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60 year old duct system with 4 inch ducts

3264 Views 5 Replies 4 Participants Last post by  sktn77a
My house used to have a low pressure GE oil furance. Nice alumn. ductwork, but all the runs to the boots are 4 inches. In 2001 the former owner put in a new Lennox oil furance. The fun cycled constantly, even though she had a smaller fan put in (5 inch). Now that I'm remodeling I want to switch to gas and was hoping to use the existing duct work. I understand that there MAY BE variable speed gas furnaces that will do OK with the 4 inch ducting? Some contractors have told me I don't have any options and that I must size up the ducting to 6 inch. I can't get to the majority of the ducting and I have lath and plaster, so I don't want to start tearing into the walls at this point. I don't want to purchase a new furnance and find out I am still going to have the same problem with the fan cycling etc. Any thoughts on this?
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I doubt if a ECM unit will work well with those pipes. My technical rep from Lennox has told us that some newer cheap built homes with undersized ductwork are having problems with the ECM motor not being able to find a constant speed from a lack of airflow. Causes the motor to hunt. May also void the unit warranty. We have a few of those 4" systems in Wpg and they are terrible, Sorry.
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