quick enough. Observing it during the day and evening it climbs to around 170-180 deg. but nowhere near the 210 deg high limit setting.
WHAT I'VE TRIED: Changed the filter, run it without a filter and removed and cleaned the combo ignitor/flame sensor. Still fails. Last night I discovered if before going to bed I unplug it a few minutes AND leave the door cracked open (it sits in hall a closet) it will run through the night. Not sure which of those is "curing" the problem. Thanks in advance for any advice for a permanent fix.
On a side note, for several months I've been hearing a distinct thump usually when the main blower turns on and always around 3-4 minutes after the burners turn off as it's cooling. Not absolutely sure but it sounds like it's coming from above A/C evaporator or even in the attic. It's more pronounced with the filter removed. I don't see an easy way to get in there and find out what's up.
Just to make sure I've got a grasp of how this thing works it's the high limit switch that cuts off gas to the burners and the pressure switch that kills the ignitor correct? Thanks again.
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