Hello- I've got a ~15 year old Tappan furnace, 95%, here's what it's been doing, step-by-step
1. Call for heat at the thermostat, burners fire up, main blower and exhaust motor kick on with no problem.
2. Heat works with no issues - literally I've had the heat running for hours, from 50deg to 70deg. The outside of the burner box is warm/hot to the touch, but the limit switch (165deg high limit switch with manual reset) mounted on the outside of that box does not trip at that point.
3. House reaches desired temp- thermostat stops calling for heat, burners turn off, exhaust motor turns off (main blower stays on).
4. About 1-2 minutes after the burners and exhaust shut off, the limit switch trips, which sends the furnace into a cycle where it kicks both motors on and will not fire the burners again until the limit switch is manually reset.
Here's what I've done so far-
1. Replaced the limit switch, it still trips at the same time in the cycle (1-2 minutes after the burners shut down).
2. Removed the air filter to ensure that airflow wasn't a problem.
3. Had a local HVAC pro come out multiple times to look at it. His diagnosis is that the furnace is "beat up," and that the exchanger is faulty, causing too much flame or too much heat.
That diagnosis makes no sense to me- First, if the exchanger was cracked, wouldn't that cause CO leakage and not an out-of-control flame? Second, if there's too much heat caused by the exchanger, why doesn't the limit switch trip when the burners are on? Literally, the furnace can run for hours without over-heating.
Here's what I plan to do next, appreciate any feedback or guidance-
1. Replace the exhaust motor- it's running but making a low-pitched growl when it does. It was exposed to some water due to a leak in the exhaust pipe that dripped right on the motor years ago. Not sure this is it because it's not supposed to run once the furnace runs to cool-down mode (I don't think it is anyway), but we'll see.
2. Clean as much as I can upstream and downstream from the air filter. I'm not sure what to do except stick a shopvac hose in the duct and go to town.
3. Do the same to the exhaust pipe- I guess literally stick the vac hose on the end of the exhaust.
Thanks...
High Limit Switch trips 1-2 minutes after burners shut down
1. Call for heat at the thermostat, burners fire up, main blower and exhaust motor kick on with no problem.
2. Heat works with no issues - literally I've had the heat running for hours, from 50deg to 70deg. The outside of the burner box is warm/hot to the touch, but the limit switch (165deg high limit switch with manual reset) mounted on the outside of that box does not trip at that point.
3. House reaches desired temp- thermostat stops calling for heat, burners turn off, exhaust motor turns off (main blower stays on).
4. About 1-2 minutes after the burners and exhaust shut off, the limit switch trips, which sends the furnace into a cycle where it kicks both motors on and will not fire the burners again until the limit switch is manually reset.
Here's what I've done so far-
1. Replaced the limit switch, it still trips at the same time in the cycle (1-2 minutes after the burners shut down).
2. Removed the air filter to ensure that airflow wasn't a problem.
3. Had a local HVAC pro come out multiple times to look at it. His diagnosis is that the furnace is "beat up," and that the exchanger is faulty, causing too much flame or too much heat.
That diagnosis makes no sense to me- First, if the exchanger was cracked, wouldn't that cause CO leakage and not an out-of-control flame? Second, if there's too much heat caused by the exchanger, why doesn't the limit switch trip when the burners are on? Literally, the furnace can run for hours without over-heating.
Here's what I plan to do next, appreciate any feedback or guidance-
1. Replace the exhaust motor- it's running but making a low-pitched growl when it does. It was exposed to some water due to a leak in the exhaust pipe that dripped right on the motor years ago. Not sure this is it because it's not supposed to run once the furnace runs to cool-down mode (I don't think it is anyway), but we'll see.
2. Clean as much as I can upstream and downstream from the air filter. I'm not sure what to do except stick a shopvac hose in the duct and go to town.
3. Do the same to the exhaust pipe- I guess literally stick the vac hose on the end of the exhaust.
Thanks...
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