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14 Posts
Hey folks, my first post! I’ll try to be detailed!
So I bought an older house with an older furnace (early 90’s), the duct work I believe is original from the very early 1900’s when the house used to have a coal fired furnace so it’s pretty restrictive. Since I got the house the furnace has been kicking off every now and then and I never had the time to investigate until now. Turns out, it’s been over heating and the control limit switch keeps opening.
I changed the filter, cleaned the blower and inducer motor, checked the heat exchangers for cracks and lastly monitored the temperature inside the air chamber with my own thermocouple. My thermocouple isn’t calibrated but the control limit trips at roughly 185F even though it’s a L160-20. When I bypass the control switch the max temperature I saw is about 200F which is shown below in the graph. What harm is there in putting an L170-20 in as a replacement? It's a two family house and the other furnace is identical and uses an L170-20.
It seems like my options are:
-increase the control limit temperature value
-tone down the BTUs of the burners
-redo my ducts to allow better airflow
Am I missing something? Any input would be greatly appreciated
Temperature graph link: h t t p : / / i50.tinypic.com/hvapvp.png
So I bought an older house with an older furnace (early 90’s), the duct work I believe is original from the very early 1900’s when the house used to have a coal fired furnace so it’s pretty restrictive. Since I got the house the furnace has been kicking off every now and then and I never had the time to investigate until now. Turns out, it’s been over heating and the control limit switch keeps opening.
I changed the filter, cleaned the blower and inducer motor, checked the heat exchangers for cracks and lastly monitored the temperature inside the air chamber with my own thermocouple. My thermocouple isn’t calibrated but the control limit trips at roughly 185F even though it’s a L160-20. When I bypass the control switch the max temperature I saw is about 200F which is shown below in the graph. What harm is there in putting an L170-20 in as a replacement? It's a two family house and the other furnace is identical and uses an L170-20.
It seems like my options are:
-increase the control limit temperature value
-tone down the BTUs of the burners
-redo my ducts to allow better airflow
Am I missing something? Any input would be greatly appreciated

Temperature graph link: h t t p : / / i50.tinypic.com/hvapvp.png