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60 year old duct system with 4 inch ducts

3258 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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How big a furnace do you need? If you have enough 4" ducts, it could work, but currently you have 132 sq in of supply from the 4" ducts and that's less than half of what you would need for a 70,000BTU furnace. Not familiar with the term "robber" but if it's a supply duct, you are getting another 144 sq in of supply from those. Problem with 4" ductwork with a gas furnace is the velocities will be quite high, producing drafts and noise.
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