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60 year old duct system with 4 inch ducts

3266 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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Upstairs: 8 o- 4 inch
downstairs: 3 - 4 inch
downstairs: 2 -6 x12 robbers
4 return total - 6 x 12 (2 up and 2 down)

I can add some supply(s) if needed as well as returns - just don't want to switch out the 4 inch I have now. Some exposed duct work in the downstairs I could access. The house is just under 2000 square feet.
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